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  • 1.
    Aakash, förnamn
    et al.
    Department of Electronics and Communication Engg., Birla Institute of Technology.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mohan, Rajendra
    Department of Physics, National Institute of Technology Patna.
    Mukherjee, Samrat
    Department of Physics, National Institute of Technology Patna.
    Structural, magnetic and hyperfine characterizations of nanocrystalline Zn-Cd doped nickel ferrites2017In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 441, p. 710-717Article in journal (Refereed)
    Abstract [en]

    In our present work, we have synthesized a series of Cd-Zn doped nickel ferrite ((Cd-0.5 Zn-x(x)) Ni0(.5)Fe(2)O(4); x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) through standard chemical co-precipitation method to study the influence of diamagnetic ions (Cd, Zn) on the magnetic properties of ferrites. XRD and Raman spectroscopy were employed for the structural characterizations. The refinement of the X-ray diffractogram data augmented by the Williamson-Hall plots showed the presence of Cd2+ vacancies and a strained crystal structure. The vibrational spectroscopy indicated the presence of lower space-group symmetry and a distorted crystal structure. Magnetic measurements showed the samples possessed low magnetic anisotropy along with a canted spin structure. The Mossbauer measurements confirmed the cation distribution and gave evidence of super transferred hyperfine interactions arising due to canted spin structure of the system.

  • 2.
    Abou-Ras, Daniel
    et al.
    Helmholtz Zentrum Berlin Mat & Energie GmbH, Hahn Meitner Pl 1, D-14109 Berlin, Germany..
    Wagner, Sigurd
    Princeton Univ, Dept Elect Engn, Princeton, NJ 08544 USA..
    Stanbery, Bill J.
    Siva Power, 5102 Calle Sol, Santa Clara, CA 95054 USA..
    Schock, Hans-Werner
    Helmholtz Zentrum Berlin Mat & Energie GmbH, Hahn Meitner Pl 1, D-14109 Berlin, Germany..
    Scheer, Roland
    Martin Luther Univ Halle Wittenberg, Inst Phys, Photovolta Grp, D-06120 Halle, Saale, Germany..
    Stolt, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. Solibro Res AB, Sweden.
    Siebentritt, Susanne
    Univ Luxembourg, Phys & Mat Sci Res Unit, Lab Photovolta, Belvaux, Luxembourg..
    Lincot, Daniel
    CNRS EDF Chim Paristech PSL, Inst Photovolta Ile France IPVF, IRDEP, 6 Quai Watier, F-78401 Chatou, France..
    Eberspacher, Chris
    Solopower Syst Inc, Corp & Mfg Headquarters, 6308 North Marine Dr, Portland, OR 97203 USA..
    Kushiya, Katsumi
    Solar Frontier KK, 123-1 Shimo Kawairi, Atsugi, Kanagawa, Japan..
    Tiwari, Ayodhya N.
    Empa Swiss Fed Labs Mat Sci & Technol, Lab Thin Films & Photovolta, Ueberlandstr 129, CH-8600 Dubendorf, Switzerland..
    Innovation highway: Breakthrough milestones and key developments in chalcopyrite photovoltaics from a retrospective viewpoint2017In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 633, p. 2-12Article in journal (Refereed)
    Abstract [en]

    The present contribution is a summary of an event that was organized as a special evening session in Symposium V "Chalcogenide Thin-Film Solar Cells" at the E-MRS 2016 Spring Meeting, Lille, France. The presentations in this session were given by the coauthors of this paper. These authors present retrospectives of key developments in the field of Cu(In,Ga)(S,Se)(2) solar cells as they themselves had witnessed in their laboratories or companies. Also, anecdotes are brought up, which captured interesting circumstances in that evolutionary phase of the field. Because the focus was on historical perspectives rather than a comprehensive review of the field, recent developments intentionally were not addressed.

  • 3. Adeyev, V.M.
    et al.
    Vasylyev, M.O.
    Pogorelov, Ye.
    Features of auger-investigations of ultrathin periodic metallic film structures2006In: Metallofizika i novejsie tehnologii, ISSN 1024-1809, Vol. 28, no 2, p. 193-200Article in journal (Refereed)
    Abstract [en]

    In the presented work the applicability of Auger Electron Spectroscopy for studying layered metal film structures Fe/(Au, Cu)/Tb on Si substrate with thickness of layers up to 30 Å was shown. It was ascertained that Fe layer is interacting with Si substrate and as a result Fe-silicide is formed. The thickness of buffer layers aimed to protect Fe layer appeared to be insufficient for complete suppression of silicide formation

  • 4.
    Adlmann, Franz Alois
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Opportunities and challenges of surface scattering at next generation neutron sources2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Complex fluids and soft matter are ubiquitously found in the world and all contacts in life are made over surfaces. To describe the mechanical behavior of such substances, rheological methods are used. Flow instabilities are a big challenge in rheology since they will be reflected in the macroscopic variables probed, like e. g. the viscosity. Many such discontinuities may actually originate at the surface. Investigating the properties of liquids in contact with the surface under mechanical load is the main course of the thesis. Neutron reflectometry and GISANS are perfect tools in this context to access the solid liquid interfaces, since they can penetrate many engineering materials and show a comparably large scattering potential at deuterated samples. In this thesis shear was applied on a model solution and neutron scattering techniques were used to investigate the structure under load. The focus was set on the development of the measurement methods themselves to enable new scientific insights in the future. First, by stroboscopic measurements the flux limitations are overcome for oscillatory rheology. By reintegration in the post processing it is shown that kinetic measurements with neutrons are possible with a time resolution below one millisecond. Second, the transformation of grazing incidence data from the laboratory system into q-space is strongly non-linear resulting in a need for re-binning. In this thesis a universal tool has been developed for this purpose. Finally, there is an ongoing discussion on depth sensitive neutron scattering experiments from solid-liquid boundaries. By using emission densities we show that such experiments face severe limitations due to the low absorption cross section of the neutron.

    List of papers
    1. Depth-resolved grazing-incidence time-of-flight neutron scattering from a solid-liquid interface
    Open this publication in new window or tab >>Depth-resolved grazing-incidence time-of-flight neutron scattering from a solid-liquid interface
    Show others...
    2014 (English)In: Journal of applied crystallography, ISSN 0021-8898, E-ISSN 1600-5767, Vol. 47, p. 130-135Article in journal (Refereed) Published
    Abstract [en]

    Small-angle scattering in grazing-incidence beam geometry has been applied on a time-of-flight neutron instrument to investigate a solid-liquid boundary. Owing to the broad wavelength distribution provided for a specific incident beam angle, the penetration depth of the neutron beam is varied over a wide range in a single measurement. The near surface structures of block copolymer micelles close to silicon substrates with distinct surface energies are resolved. It is observed that the very near surface structure strongly depends on the surface coating, whereas further away from the surface, bulk-like ordering is found.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-219976 (URN)10.1107/S1600576713028665 (DOI)000330485100018 ()
    Available from: 2014-03-12 Created: 2014-03-09 Last updated: 2018-04-11Bibliographically approved
    2. Polymer Brush Collapse under Shear Flow
    Open this publication in new window or tab >>Polymer Brush Collapse under Shear Flow
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    2017 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 50, no 3, p. 1215-1224Article in journal (Refereed) Published
    Abstract [en]

    Shear responsive surfaces offer potential advances in a number of applications. Surface functionalization using polymer brushes is one route to such properties, particularly in the case of entangled polymers. We report on neutron reflectometry measurements of polymer brushes in entangled polymer solutions performed under controlled shear as well as coarse-grained computer simulations corresponding to these interfaces. Here we show a reversible and reproducible collapse of the brushes, increasing with the shear rate. Using two brushes of greatly different chain lengths and grafting densities, we demonstrate that the dynamics responsible for the structural change of the brush are governed by the free chains in solution rather than the brush itself, within the range of parameters examined. The phenomenon of the brush collapse could find applications in the tailoring of nanosensors and as a way to dynamically control surface friction and adhesion.

    Place, publisher, year, edition, pages
    American Chemical Society (ACS), 2017
    National Category
    Materials Engineering
    Identifiers
    urn:nbn:se:uu:diva-320464 (URN)10.1021/acs.macromol.6b02525 (DOI)000394410200053 ()
    Available from: 2017-04-26 Created: 2017-04-26 Last updated: 2018-04-11Bibliographically approved
    3. Probing the dynamics of high-viscosity entangled polymers under shear using Neutron Spin Echo spectroscopy
    Open this publication in new window or tab >>Probing the dynamics of high-viscosity entangled polymers under shear using Neutron Spin Echo spectroscopy
    Show others...
    2016 (English)In: VI European Conference On Neutron Scattering (ECNS2015), 2016, article id UNSP 012014Conference paper, Published paper (Refereed)
    Abstract [en]

    Neutron Spin Echo spectroscopy provides unique insight into molecular and submolecular dynamics as well as intra-and inter-molecular interactions in soft matter. These dynamics may change drastically under shear flow. In particular in polymer physics a stress plateau is observed, which might be explained by an entanglement-disentanglement transition. However, such a transition is difficult to identify directly by experiments. Neutron Spin Echo has been proven to provide information about entanglement length and degree by probing the local dynamics of the polymer chains. Combining shear experiments and neutron spin echo is challenging since, first the beam polarisation has to be preserved during scattering and second, Doppler scattered neutrons may cause inelastic scattering. In this paper we present a new shear device adapted for these needs. We demonstrate that a high beam polarisation can be preserved and present first data on an entangled polymer solution under shear. To complement the experiments on the dynamics we present novel SANS data revealing shear-induced conformational changes in highly entangled polymers.

    Series
    Journal of Physics Conference Series, ISSN 1742-6588 ; 746
    National Category
    Accelerator Physics and Instrumentation
    Identifiers
    urn:nbn:se:uu:diva-346197 (URN)10.1088/1742-6596/746/1/012014 (DOI)000409470900014 ()
    Conference
    6th European Conference on Neutron Scattering (ECNS), AUG 30-SEP 04, 2015, Univ Zaragoza, CSIC, Mat Sci Inst Aragon, Zaragoza, SPAIN
    Available from: 2018-03-15 Created: 2018-03-15 Last updated: 2018-04-11Bibliographically approved
    4. Depth resolved grazing incidence neutron scattering experiments from semi-infinite interfaces: a statistical analysis of the scattering contributions
    Open this publication in new window or tab >>Depth resolved grazing incidence neutron scattering experiments from semi-infinite interfaces: a statistical analysis of the scattering contributions
    Show others...
    2018 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, article id 165901Article in journal, Letter (Refereed) Published
    Abstract [en]

    Grazing incidence neutron scattering experiments offer surface sensitivity by reflecting from an interface at momentum transfers close to total external reflection. Under these conditions the penetration depth is strongly non-linear and may change by many orders of magnitude. This fact imposes severe challenges for depth resolved experiments, since the brilliance of neutron beams is relatively low in comparison to e.g. synchrotron radiation. In this article we use probability density functions to calculate the contribution of scattering at different distances from an interface to the intensities registered on the detector. Our method has the particular advantage that the depth sensitivity is directly extracted from the scattering pattern itself. Hence for perfectly known samples exact resolution functions can be calculated and visa versa. We show that any tails in the resolution function, e.g. Gaussian shaped, hinders depth resolved experiments. More importantly we provide means for a descriptive statistical analysis of detector images with respect to the scattering contributions and show that even for perfect resolution near surface scattering is hardly accessible.

    Place, publisher, year, edition, pages
    Institute of Physics Publishing (IOPP), 2018
    Keywords
    GISANS, neutrons, interfaces, depth profiling, resolution
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-348219 (URN)10.1088/1361-648X/aab573 (DOI)000428466400001 ()29521272 (PubMedID)
    Funder
    Swedish Research Council, 2016-04645The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), IG-2011-2067
    Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2018-06-20Bibliographically approved
    5. Normalization of stroboscopic neutron scattering experiments
    Open this publication in new window or tab >>Normalization of stroboscopic neutron scattering experiments
    Show others...
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    Neutron scattering studies of kinetic processes are often ux limited. For processes,which can be repeated many times, this can be mitigated by the so-called strobo-scopic reintegration. The recorded scattering signals are assigned to dierent timeslices of the process and corresponding slices from all repetitions are summed up. Thenormalisation of such data with respect to the incident beam spectrum can be chal-lenging since the incident neutron ux may not necessarily be evenly distributed overall time slices. In this paper we discuss how this can result in misleading informationand a technique to amend for these issues is presented.

    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-348265 (URN)
    Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2018-04-13Bibliographically approved
    6. Överlåtaren: a fast way to transfer and orthogonalize two-dimensional off-specular reflectivity data
    Open this publication in new window or tab >>Överlåtaren: a fast way to transfer and orthogonalize two-dimensional off-specular reflectivity data
    Show others...
    2016 (English)In: Journal of applied crystallography, ISSN 0021-8898, E-ISSN 1600-5767, Vol. 49, p. 2091-2099Article in journal (Refereed) Published
    Abstract [en]

    Reflectivity measurements offer unique opportunities for the study of surfaces and interfaces, and specular reflectometry has become a standard tool in materials science to resolve structures normal to the surface of a thin film. Off-specular scattering, which probes lateral structures, is more difficult to analyse, because the Fourier space being probed is highly anisotropic and the scattering pattern is truncated by the interface. As a result, scattering patterns collected with (especially time-of-flight) neutron reflectometers are difficult to transform into reciprocal space for comparison with model calculations. A program package is presented for a generic two-dimensional transformation of reflectometry data into q space and back. The data are represented on an orthogonal grid, allowing cuts along directions relevant for theoretical modelling. This treatment includes background subtraction as well as a full characterization of the resolution function. The method is optimized for computational performance using repeatable operations and standardized instrument settings.

    Keywords
    off-specular scattering, neutron reflectometry, interfaces, surfaces
    National Category
    Atom and Molecular Physics and Optics Other Physics Topics Computational Mathematics
    Identifiers
    urn:nbn:se:uu:diva-315086 (URN)10.1107/S1600576716014382 (DOI)000391195900025 ()
    Funder
    Swedish Research Council, C0511501The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), IG-2011-2067
    Available from: 2017-02-09 Created: 2017-02-09 Last updated: 2018-04-11Bibliographically approved
    7. Towards neutron scattering experiments with sub-millisecond time resolution
    Open this publication in new window or tab >>Towards neutron scattering experiments with sub-millisecond time resolution
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    2015 (English)In: Journal of applied crystallography, ISSN 0021-8898, E-ISSN 1600-5767, Vol. 48, p. 220-226Article in journal (Refereed) Published
    Abstract [en]

    Neutron scattering techniques offer several unique opportunities in materials research. However, most neutron scattering experiments suffer from the limited flux available at current facilities. This limitation becomes even more severe if time-resolved or kinetic experiments are performed. A new method has been developed which overcomes these limitations when a reversible process is studied, without any compromise on resolution or beam intensity. It is demonstrated that, by recording in absolute time the neutron detector events linked to an excitation, information can be resolved on sub-millisecond timescales. Specifically, the concept of the method is demonstrated by neutron reflectivity measurements in time-of-flight mode at the Liquids Reflectometer located at the Spallation Neutron Source, Oak Ridge National Laboratory, Tennessee, USA, combined with in situ rheometry. The opportunities and limitations of this new technique are evaluated by investigations of a micellar polymer solution offering excellent scattering contrast combined with high sensitivity to shear.

    Keywords
    neutron scattering, in situ rheology, block copolymers, time-resolved studies, surface kinetics, large amplitude oscillatory shear (LAOS), time-of-flight reflectometry
    National Category
    Other Physics Topics
    Identifiers
    urn:nbn:se:uu:diva-246826 (URN)10.1107/S1600576714027848 (DOI)000349210700028 ()
    Available from: 2015-03-10 Created: 2015-03-10 Last updated: 2018-04-11Bibliographically approved
  • 5.
    Adlmann, Franz
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Busch, S.
    Vacaliuc, B.
    Nelson, A.
    Ankner, J.F.
    Browning, J.F.
    Parizzi, A.
    Bilheux, J. -K.
    Wolff, Max
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Normalization of stroboscopic neutron scattering experimentsManuscript (preprint) (Other academic)
    Abstract [en]

    Neutron scattering studies of kinetic processes are often ux limited. For processes,which can be repeated many times, this can be mitigated by the so-called strobo-scopic reintegration. The recorded scattering signals are assigned to dierent timeslices of the process and corresponding slices from all repetitions are summed up. Thenormalisation of such data with respect to the incident beam spectrum can be chal-lenging since the incident neutron ux may not necessarily be evenly distributed overall time slices. In this paper we discuss how this can result in misleading informationand a technique to amend for these issues is presented.

  • 6.
    Adlmann, Franz
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Herbel, Jörg
    ETH Zürich, Zürich, Switzerland.
    Korolkovas, Airidas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Bliersbach, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Toperverg, Boris
    Petersburg Nuclear Physics Institute, Gatchina, Russia.
    Van Herck, Walter
    Forschungszentrum Jülich GmbH, Garching, Germany.
    Pálsson, Gunnar K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Kitchen, Brian
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Wolff, Max
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Depth resolved grazing incidence neutron scattering experiments from semi-infinite interfaces: a statistical analysis of the scattering contributions2018In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, article id 165901Article in journal (Refereed)
    Abstract [en]

    Grazing incidence neutron scattering experiments offer surface sensitivity by reflecting from an interface at momentum transfers close to total external reflection. Under these conditions the penetration depth is strongly non-linear and may change by many orders of magnitude. This fact imposes severe challenges for depth resolved experiments, since the brilliance of neutron beams is relatively low in comparison to e.g. synchrotron radiation. In this article we use probability density functions to calculate the contribution of scattering at different distances from an interface to the intensities registered on the detector. Our method has the particular advantage that the depth sensitivity is directly extracted from the scattering pattern itself. Hence for perfectly known samples exact resolution functions can be calculated and visa versa. We show that any tails in the resolution function, e.g. Gaussian shaped, hinders depth resolved experiments. More importantly we provide means for a descriptive statistical analysis of detector images with respect to the scattering contributions and show that even for perfect resolution near surface scattering is hardly accessible.

  • 7.
    Ahlberg, Martina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Critical Phenomena and Exchange Coupling in Magnetic Heterostructures2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The continuous phase transition in thin magnetic films and superlattices has been studied using the magneto-optical Kerr effect (MOKE) and polarized neutron scattering (PNR).  It has been shown that the critical behavior of amorphous thin films belonging to the 2D XY universality class can be described within the same theory as crystalline sample. This means that quenched disorder only serves as a marginal perturbation in systems with this symmetry.

    The connection between interlayer exchange coupling and the observed critical behavior in Fe/V superlattices was explored. The results prove that the origin of unusually high values of the exponent β can be traced to a position dependence of the magnetization at elevated temperatures. The magnetization of the outermost layers within the superlattice shows a more pronounced decrease at lower temperatures, compared to the inner layers, which in turn have a more abrupt decrease in the vicinity of the critical temperature. This translates to a high exponent, especially when the layers are probed by a technique where more weight is given to the layers close to the surface, e.g.MOKE.  The interlayer exchange coupling as a function of spacer thickness and temperature was also studied in its own right. The data was compared to the literature, and a dependence on the thickness of the magnetic layers was concluded.

    The phase transition in amorphous FeZr/CoZr multilayers, where the magnetization emanates from ferromagnetic proximity effects, was investigated. Even though the determined exponents of the zero-field magnetization, the susceptibility and the critical isotherm did not correspond to any universality class, scaling plots displayed an excellent data collapse.

    Samples consisting of Fe δ-layers (0.3-1.4 monolayers) embedded in Pd were studied using element-specific resonant x-ray magnetic scattering. The magnetization of the two constituents showed distinctly different temperature dependences.

    List of papers
    1. Influence of boundaries on magnetic ordering in Fe/V superlattices
    Open this publication in new window or tab >>Influence of boundaries on magnetic ordering in Fe/V superlattices
    Show others...
    2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 21, p. 214429-Article in journal (Refereed) Published
    Abstract [en]

    We study the role of surface boundaries on the magnetic properties of [Fe/V](n) superlattice structures, with n=2-10. Using the magneto-optical Kerr effect and polarized neutron reflectivity measurements, we examine the evolution of both the total and the layer-resolved magnetizations as a function of temperature. By varying n, we observe a large shift in the transition temperatures T-c and a substantial change in the total magnetization critical exponent beta. In particular, the thicker samples exhibit nonuniversal exponent values. By resolving the magnetization as a function of position within the superlattice, we show that this behavior arises from contributions of the surfaces. Furthermore, we attribute the large shift in T-c to long-ranged interactions present in the superlattice.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-136258 (URN)10.1103/PhysRevB.81.214429 (DOI)000278936900004 ()
    Available from: 2010-12-11 Created: 2010-12-11 Last updated: 2017-12-11Bibliographically approved
    2. Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
    Open this publication in new window or tab >>Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
    2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 22, p. 224404-Article in journal (Refereed) Published
    Abstract [en]

    We present an experimental realization of a magnetic two-dimensional XY system using amorphous materials. The classification of the dimensionality is based on the critical behavior of amorphous Co68Fe24Zr8(d)/Al70O30 (20 angstrom) multilayers, where d = 11 - 16 angstrom. Analysis of the remanent magnetization, the magnetic isotherms, the initial susceptibility, and the magnetic correlation length shows that the magnetic phase transition can be described by the 2D XY model. The samples are not paramagnetic above the critical temperature but are characterized by local magnetic order manifested in the field and temperature dependence of the magnetization. Furthermore, an average spin-spin interaction length of 8.1 angstrom was estimated using the thickness dependence of the Curie temperature.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-155916 (URN)10.1103/PhysRevB.83.224404 (DOI)000291727600006 ()
    Available from: 2011-07-05 Created: 2011-07-04 Last updated: 2017-12-11Bibliographically approved
    3. Experimental realization of amorphous two-dimensional XY magnets
    Open this publication in new window or tab >>Experimental realization of amorphous two-dimensional XY magnets
    2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, p. 024430-Article in journal (Refereed) Published
    Abstract [en]

    The temperature dependence of the magnetization of thin amorphous Fe(89)Zr(11)/Al(78)Zr(22) layerswas investigated. Dimensionality analysis of the ferromagnetic transition of 15 A thick layers yielded critical exponents characteristic of the 2D XY (planar rotor) model. Above the ordering temperature significant polarizability with an applied field is observed, due to the existence of large-scale magnetic correlations, of which the extent and origin have been determined.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-156953 (URN)10.1103/PhysRevB.84.024430 (DOI)000292873700002 ()
    Available from: 2011-08-20 Created: 2011-08-11 Last updated: 2017-12-08Bibliographically approved
    4. Temperature dependence of magnetic properties in weakly exchange coupled Fe/V superlattices
    Open this publication in new window or tab >>Temperature dependence of magnetic properties in weakly exchange coupled Fe/V superlattices
    2013 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 341, p. 142-147Article in journal (Refereed) Published
    Abstract [en]

    We use Fe(3)/V(x)Fe(3)/V(x) superlattices, x = 16-27 monolayers (ML) to explore the interlayer exchange coupling (IEC) as a function of both spacer layer thickness and temperature. Fe/V is a common model system for studies addressing the IEC, but the behavior in the weakly exchange coupled regime, complemented with the temperature dependence of the magnetic properties, have remained unexplored. We observe clear regions with ferro- and antiferromagnetic coupling, which is manifested in oscillations in the saturation field, the remanence and the critical temperature. The oscillation in the interlayer coupling (J') has a period of 5.8(1) ML. This is approximate to 0.8 ML smaller than observed for Fe(7)/V(x) superlattices which illustrates the influence of the magnetic layer thickness on the interlayer coupling. The temperature dependence of the remanent magnetization was proven to be significantly affected by the strength of the ferromagnetic coupling, leading to high values of the effective critical exponent beta(eff).

    Keywords
    Magnetic superlattice, Magnetic phase transition, Exchange interaction
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-168308 (URN)10.1016/j.jmmm.2013.04.058 (DOI)000320583800026 ()
    Available from: 2012-02-16 Created: 2012-02-08 Last updated: 2017-12-07Bibliographically approved
    5. Effect of ferromagnetic proximity on critical behavior
    Open this publication in new window or tab >>Effect of ferromagnetic proximity on critical behavior
    2012 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 85, no 22, p. 224425-1-224425-6Article in journal (Refereed) Published
    Abstract [en]

    We have investigated the magnetic phase transition in amorphous Fe93Zr7(x angstrom)/Co95Zr5(1 angstrom) multilayers, where x = 25, 50, and 75. The extremely thin CoZr layer induces magnetic order at temperatures well above the inherent ordering temperature of Fe93Zr7. The changes in the critical exponent beta, associated with the temperature dependence of the magnetization, imply a crossover from two-to three-dimensional behavior as the FeZr thickness is reduced, consistent with a substantial magnetic induction in the FeZr layers. In addition we determined the exponents delta and gamma, of the critical isotherm and the susceptibility, respectively, and their values confirm the nonuniversal character of the phase transition. Scaling of the results yields an excellent data collapse and is found to hold in the crossover regime where the universality hypothesis is not applicable. 

    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-168796 (URN)10.1103/PhysRevB.85.224425 (DOI)000305532200002 ()
    Funder
    Swedish Research CouncilKnut and Alice Wallenberg Foundation
    Available from: 2012-02-16 Created: 2012-02-15 Last updated: 2017-12-07Bibliographically approved
    6. Element specific magnetization in Fe/Pd quantum well structures
    Open this publication in new window or tab >>Element specific magnetization in Fe/Pd quantum well structures
    Show others...
    (English)Manuscript (preprint) (Other academic)
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-168443 (URN)
    Available from: 2012-02-16 Created: 2012-02-10 Last updated: 2012-08-01
  • 8.
    Ahlberg, Martina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Andersson, Gabriella
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 22, p. 224404-Article in journal (Refereed)
    Abstract [en]

    We present an experimental realization of a magnetic two-dimensional XY system using amorphous materials. The classification of the dimensionality is based on the critical behavior of amorphous Co68Fe24Zr8(d)/Al70O30 (20 angstrom) multilayers, where d = 11 - 16 angstrom. Analysis of the remanent magnetization, the magnetic isotherms, the initial susceptibility, and the magnetic correlation length shows that the magnetic phase transition can be described by the 2D XY model. The samples are not paramagnetic above the critical temperature but are characterized by local magnetic order manifested in the field and temperature dependence of the magnetization. Furthermore, an average spin-spin interaction length of 8.1 angstrom was estimated using the thickness dependence of the Curie temperature.

  • 9.
    Ahlberg, Martina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Korelis, Panagiotis
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Andersson, Gabriella
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Effect of ferromagnetic proximity on critical behavior2012In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 85, no 22, p. 224425-1-224425-6Article in journal (Refereed)
    Abstract [en]

    We have investigated the magnetic phase transition in amorphous Fe93Zr7(x angstrom)/Co95Zr5(1 angstrom) multilayers, where x = 25, 50, and 75. The extremely thin CoZr layer induces magnetic order at temperatures well above the inherent ordering temperature of Fe93Zr7. The changes in the critical exponent beta, associated with the temperature dependence of the magnetization, imply a crossover from two-to three-dimensional behavior as the FeZr thickness is reduced, consistent with a substantial magnetic induction in the FeZr layers. In addition we determined the exponents delta and gamma, of the critical isotherm and the susceptibility, respectively, and their values confirm the nonuniversal character of the phase transition. Scaling of the results yields an excellent data collapse and is found to hold in the crossover regime where the universality hypothesis is not applicable. 

  • 10.
    Ahlberg, Martina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Papaioannou,, Evangelos Th
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Temperature dependence of magnetic properties in weakly exchange coupled Fe/V superlattices2013In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 341, p. 142-147Article in journal (Refereed)
    Abstract [en]

    We use Fe(3)/V(x)Fe(3)/V(x) superlattices, x = 16-27 monolayers (ML) to explore the interlayer exchange coupling (IEC) as a function of both spacer layer thickness and temperature. Fe/V is a common model system for studies addressing the IEC, but the behavior in the weakly exchange coupled regime, complemented with the temperature dependence of the magnetic properties, have remained unexplored. We observe clear regions with ferro- and antiferromagnetic coupling, which is manifested in oscillations in the saturation field, the remanence and the critical temperature. The oscillation in the interlayer coupling (J') has a period of 5.8(1) ML. This is approximate to 0.8 ML smaller than observed for Fe(7)/V(x) superlattices which illustrates the influence of the magnetic layer thickness on the interlayer coupling. The temperature dependence of the remanent magnetization was proven to be significantly affected by the strength of the ferromagnetic coupling, leading to high values of the effective critical exponent beta(eff).

  • 11.
    Ahmadi, Sareh
    et al.
    Materialfusik,KTH-Electrum.
    Shariati, Masumeh-Nina
    Yu, Shun
    Göthelid, Mats
    Charge transfer and chemical interaction of ZnPc and FePc with Au(111)Manuscript (preprint) (Other academic)
  • 12.
    Aijaz, Asim
    et al.
    Linkoping Univ, Dept Phys Chem & Biol, IFM Mat Phys, SE-58183 Linkoping, Sweden.;Uppsala Univ, Dept Engn Sci, Angstrom Lab, POB 534, SE-75121 Uppsala, Sweden..
    Louring, Sascha
    Aarhus Univ, Interdisciplinary Nanosci Ctr iNANO, Ny Munkegade 120, DK-8000 Aarhus C, Denmark.;Danish Technol Inst, Tribol Ctr, Teknol Pk,Kongsvang Alle 29, DK-8000 Aarhus C, Denmark..
    Lundin, Daniel
    Univ Paris Saclay, Univ Paris Sud, LPGP, CNRS,UMR 8578, F-91405 Orsay, France..
    Kubart, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Jensen, Jens
    Linkoping Univ, Dept Phys Chem & Biol, IFM Mat Phys, SE-58183 Linkoping, Sweden..
    Sarakinos, Kostas
    Linkoping Univ, Dept Phys Chem & Biol, IFM Mat Phys, SE-58183 Linkoping, Sweden..
    Helmersson, Ulf
    Linkoping Univ, Dept Phys Chem & Biol, IFM Mat Phys, SE-58183 Linkoping, Sweden..
    Synthesis of hydrogenated diamondlike carbon thin films using neon-acetylene based high power impulse magnetron sputtering discharges2016In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 34, no 6, article id 061504Article in journal (Refereed)
    Abstract [en]

    Hydrogenated diamondlike carbon (DLC:H) thin films exhibit many interesting properties that can be tailored by controlling the composition and energy of the vapor fluxes used for their synthesis. This control can be facilitated by high electron density and/or high electron temperature plasmas that allow one to effectively tune the gas and surface chemistry during film growth, as well as the degree of ionization of the film forming species. The authors have recently demonstrated by adding Ne in an Ar-C high power impulse magnetron sputtering (HiPIMS) discharge that electron temperatures can be effectively increased to substantially ionize C species [Aijaz et al., Diamond Relat. Mater. 23, 1 (2012)]. The authors also developed an Ar-C2H2 HiPIMS process in which the high electron densities provided by the HiPIMS operation mode enhance gas phase dissociation reactions enabling control of the plasma and growth chemistry [Aijaz et al., Diamond Relat. Mater. 44, 117 (2014)]. Seeking to further enhance electron temperature and thereby promote electron impact induced interactions, control plasma chemical reaction pathways, and tune the resulting film properties, in this work, the authors synthesize DLC: H thin films by admixing Ne in a HiPIMS based Ar/C2H2 discharge. The authors investigate the plasma properties and discharge characteristics by measuring electron energy distributions as well as by studying discharge current characteristics showing an electron temperature enhancement in C2H2 based discharges and the role of ionic contribution to the film growth. These discharge conditions allow for the growth of thick (>1 mu m) DLC: H thin films exhibiting low compressive stresses (similar to 0.5 GPa), high hardness (similar to 25 GPa), low H content (similar to 11%), and density in the order of 2.2 g/cm(3). The authors also show that film densification and change of mechanical properties are related to H removal by ion bombardment rather than subplantation.

  • 13.
    Akansel, Serkan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Magnetization Dynamics in Ferromagnetic Thin Films: Evaluation of Different Contributions to Damping in Co2FeAl and FeCo Film Structures2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Static and dynamic magnetic properties of Co2FeAl and Fe65Co35 alloys have been investigated. Co2FeAl films were deposited at different temperatures and the deposition parameters were optimized with respect to structural and magnetic properties. As a result, a film with B2 crystalline phase was obtained without any post-annealing process. A lowest magnetic damping parameter of  was obtained for the film deposited at 573K. This obtained low value is comparable to the lowest values reported in research literature.  After optimizing the deposition parameters of this alloy, different seed layers and capping layers were added adjacent to the Co2FeAl layer and the effect of these layers on the magnetic relaxation was investigated. In addition to adding nonmagnetic layers to Co2FeAl, the dependence of the magnetic damping parameter with respect to the thickness of Co2FeAl was investigated by depositing films with different thicknesses. A temperature dependent study of the magnetic damping parameter was also performed and the measured damping parameters were compared with theoretically calculated intrinsic Gilbert damping parameters. Different extrinsic contributions to the magnetic damping, such as two magnon scattering, spin pumping, eddy-current damping and radiative damping, were identified and subtracted from the experimentally obtained damping parameter. Hence, it was possible to obtain the intrinsic damping parameter, that is called the Gilbert damping parameter.

    In the second part of the thesis, Fe65Co35 alloys were investigated in terms of static and dynamic magnetic properties. Fe65Co35 films were deposited without and with different seed layers in order to first understand the effect of the seed layer on static magnetic properties of the films, such as the coercivity of the films. Then the films with seed layers yielding the lowest coercivity were investigated in terms of dynamic magnetic properties. Fe65Co35 films with different rhenium dopant concentrations and with ruthenium as the seed and capping layer were also investigated. The purpose of this study was to increase the damping parameter of the films and an increase of about ~230% was obtained by adding the dopant to the structure. This study was performed at different temperatures and after subtraction of the extrinsic contributions to the damping, the experimental values were compared with theoretically calculated values of the Gilbert damping parameter. During the thesis work, magnetic looper and superconducting quantum interference device magnetometers set-ups were used for static magnetic measurements and cavity, broadband in-plane and broadband out-of-plane ferromagnetic resonance set-ups were used for dynamic measurements.

    List of papers
    1. Thickness dependent enhancement of damping in Co2FeAl/β-Ta thin films
    Open this publication in new window or tab >>Thickness dependent enhancement of damping in Co2FeAl/β-Ta thin films
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    2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 13, article id 134421Article in journal (Refereed) Published
    Abstract [en]

    In the present work Co2FeAl (CFA) thin films were deposited by ion beam sputtering on Si (100) substrates at the optimized deposition temperature of 300°C. A series of CFA films with different thickness (tCFA ); 8, 10, 12, 14, 16, 18 and 20 nm were prepared and all samples were capped with a 5 nm thick b-Ta layer. The thickness dependent static and dynamic properties of the films were studied by SQUID magnetometry, in-plane as well as out-of-plane broadband VNA-FMR measurements and angle dependent cavity FMR measurements. The saturation magnetization and the coercive field were found to be weakly thickness dependent and lie in the range 900 – 950 kA/m and 0.53 – 0.87 kA/m, respectively. The effective damping parameter ( αeff) extracted from in-plane and out-of-plane FMR results reveal a 1/tCFA dependence, the values for the in-plane αeff being larger due to two-magnon scattering (TMS). The origin of the αeff thickness dependence is spin pumping into the non-magnetic b-Ta layer and in case of the in-plane  αeff also a thickness dependent TMS contribution. From the out-of-plane FMR results, it was possible to disentangle the different contributions to αeff   and to the extract values for the intrinsic Gilbert damping (αG ) and the effective spin-mixing conductance (g_eff^↑↓ ) of the CFA/ b-Ta interface, yielding αG=1.1X10-3 and g_eff^↑↓=2.90x1019 m-2.

    Place, publisher, year, edition, pages
    American Physical Society, 2018
    Keywords
    Magnetization dynamics, magnetic thin films, Gilbert damping, ferromagnetic resonance
    National Category
    Condensed Matter Physics Materials Engineering
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-345848 (URN)10.1103/PhysRevB.97.134421 (DOI)000430544300002 ()
    Funder
    Knut and Alice Wallenberg Foundation, 2012.0031
    Available from: 2018-03-12 Created: 2018-03-12 Last updated: 2018-06-26Bibliographically approved
    2. Enhanched Gilbert Damping in Re doped FeCo Films: A combined experimental and theoretical study
    Open this publication in new window or tab >>Enhanched Gilbert Damping in Re doped FeCo Films: A combined experimental and theoretical study
    Show others...
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    The effects of rhenium doping in the range 0 – 10 at% on the static and dynamic magnetic properties of Fe65Co35 thin films have been studied experimentally as well as with first principles electronic structure calculations focussing on the change of the saturation magnetization (Ms ) and the Gilbert damping parameter (α ). Both experimental and theoretical results show that Ms decreases with increasing Re doping level, while at the same time α increases.  The experimental low temperature saturation magnetic induction exhibits a 29% decrease, from 2.31T to 1.64T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample 2.7X103, which is close to the Gilbert damping parameter extracted from the theoretical calculations. The room temperature experimental value for the damping parameter increases to  9X103 when doping with 10 at% Re; the corresponding increase of the Gilbert damping parameter obtained from theoretical calculations is 7.3X103. Both experimental and theoretical values for the damping parameter weakly decrease with decreasing temperature.

    Keywords
    Magnetization dynamics, magnetic thin films, Gilbert damping, ferromagnetic resonance
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-345854 (URN)
    Funder
    Knut and Alice Wallenberg Foundation, 2012.0031
    Available from: 2018-03-12 Created: 2018-03-12 Last updated: 2018-03-12
    3. Effect of seed layers on dynamic and static magnetic properties of Fe65Co35 thin films
    Open this publication in new window or tab >>Effect of seed layers on dynamic and static magnetic properties of Fe65Co35 thin films
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    2018 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 30, article id 305001Article in journal (Refereed) Published
    Abstract [en]

    Fe65Co35 thin films have been deposited on SiO2 substrates using sputtering technique with different choices of seed layer; Ru, Ni82.5Fe17.5, Rh, Y and Zr. Best soft magnetic properties were observed with seed layers of Ru, Ni82.5Fe17.5 and Rh. Adding these seed layers, the coercivity of the Fe65Co35 films decreased to values of around 1.5 mT, which can be compared to the value of 12.5 mT obtained for films deposited without seed layer. Further investigations were performed on samples with these three seed layers in terms of dynamic magnetic properties, both on as prepared and annealed samples, using constant frequency cavity and broadband ferromagnetic resonance measurements. Damping parameters of around 8.0X10-3 and 4.5X10-3 were obtained from in-plane and out-of-plane measurements, respectively, for as prepared samples, values that were reduced to 6.5X10-3 and 4.0X10-3 for annealed samples.

    Keywords
    Magnetization dynamics, magnetic thin films, Gilbert damping, ferromagnetic resonance
    National Category
    Condensed Matter Physics Engineering and Technology
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-345853 (URN)10.1088/1361-6463/aaccc0 (DOI)000437408700001 ()
    Funder
    Knut and Alice Wallenberg Foundation, 2012.0031
    Available from: 2018-03-12 Created: 2018-03-12 Last updated: 2018-10-12Bibliographically approved
    4. Growth of Co2FeAl Heusler alloy thin films on Si(100) having very small Gilbert damping by Ion beam sputtering
    Open this publication in new window or tab >>Growth of Co2FeAl Heusler alloy thin films on Si(100) having very small Gilbert damping by Ion beam sputtering
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    2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 28692Article in journal (Refereed) Published
    Abstract [en]

    The influence of growth temperature T-s (300-773 K) on the structural phase ordering, static and dynamic magnetization behaviour has been investigated in ion beam sputtered full Heusler alloy Co2FeAl (CFA) thin films on industrially important Si(100) substrate. The B2 type magnetic ordering is established in these films based on the clear observation of the (200) diffraction peak. These ion beam sputtered CFA films possess very small surface roughness of the order of subatomic dimensions (<3 angstrom) as determined from the fitting of XRR spectra and also by AFM imaging. This is supported by the occurrence of distinct Kiessig fringes spanning over the whole scanning range (similar to 4 degrees) in the x-ray reflectivity (XRR) spectra. The Gilbert damping constant alpha and effective magnetization 4 pi M-eff are found to vary from 0.0053 +/- 0.0002 to 0.0015 +/- 0.0001 and 13.45 +/- 00.03 kG to 14.03 +/- 0.04 kG, respectively. These Co2FeAl films possess saturation magnetization ranging from 4.82 +/- 0.09 to 5.22 +/- 0.10 mu(B)/f.u. consistent with the bulk L2(1)-type ordering. A record low alpha-value of 0.0015 is obtained for Co2FeAl films deposited on Si substrate at T-s similar to 573 K.

    National Category
    Other Physics Topics Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-299867 (URN)10.1038/srep28692 (DOI)000378783900001 ()27357004 (PubMedID)
    Available from: 2016-07-28 Created: 2016-07-28 Last updated: 2018-03-12Bibliographically approved
    5. Static and dynamic properties of Co2FeAl thin films: Effect of MgO and Ta as capping layers
    Open this publication in new window or tab >>Static and dynamic properties of Co2FeAl thin films: Effect of MgO and Ta as capping layers
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    2017 (English)In: 61ST DAE-SOLID STATE PHYSICS SYMPOSIUM, 2017, Vol. 1832, article id 080007-1Conference paper, Published paper (Refereed)
    Abstract [en]

    The influence of MgO and Ta capping layers on the static and dynamic magnetic properties of Co2FeAl (CFA) Heusler alloy thin films has been investigated. It is observed that the CFA film deposited with MgO capping layer is preeminent compared to the uncapped or Ta capped CFA film. In particular, the magnetic inhomogeneity contribution to the ferromagnetic resonance line broadening and damping constant are found to be minimal for the MgO capped CFA thin film i.e., 0.12±0.01 Oe and 0.0074±0.00014, respectively. The saturationmagnetization was found to be 960±25emu/cc.

    Series
    AIP Conference Proceedings, ISSN 0094-243X
    Keywords
    Heusler alloy, MgO, Gilbert damping
    National Category
    Physical Sciences Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-332482 (URN)10.1063/1.4980467 (DOI)000412824400289 ()978-0-7354-1500-3 (ISBN)
    Conference
    61st DAE-Solid State Physics Symposium, DEC 26-30, 2016, KIIT Univ, Bhubaneswar, INDIA
    Projects
    Knut and Alice Wallenberg (KAW) Foundation, Grant No. KAW 2012.0031.
    Available from: 2017-10-27 Created: 2017-10-27 Last updated: 2018-03-29Bibliographically approved
    6. Temperature-dependent Gilbert damping of Co2FeAl thin films with different degree of atomic order
    Open this publication in new window or tab >>Temperature-dependent Gilbert damping of Co2FeAl thin films with different degree of atomic order
    Show others...
    2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 224425Article in journal (Refereed) Published
    Abstract [en]

    Half-metallicity and low magnetic damping are perpetually sought for spintronics materials, and full Heusler compounds in this respect provide outstanding properties. However, it is challenging to obtain the well-ordered half-metallic phase in as-deposited full Heusler compound thin films, and theory has struggled to establish a fundamental understanding of the temperature-dependent Gilbert damping in these systems. Here we present a study of the temperature-dependent Gilbert damping of differently ordered as-deposited Co2FeAl full Heusler compound thin films. The sum of inter- and intraband electron scattering in conjunction with the finite electron lifetime in Bloch states governs the Gilbert damping for the well-ordered phase, in contrast to the damping of partially ordered and disordered phases which is governed by interband electronic scattering alone. These results, especially the ultralow room-temperature intrinsic damping observed for the well-ordered phase, provide fundamental insights into the physical origin of the Gilbert damping in full Heusler compound thin films.

    Place, publisher, year, edition, pages
    American Physical Society, 2017
    Keywords
    Gilbert damping, full Heusler compound, Temperature Dependent Gilbert damping, Thin Films, Density of states
    National Category
    Condensed Matter Physics Engineering and Technology
    Research subject
    Engineering Science with specialization in Solid State Physics
    Identifiers
    urn:nbn:se:uu:diva-337305 (URN)10.1103/PhysRevB.96.224425 (DOI)000418572700007 ()
    Funder
    Knut and Alice Wallenberg Foundation, KAW 2012.0031Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology, GGS1403ASwedish National Infrastructure for Computing (SNIC)
    Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2018-03-12
  • 14.
    Akansel, Serkan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Kumar, Ankit
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Behera, Nilamani
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Husain, Sajid
    Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India.
    Brucas, Rimantas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Chaudhary, Sujeet
    Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Thickness dependent enhancement of damping in Co2FeAl/β-Ta thin films2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 13, article id 134421Article in journal (Refereed)
    Abstract [en]

    In the present work Co2FeAl (CFA) thin films were deposited by ion beam sputtering on Si (100) substrates at the optimized deposition temperature of 300°C. A series of CFA films with different thickness (tCFA ); 8, 10, 12, 14, 16, 18 and 20 nm were prepared and all samples were capped with a 5 nm thick b-Ta layer. The thickness dependent static and dynamic properties of the films were studied by SQUID magnetometry, in-plane as well as out-of-plane broadband VNA-FMR measurements and angle dependent cavity FMR measurements. The saturation magnetization and the coercive field were found to be weakly thickness dependent and lie in the range 900 – 950 kA/m and 0.53 – 0.87 kA/m, respectively. The effective damping parameter ( αeff) extracted from in-plane and out-of-plane FMR results reveal a 1/tCFA dependence, the values for the in-plane αeff being larger due to two-magnon scattering (TMS). The origin of the αeff thickness dependence is spin pumping into the non-magnetic b-Ta layer and in case of the in-plane  αeff also a thickness dependent TMS contribution. From the out-of-plane FMR results, it was possible to disentangle the different contributions to αeff   and to the extract values for the intrinsic Gilbert damping (αG ) and the effective spin-mixing conductance (g_eff^↑↓ ) of the CFA/ b-Ta interface, yielding αG=1.1X10-3 and g_eff^↑↓=2.90x1019 m-2.

  • 15.
    Akansel, Serkan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Kumar, Ankit
    Venugopal, Vijayaharan
    Banerjee, Rudra
    Autieri, Carmine
    Brucas, Rimantas
    Behera, Nilamani
    Sortica, Mauricio
    Primetzhofer, Daniel
    Basu, Swaraj
    Gubbins, Mark
    Sanyal, Biplab
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Enhanched Gilbert Damping in Re doped FeCo Films: A combined experimental and theoretical studyManuscript (preprint) (Other academic)
    Abstract [en]

    The effects of rhenium doping in the range 0 – 10 at% on the static and dynamic magnetic properties of Fe65Co35 thin films have been studied experimentally as well as with first principles electronic structure calculations focussing on the change of the saturation magnetization (Ms ) and the Gilbert damping parameter (α ). Both experimental and theoretical results show that Ms decreases with increasing Re doping level, while at the same time α increases.  The experimental low temperature saturation magnetic induction exhibits a 29% decrease, from 2.31T to 1.64T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample 2.7X103, which is close to the Gilbert damping parameter extracted from the theoretical calculations. The room temperature experimental value for the damping parameter increases to  9X103 when doping with 10 at% Re; the corresponding increase of the Gilbert damping parameter obtained from theoretical calculations is 7.3X103. Both experimental and theoretical values for the damping parameter weakly decrease with decreasing temperature.

  • 16.
    Akansel, Serkan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Venugopal, Vijayaharan
    Kumar, Ankit
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Gupta, Rahul
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Brucas, Rimantas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    George, Sebastian
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Neagu, Alexandra
    Tai, Cheuk-Wai
    Gubbins, Mark
    Andersson, Gabriella
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Effect of seed layers on dynamic and static magnetic properties of Fe65Co35 thin films2018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 30, article id 305001Article in journal (Refereed)
    Abstract [en]

    Fe65Co35 thin films have been deposited on SiO2 substrates using sputtering technique with different choices of seed layer; Ru, Ni82.5Fe17.5, Rh, Y and Zr. Best soft magnetic properties were observed with seed layers of Ru, Ni82.5Fe17.5 and Rh. Adding these seed layers, the coercivity of the Fe65Co35 films decreased to values of around 1.5 mT, which can be compared to the value of 12.5 mT obtained for films deposited without seed layer. Further investigations were performed on samples with these three seed layers in terms of dynamic magnetic properties, both on as prepared and annealed samples, using constant frequency cavity and broadband ferromagnetic resonance measurements. Damping parameters of around 8.0X10-3 and 4.5X10-3 were obtained from in-plane and out-of-plane measurements, respectively, for as prepared samples, values that were reduced to 6.5X10-3 and 4.0X10-3 for annealed samples.

  • 17.
    Akhtar, Sultan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Rubino, Stefano
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    A simple TEM method for fast thickness characterization of suspendedgraphene flakesManuscript (preprint) (Other academic)
  • 18.
    Akhter, Shahida
    et al.
    Univ Chittagong, Dept Phys, Chittagong 4331, Bangladesh..
    Hakim, M. A.
    BUET, Dept Glass & Ceram Engn, Dhaka, Bangladesh..
    Hoque, S. M.
    Atom Energy Ctr, Mat Sci Div, Dhaka 1000, Bangladesh..
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Glassy behavior of diluted Cu-Zn ferrites2018In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 452, p. 261-265Article in journal (Refereed)
    Abstract [en]

    The magnetic behavior of Zn substituted Cu-Zn spinel ferrites having chemical formula Cu1-xZnxFe2O4 (x = 0.7, 0.8, 0.9 and 1.0) has been studied by SQUID magnetometry, by means of magnetic hysteresis, field-cooled (FC) and zero-field-cooled (ZFC) magnetization, memory effect and low field ac susceptibility measurements. These measurements suggest that the ferrimagnetic phase of the x <= 0.8 samples is gradually turned into a spin glass (x >= 0.9). The compound with x = 0.9 exhibits the typical dynamical behavior of spin glasses, with indication of aging, rejuvenation and memory effects. The evolution of the magnetic properties of Cu-Zn spinel ferrites with substitution of Zn for Cu is discussed.

  • 19. Akhter, Shahida
    et al.
    Paul, D. P.
    Hoque, S. M.
    Hakim, M. A.
    Hudl, Matthias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Magnetic and magnetocaloric properties of Cu1-xZnxFe2O4 (x=0.6, 0.7, 0.8) ferrites2014In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 367, p. 75-80Article in journal (Refereed)
    Abstract [en]

    The effect of Zn substitution on the magnetic and magnetocaloric properties of Cu1-xZnxFe2O4 (x=0.6, 0.7, 0.8) ferrites over a wide temperature range has been investigated. The polycrystalline samples were synthesized using the solid-state reaction at sintering temperature 1050 degrees C (1323 K) for 2 h and has been characterized by SQUID magnetometry. Magnetization versus temperature showed that all samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T-c is found to decrease from 373 K for x=0.6 to 140 K for x=0.8 as well as the saturation magnetization Ms which shifts from 100 to 44 emu/gm. The magnetocaloric effect was obtained by measuring a family of M-I1 curves at set temperature intervals and calculating the entropy change, LIS for this system using the Maxwell relation. The AS of all samples increased with increasing applied field and showed a maximum around their respective 'Fe. The entropy change (Delta S) decreased with increasing Zn content, whereas the relative cooling power (RCP) slightly increased. The large RCP and Delta S found in Zn substitution Cu-Zn ferrites will be interesting for magnetic refrigeration near room temperature.

  • 20.
    Alippi, Paola
    et al.
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    Lanzilotto, Valeria
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics. CNR IOM, Lab Nazl TASC, C Area Sci Pk SS14 Km 163-5, I-34129 Trieste, Italy.;Univ Trieste, Dept Phys, Via Valerio 2, I-34127 Trieste, Italy..
    Paoletti, Anna Maria
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    Mattioli, Giuseppe
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    Zanotti, Gloria
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    Pennesi, Giovanna
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    Filippone, Francesco
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    Cossaro, Albano
    CNR IOM, Lab Nazl TASC, C Area Sci Pk SS14 Km 163-5, I-34129 Trieste, Italy..
    Verdini, Alberto
    CNR IOM, Lab Nazl TASC, C Area Sci Pk SS14 Km 163-5, I-34129 Trieste, Italy..
    Morgante, Alberto
    CNR IOM, Lab Nazl TASC, C Area Sci Pk SS14 Km 163-5, I-34129 Trieste, Italy.;Univ Trieste, Dept Phys, Via Valerio 2, I-34127 Trieste, Italy..
    Bonapasta, Aldo Amore
    CNR ISM, Ist Struttura Mat Consiglio Nazl Ric, Via Salaria,Km 29-300, I-00015 Rome, Italy..
    A Ru-Ru pair housed in ruthenium phthalocyanine: the role of a "cage" architecture in the molecule coupling with the Ag(111) surface2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 2, p. 1449-1457Article in journal (Refereed)
    Abstract [en]

    A number of studies have investigated the properties of monomeric and double-decker phthalocyanines (Pcs) adsorbed on metal surfaces, in view of applications in spintronics devices. In a combined experimental and theoretical study, we consider here a different member of the Pcs family, the (RuPc)(2) dimer, whose structure is characterized by two paired up magnetic centers embedded in a double-decker architecture. For (RuPc)(2) on Ag(111), we show that this architecture works as a preserving cage by shielding the Ru-Ru pair from a direct interaction with the surface atoms. In fact, while noticeable surface-to-molecule charge transfer occurs with the ensuing quenching of the molecular magnetic moment, such phenomena occur here in the absence of a direct Ru-Ag coupling or structural rearrangement, at variance with other Pcs and thanks to the above shielding effect. These unique properties of the (RuPc)(2) architecture are expected to permit an easy control of the surface-to-molecule charge-transfer process as well as of the molecular magnetic properties, thus making the (RuPc)(2) dimer a significant paradigm for innovative "cage" structures as well as a promising candidate for applications in spintronics nano or single-molecule devices.

  • 21. Alkhadi, H.S.
    et al.
    Tran, Tuan
    Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, Australia.
    Kremer, F.
    Williams, J.S.
    The influence of capping layers on pore formation in Ge during ion implantation2016In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 120, no 21, article id 215706Article in journal (Refereed)
    Abstract [en]

    Ion induced porosity in Ge has been investigated with and without a cap layer for two ion species, Ge and Sn, with respect to ion fluence and temperature. Results without a cap are consistent with a previous work in terms of an observed ion fluence and temperature dependence of porosity, but with a clear ion species effect where heavier Sn ions induce porosity at lower temperature (and fluence) than Ge. The effect of a cap layer is to suppress porosity for both Sn and Ge at lower temperatures but in different temperatures and fluence regimes. At room temperature, a cap does not suppress porosity and results in a more organised pore structure under conditions where sputtering of the underlying Ge does not occur. Finally, we observed an interesting effect in which a barrier layer of a-Ge that is denuded of pores formed directly below the cap layer. The thickness of this layer (∼ 8 nm) is largely independent of ion species, fluence, temperature, and cap material, and we suggest that this is due to viscous flow of a-Ge under ion irradiation and wetting of the cap layer to minimize the interfacial free energy.

  • 22.
    Allen, Andrew J.
    et al.
    NIST, Mat Measurement Sci Div Gaithersburg, MD USA.
    Hajdu, Janos
    Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology, Molecular biophysics. AS CR, European Extreme Light Infrastruct, Inst Phys, Prague, Czech Republic..
    McIntyre, Garry J.
    Australian Nucl Sci & Technol Org, New Illawarra Rd, Lucas Heights, NSW, Australia.
    Journal of Applied Crystallography: the first 50 years and beyond2018In: Journal of applied crystallography, ISSN 0021-8898, E-ISSN 1600-5767, Vol. 51, no Part: 2, p. 233-234Article in journal (Other academic)
    Abstract [en]

    The Editors of Journal of Applied Crystallography mark the journal's 50th anniversary.

  • 23.
    Al-Zoubi, N.
    et al.
    Tafila Tech Univ, Dept Appl Phys, Tafila, Jordan..
    Schönecker, S.
    KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, Stockholm, Sweden..
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, Stockholm, Sweden..
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, Stockholm, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, Budapest, Hungary..
    Assessing the Exact Muffin-Tin Orbitals method for the Bain path of metals2017In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 15, p. 1243-1264Article in journal (Refereed)
    Abstract [en]

    We scrutinise the muffin-tin approximation and the screening within the framework of the Exact Muffin-Tin Orbitals method in the case of cubic and tetragonal crystal symmetries. Systematic total energy calculations are carried out for the Bain path including the body-centred cubic and face-centred cubic structures for a set of simple and transition metals. The present converged results in terms of potential sphere radius (S) and hard sphere radius (b) are in good agreement with previous theoretical calculations. We demonstrate that for all structures considered here, potential sphere radii around and slightly larger than the average Wigner-Seitz radius (w) yield accurate total energy results whereas S values smaller than w give large errors. It is shown that for converged total energies hard spheres with radii b = 0.7-0.8w should be used for an efficient screening within real space clusters consisting typically of 70-90 lattice sites. The less efficient convergence of the total energy in the case of small hard spheres is ascribed to the delocalisation of the screened spherical waves, which leads to inaccurate interstitial overlap matrix. The above conclusions are not significantly affected by the volume of the system.

  • 24.
    Amft, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Density Functional Theory Studies of Small Supported Gold Clusters and Related Questions: What a Difference an Atom Makes2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    During the last decades the specific manipulation of matter on the (sub-) nanometer scale, also known as nanoscience, became possible by technologies such as the scanning tunneling microscope. Nanocatalysts, i.e. catalytic active structures of up to a few nanometers in size, belong to this rather new class of materials. Unlike ordinary ’macroscopic’ catalytic materials, the performance of nanocatalysts does not simply scale, for instance, with the surface to volume ratio of the active material.

    In this Thesis model nanocatalysts are investigated by means of ab-initio density functional theory calculations.

    In paper I, we explain the experimentally observed catalytic characteristics of small gold clusters, Au1-4, on a regular magnesium oxide terrace towards the oxidation of carbon monoxide by thoroughly studying the adsorption of CO and O2 on these clusters.

    In the subsequent paper II, we study the feasibility of a catalytic water-mediated CO oxidation reaction on Au1-4/MgO and find that this reaction mechanism is not assessable for Au2,4/MgO and unlikely for Au1,3/MgO.

    Papers III and IV concentrate on the reactivity of clusters in the gas phase. Particularly, we focus on the relative stability of Au13 isomers and its potential for O2 dissociation (paper III). We find the lowest energy isomers, which contain a triangular prism at their center surrounded by a ring of the remaining seven atoms, to be generally stable upon O2 adsorption. The dissociation of O2 at certain sites of Au13 is found to be exothermic.

    In paper IV we performed scans of the Born-Oppenheimer potential energy surfaces of neutral and charged Cu3, Ag3, and Au3 to explore the thermally excited vibrations of these trimers. While the Born-Oppenheimer surface of Cu3 exhibits one fairly deep energy minimum, it is comparatively flat with two shallow minima in the case of Ag3. Hence for Ag3 there exist many thermally accessible geometries in a wide range of angles and bond lengths. For Au3, two distinct energy minima appear, being well-separated by a barrier of 180 meV. Already at room temperature, we find bond lengths changes of up to 5% for the studied trimers. Choosing Au3 as a case study for the changed reactivity of thermally excited modes, we find CO to bind up to 150 meV stronger to the excited cluster.

    Gold deposited on graphene and graphite was observed to form larger aggregates. In paper V, we study the electronic structures, high mobility, and substrate-mediated clustering processes of Au1-4 on graphene.

    Already in the 1970s is was speculated that dispersion forces, i.e. van der Waals forces, significantly contribute to the adsorption energies of gold atoms on graphite. We accounted for van der Waals interactions in our density functional theory calculations (paper VI) and investigated the influence of these dispersion forces on the binding of copper, silver, and gold adatoms on graphene. While copper and gold show a mixed adsorption mechanism, i.e. chemical binding plus attraction due to the van der Waals forces, silver is purely physisorbed on graphene.

    List of papers
    1. Catalytic activity of small MgO-supported Au clusters towards CO oxidation: A density functional study
    Open this publication in new window or tab >>Catalytic activity of small MgO-supported Au clusters towards CO oxidation: A density functional study
    2010 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 19, p. 195443-Article in journal (Refereed) Published
    Abstract [en]

    In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-133240 (URN)10.1103/PhysRevB.81.195443 (DOI)000278142000132 ()
    Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
    2. Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?
    Open this publication in new window or tab >>Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?
    2010 (English)In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694Article in journal (Other academic) Submitted
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-133241 (URN)
    Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
    3. The relative stability of Au13 isomers and their potential for O2 dissociation
    Open this publication in new window or tab >>The relative stability of Au13 isomers and their potential for O2 dissociation
    (English)Manuscript (preprint) (Other academic)
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-133242 (URN)
    Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2012-03-29
    4. Thermally Excited Vibrations in Copper, Silver, and Gold Trimers and Enhanced Binding of CO
    Open this publication in new window or tab >>Thermally Excited Vibrations in Copper, Silver, and Gold Trimers and Enhanced Binding of CO
    2010 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126Article in journal (Other academic) Submitted
    National Category
    Inorganic Chemistry
    Research subject
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-133243 (URN)
    Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
    5. Small gold clusters on graphene, their mobility and clustering: a DFT study
    Open this publication in new window or tab >>Small gold clusters on graphene, their mobility and clustering: a DFT study
    2011 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 23, no 20, p. 205301-Article in journal (Refereed) Published
    Abstract [en]

    Motivated by the experimentally observed high mobility of gold atoms on graphene and their tendency to form nanometer-sized clusters, we present a density functional theory study of the ground state structures of small gold clusters on graphene, their mobility and clustering. Our detailed analysis of the electronic structures identifies the opportunity to form strong gold-gold bonds and the graphene-mediated interaction of the pre-adsorbed fragments as the driving forces behind gold's tendency to aggregate on graphene. While clusters containing up to three gold atoms have one unambiguous ground state structure, both gas phase isomers of a cluster with four gold atoms can be found on graphene. In the gas phase the diamond-shaped Au-4(D) cluster is the ground state structure, whereas the Y-shaped Au-4(Y) becomes the actual ground state when adsorbed on graphene. As we show, both clusters can be produced on graphene by two distinct clustering processes. We also studied in detail the stepwise formation of a gold dimer out of two pre-adsorbed adatoms, as well as the formation of Au-3. All reactions are exothermic and no further activation barriers, apart from the diffusion barriers, were found. The diffusion barriers of all studied clusters range from 4 to 36 meV only, and are substantially exceeded by the adsorption energies of -0.1 to -0.59 eV. This explains the high mobility of Au1-4 on graphene along the C-C bonds.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-133244 (URN)10.1088/0953-8984/23/20/205301 (DOI)000290212300005 ()
    Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
    6. Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
    Open this publication in new window or tab >>Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
    2011 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 39, article id 395001Article in journal (Refereed) Published
    Abstract [en]

    We performed a systematic density functional (DF) study of the adsorption of copper, silver, and gold adatoms on pristine graphene, especially accounting for van der Waals (vdW) interactions by the vdW-DF and PBE + D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene layer can be significant. Only the results for silver are qualitatively at variance with those obtained with the generalized gradient approximation, which gives no binding in this case. However in all three cases, we observe some quantitative differences between the vdW-DF and PBE + D2 methods. For instance the adsorption energies calculated with the PBE + D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE + D2 are shorter than those calculated with the vdW-DF method.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-133245 (URN)10.1088/0953-8984/23/39/395001 (DOI)000295035200010 ()
    Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
  • 25.
    Andersson, Gabriella
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics III.
    Burkert, Till
    Warnicke, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Björck, Matts
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics III.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Chacon, Cyril
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Zlotea, Claudia
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Perpendicular magnetocrystalline anisotropy in tetragonally distorted Fe-Co alloys2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, no 3, article id 037205Article in journal (Refereed)
    Abstract [en]

    We report on the experimental realization of tetragonal Fe-Co alloys as a constituent of Fe(0.36)Co(0.64)/Pt superlattices with huge perpendicular magnetocrystalline anisotropy energy, reaching 210 mu eV/atom, and a saturation magnetization of 2.5 mu(B)/atom at 40 K, in qualitative agreement with theoretical predictions. At room temperature the corresponding values 150 mu eV/atom and 2.2 mu(B)/atom are achieved. This suggests that Fe-Co alloys with carefully chosen combinations of composition and distortion are good candidates for high-density perpendicular storage materials.

  • 26.
    ANDERSSON, JO
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    GUNNARSSON, K
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    SVEDLINDH, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    NORDBLAD, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    LUNDGREN, L
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    ARUGA, H
    ITO, A
    DYNAMIC SCALING IN THE ISING REENTRANT SPIN-GLASS FE0.62MN0.38TIO31990In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 165, p. 183-184Article in journal (Refereed)
  • 27.
    ANDERSSON, JO
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    MATTSSON, J
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    NORDBLAD, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    OVERLAP LENGTH IN A CU-MN SPIN-GLASS PROBED BY AC SUSCEPTIBILITY1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 48, p. 13977-13980Article in journal (Refereed)
    Abstract [en]

    An important theoretical concept in the ‘’droplet” model of spin glasses is the existence of an overlap length. Experimentally this concept was verified by dc-magnetization measurements. However, recent ac-susceptibility measurements by Lefloch et al. [Europhys. Lett. 18, 647 (1992)] have been interpreted to contradict this result. In this paper, ac-susceptibility and dc-magnetization measurements on a Cu (2% Mn) spin glass are reported. The results of the ac-susceptibility measurements are consistent with dc measurements and provide evidence for the existence of an overlap length in 3d spin glasses at low temperatures.

  • 28.
    Andersson, Mikael Svante
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Interacting Magnetic Nanosystems: An Experimental Study Of Superspin Glasses2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis presents experimental results on strongly interacting γ-Fe2O3 magnetic nanoparticles and their collective properties. The main findings are that very dense randomly packed (≈60%) γ-Fe2O3 nanoparticles form a replica of a spin glass. The magnetic properties of the nanoparticle system are in most regards the same as those of an atomic spin glass. The system is therefore proposed as a model superspin glass. In superspin glasses the interacting building blocks that form the collective state are single domain nanoparticles, superspins with a magnetic moment of about 10000 μB, which can be compared to the atomic magnetic moment in spin glasses of approximately 1 μB.  It was found that the relaxation time of the individual nanoparticles impacts the collective properties and governs the superspin dimensionality. Several dense compacts, each prepared with nanoparticles of a specific size, with diameters 6, 8, 9 and 11.5 nm, were studied. All the studied compacts were found to form a superspin glass state. Non-interacting reference samples, consisting of the same particles but coated with a silica shell, were synthesized to determine the single particle magnetic properties.  It was also found that the effects of the nanoparticle size distribution, which lead to a variation of the magnetic properties, can be mitigated by having strong enough interparticle interactions. The majority of the work was carried out using SQUID magnetometry.

    List of papers
    1. Ageing dynamics of a superspin glass
    Open this publication in new window or tab >>Ageing dynamics of a superspin glass
    Show others...
    2014 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 108, no 1, p. 17004-Article in journal (Refereed) Published
    Abstract [en]

    Magnetization dynamics of a model superspin glass system consisting of nearly monodispersed close-packed maghemite particles of diameter 8 nm is investigated. The observed non-equilibrium features of the dynamics are qualitatively similar to those of atomic spin glass systems. The intrinsic relaxation function, as observed in zero-field-cooled magnetization relaxation experiments, depends on the time the sample has been kept at constant temperature (ageing). Accompanying low-field experiments show that the archetypal spin glass characteristics -ageing, memory and rejuvenation- are reproduced in this dense system of dipolar-dipolar interacting superspins.  

    National Category
    Materials Engineering Physical Sciences Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-238588 (URN)10.1209/0295-5075/108/17004 (DOI)000343970300018 ()
    Available from: 2014-12-16 Created: 2014-12-14 Last updated: 2017-12-05Bibliographically approved
    2. Size-dependent surface effects in maghemite nanoparticles and its impact on interparticle interactions in dense assemblies
    Open this publication in new window or tab >>Size-dependent surface effects in maghemite nanoparticles and its impact on interparticle interactions in dense assemblies
    Show others...
    2015 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 26, no 47, article id 475703Article in journal (Refereed) Published
    Abstract [en]

    The question of the dominant interparticle magnetic interaction type in random closely packed assemblies of different diameter (6.2-11.5 nm) bare maghemite nanoparticles (NPs) is addressed. Single-particle magnetic properties such as particle anisotropy and exchange bias field are first of all studied in dilute (reference) systems of these same NPs, where interparticle interactions are neglible. Substantial surface spin disorder is revealed in all particles except the smallest, viz. for diameters d = 8-11.5 nm but not for d = 6.2-6.3 nm. X-ray diffraction analysis points to a crystallographic origin of this effect. The study of closely packed assemblies of the d >= 8 nm particles observes collective (superspin) freezing that clearly appears to be governed by interparticle dipole interactions. However, the dense assemblies of the smallest particles exhibit freezing temperatures that are higher than expected from a simple (dipole) extrapolation of the corresponding temperatures found in the d >= 8 nm assemblies. It is suggested that the nature of the dominant interparticle interaction in these smaller particle assemblies is superexchange, whereby the lack of significant surface spin disorder allows this mechanism to become important at the level of interacting superspins.

    Keywords
    nanoparticles, magnetism, dipolar, superexchange
    National Category
    Nano Technology
    Identifiers
    urn:nbn:se:uu:diva-272272 (URN)10.1088/0957-4484/26/47/475703 (DOI)000366209100010 ()
    Funder
    Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
    Available from: 2016-01-18 Created: 2016-01-13 Last updated: 2017-11-30Bibliographically approved
    3. Particle size-dependent superspin glass behavior in random compacts of monodisperse maghemite nanoparticles
    Open this publication in new window or tab >>Particle size-dependent superspin glass behavior in random compacts of monodisperse maghemite nanoparticles
    Show others...
    2016 (English)In: MATERIALS RESEARCH EXPRESS, ISSN 2053-1591, Vol. 3, no 4, article id 045015Article in journal (Refereed) Published
    Abstract [en]

    Dense random assemblies made from highly monodisperse gamma-Fe2O3 nanoparticles with sizes ranging from 6.2 to 11.5 nm have been investigated by DC and AC magnetometry. It is found that all assemblies undergo superspin glass phase transitions. The superspin glass phase transition temperature is strongly dependent on the particle size and the nature of the interparticle interaction. However the transition from superparamagnet to superspin glass, as evidenced by the shape of the ac-susceptibility curves and the dynamic critical exponents associated with the transition, is similar in all systems.

    Keywords
    magnetic nanoparticles, maghemite, superspin glass, phase transition
    National Category
    Materials Engineering
    Identifiers
    urn:nbn:se:uu:diva-299615 (URN)10.1088/2053-1591/3/4/045015 (DOI)000377811500016 ()
    Funder
    Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
    Available from: 2016-07-25 Created: 2016-07-25 Last updated: 2017-04-07Bibliographically approved
    4. Effects of the individual particle relaxation time on superspin glass dynamics
    Open this publication in new window or tab >>Effects of the individual particle relaxation time on superspin glass dynamics
    Show others...
    2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 5, article id 054407Article in journal (Refereed) Published
    Abstract [en]

    The low temperature dynamic magnetic properties of two dense magnetic nanoparticle assemblies with similar superspin glass transition temperatures T-g similar to 140 K are compared. The two samples are made from batches of 6 and 8 nm monodisperse gamma-Fe2O3 nanoparticles, respectively. The properties of the individual particles are extracted from measurements on reference samples where the particles have been covered with a thick silica coating. The blocking temperatures of these dilute assemblies are found at 12.5 K for the 6 nm particles and at 35 K for the 8 nm particles, which implies different anisotropy energy barriers of the individual particles and vastly different temperature evolution of their relaxation times. The results of the measurements on the concentrated particle assemblies suggest a strong influence of the particle energy barrier on the details of the aging dynamics, memory behavior, and apparent superspin dimensionality of the particles.

    National Category
    Condensed Matter Physics Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-279564 (URN)10.1103/PhysRevB.93.054407 (DOI)000369726000001 ()
    Funder
    Swedish Research Council
    Available from: 2016-03-02 Created: 2016-03-02 Last updated: 2017-04-07Bibliographically approved
    5. Magnetic properties of nanoparticle compacts with controlled broadening of the particle size distribution
    Open this publication in new window or tab >>Magnetic properties of nanoparticle compacts with controlled broadening of the particle size distribution
    Show others...
    2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 184431Article in journal (Refereed) Published
    Abstract [en]

    Binary random compacts with different proportions of small (volume V) and large (volume 2V) essentially bare maghemite nanoparticles are used to investigate the effect of controllably broadening the particle size distribution on the magnetic properties of magnetic nanoparticle assemblies with strong dipolar interaction. A series of eight random mixtures of highly uniform 9.0- and 11.5-nm-diameter maghemite particles prepared by thermal decomposition is studied. In spite of the severely broadened size distributions in the mixed samples, well-defined superspin glass transition temperatures are observed across the series, their values increasing linearly with the weight fraction of large particles.

    National Category
    Engineering and Technology Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-319714 (URN)10.1103/PhysRevB.95.184431 (DOI)000405203000010 ()
    Funder
    Swedish Research Council
    Note

    Title in List of papers in Thesis: Magnetic properties of nanoparticles compacts with controlled broadening of the particle size distribution

    Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-11-29Bibliographically approved
    6. Demagnetization effects in dense nanoparticle assemblies
    Open this publication in new window or tab >>Demagnetization effects in dense nanoparticle assemblies
    Show others...
    2016 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, no 15, article id 152404Article in journal (Refereed) Published
    Abstract [en]

    We highlight the relevance of demagnetizing-field corrections in the characterization of dense magnetic nanoparticle assemblies. By an analysis that employs in-plane and out-of-plane magnetometry on cylindrical assemblies, we demonstrate the suitability of a simple analytical formula-based correction method. This allows us to identify artifacts of the demagnetizing field in temperature-dependent susceptibility curves (e.g., shoulder peaks in curves from a disordered assembly of essentially bare magnetic nanoparticles). The same analysis approach is shown to be a straightforward procedure for determining the magnetic nanoparticle packing fraction in dense, disordered assemblies.

    National Category
    Condensed Matter Physics Engineering and Technology
    Identifiers
    urn:nbn:se:uu:diva-308922 (URN)10.1063/1.4964517 (DOI)000386534800031 ()
    Funder
    Swedish Research Council
    Available from: 2016-12-01 Created: 2016-12-01 Last updated: 2017-11-29Bibliographically approved
    7. Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles
    Open this publication in new window or tab >>Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles
    Show others...
    2017 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 29, no 19, p. 8258-8268Article in journal (Refereed) Published
    Abstract [en]

    Remanence magnetization plots (e.g., Henkel or delta M plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that delta M plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the delta M plots of gamma-Fe2O3 nanoparticles isolated by thick SiO2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mossbauer measurements, that has a pronounced effect on the delta M plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the delta M plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and delta M plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles.

    Place, publisher, year, edition, pages
    Washington, D.C., USA: American Chemical Society (ACS), 2017
    National Category
    Engineering and Technology Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-319716 (URN)10.1021/acs.chemmater.7b02522 (DOI)000412965800027 ()
    Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2018-02-01Bibliographically approved
  • 29.
    Andersson, Mikael Svante
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Angel De Toro, Jose
    Univ Castilla La Mancha, IRICA, E-13071 Ciudad Real, Spain.;Univ Castilla La Mancha, Dept Fis Aplicada, E-13071 Ciudad Real, Spain..
    Lee, Su Seong
    Inst Bioengn & Nanotechnol, 31 Biopolis Way, Singapore 138669, Singapore..
    Normile, Peter S.
    Univ Castilla La Mancha, IRICA, E-13071 Ciudad Real, Spain.;Univ Castilla La Mancha, Dept Fis Aplicada, E-13071 Ciudad Real, Spain..
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Effects of the individual particle relaxation time on superspin glass dynamics2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 5, article id 054407Article in journal (Refereed)
    Abstract [en]

    The low temperature dynamic magnetic properties of two dense magnetic nanoparticle assemblies with similar superspin glass transition temperatures T-g similar to 140 K are compared. The two samples are made from batches of 6 and 8 nm monodisperse gamma-Fe2O3 nanoparticles, respectively. The properties of the individual particles are extracted from measurements on reference samples where the particles have been covered with a thick silica coating. The blocking temperatures of these dilute assemblies are found at 12.5 K for the 6 nm particles and at 35 K for the 8 nm particles, which implies different anisotropy energy barriers of the individual particles and vastly different temperature evolution of their relaxation times. The results of the measurements on the concentrated particle assemblies suggest a strong influence of the particle energy barrier on the details of the aging dynamics, memory behavior, and apparent superspin dimensionality of the particles.

  • 30.
    Andersson, Mikael Svante
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Normile, Peter S.
    Lee, Su Seong
    Singh, Gurvinder
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    De Toro, José A.
    Particle size-dependent superspin glass behavior in random compact of monodisperse maghemite nanoparticles2016In: Materials Research Express, Vol. 3, article id 054407Article in journal (Refereed)
  • 31.
    Andersson, Mikael Svante
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Normile, Peter S.
    Univ Castilla La Mancha, Spain.
    Lee, Su Seong
    Inst Bioengn & Nanotechnol, Singapore.
    Singh, Gurvinder
    Norwegian Univ Sci & Technol NTNU, Dept Mat Sci & Engn, Norway.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    De Toro, José A.
    Univ Castilla La Mancha, Spain.
    Magnetic properties of nanoparticle compacts with controlled broadening of the particle size distribution2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 184431Article in journal (Refereed)
    Abstract [en]

    Binary random compacts with different proportions of small (volume V) and large (volume 2V) essentially bare maghemite nanoparticles are used to investigate the effect of controllably broadening the particle size distribution on the magnetic properties of magnetic nanoparticle assemblies with strong dipolar interaction. A series of eight random mixtures of highly uniform 9.0- and 11.5-nm-diameter maghemite particles prepared by thermal decomposition is studied. In spite of the severely broadened size distributions in the mixed samples, well-defined superspin glass transition temperatures are observed across the series, their values increasing linearly with the weight fraction of large particles.

  • 32.
    Andersson, Ole
    et al.
    Dept. of Physics, Stockholm University, Sweden.
    Bengtsson, Ingemar
    Dept. of Physics, Stockholm University, Sweden.
    Ericsson, Marie
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Quantum Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Sjöqvist, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala universitet.
    Geometric phases for mixed states of the Kitaev chain2016In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 374, no 2069, article id 20150231Article in journal (Refereed)
    Abstract [en]

    The Berry phase has found applications in building topological order parameters for certain condensed matter systems. The question whether some geometric phase for mixed states can serve the same purpose has been raised, and proposals are on the table. We analyze the intricate behaviour of Uhlmann’s geometric phase in the Kitaev chain at finite temperature, and then argue that it captures quite different physics from that intended. We also analyze the behaviour of a geometric phase introduced in the context of interferometry. For the Kitaev chain, this phase closely mirrors that of the Berry phase, and we argue that it merits further investigation. 

  • 33.
    Andersson, T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Zhang, C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Björneholm, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Mikkela, M-H
    Oulu Univ, Dept Phys Sci, Box 3000, FI-90014 Oulu, Finland..
    Jankala, K.
    Oulu Univ, Dept Phys Sci, Box 3000, FI-90014 Oulu, Finland..
    Anin, D.
    Oulu Univ, Dept Phys Sci, Box 3000, FI-90014 Oulu, Finland..
    Urpelainen, S.
    Oulu Univ, Dept Phys Sci, Box 3000, FI-90014 Oulu, Finland..
    Huttula, M.
    Oulu Univ, Dept Phys Sci, Box 3000, FI-90014 Oulu, Finland..
    Tchaplyguine, M.
    Lund Univ, Max Lab, Box 118, SE-22363 Lund, Sweden..
    Electronic structure transformation in small bare Au clusters as seen by x-ray photoelectron spectroscopy2017In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 50, no 1, article id 015102Article in journal (Refereed)
    Abstract [en]

    Free bare gold clusters in the size range from few tens to few hundred atoms (<= 1 nm dimensions) have been produced in a beam, and the size-dependent development of their full valence band including the 5d and 6s parts has been mapped 'on the fly' by synchrotron-based photoelectron spectroscopy. The Au 4f core level has been also probed, and the cluster-specific Au 4f ionization energies have been used to estimate the cluster size. The recorded in the present work valence spectra of the small clusters are compared with the spectra of the large clusters (N similar to 10(3)) created by us using a magnetron-based gas aggregation source. The comparison shows a substantially narrower 5d valence band and the decrease in its splitting for gold clusters in the size range of few hundred atoms and below. Our DFT calculations involving the pseudopotential method show that the 5d band width of the ground state increases with the cluster size and by the size N = 20 becomes comparable with the experimental width of the valence photoelectron spectrum. Similar to the earlier observations on supported clusters we interpret our experimental and theoretical results as due to the undercoordination of a large fraction of atoms in the clusters with N similar to 10(2) and below. The consequences of such electronic structure of small gold clusters are discussed in connection with their specific physical and chemical properties related to nanoplasmonics and nanocatalysis.

  • 34.
    Andersson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Surface and Interface Science.
    Free Metal Clusters Studied by Photoelectron Spectroscopy2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Clusters are aggregates of a finite number of atoms or molecules. In the present work, free clusters out of metallic parent materials have been created and studied by synchrotron radiation-based photoelectron spectroscopy. The clusters have been formed and studied in a beam and the electronic structure of the clusters has been investigated. Conclusions have been drawn about the spatial distribution of atoms of different elements in bi-component clusters, about the development of metallicity in small clusters, and about the excitation of plasmons.

    Bi-component alloy clusters of sodium and potassium and of copper and silver have been produced. The site-sensitivity of the photoelectron spectroscopy technique has allowed us to probe the geometric distribution of the atoms of the constituent elements by comparing the responses from the bulk and surface of the clusters. In both cases, we have found evidence for a surface-segregated structure, with the element with the largest atoms and lowest cohesive energy (potassium and silver, correspondingly) dominating the surface and with a mixed bulk.

    Small clusters of tin and lead have been probed to investigate the development of metallicity. The difference in screening efficiency between metals and non-metals has been utilized to determine in what size range an aggregate of atoms of these metallic parent materials stops to be metallic. For tin this has been found to occur below ~40 atoms while for lead it happened somewhere below 20-30 atoms.

    The excitation of bulk and surface plasmons has been studied in clusters of sodium, potassium, magnesium and aluminium, with radii in the nanometer range. The excitation energies have been found to be close to those of the corresponding macroscopic solids. We have also observed spectral features corresponding to multi-quantum plasmon excitation in clusters of Na and K. Such features have in macroscopic solids been interpreted as due to harmonic plasmon excitation. Our observations of features corresponding to the excitation of one bulk and one surface plasmon however suggest the presence of sequential excitation in clusters.

    List of papers
    1. Single-component surface in binary self-assembled NaK nanoalloy clusters
    Open this publication in new window or tab >>Single-component surface in binary self-assembled NaK nanoalloy clusters
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    2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 3, p. 033405-Article in journal (Refereed) Published
    Abstract [en]

    Free metallic nanoalloy clusters created in a self-assembling process out of sodium and potassium mixed vapor have been studied by synchrotron-based photoelectron spectroscopy. The clusters are shown to consist of an alloy core surrounded by a surface layer containing only K in a range of conditions from K-rich to Na-rich nanoalloys. The size of the clusters as well as the fraction of the elements has been estimated from the spectra using our results on pure clusters. The mechanism behind the observed structure is discussed in terms of the total cohesive-energy minimization.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-128322 (URN)10.1103/PhysRevB.80.033405 (DOI)000268617800026 ()
    Available from: 2010-07-22 Created: 2010-07-20 Last updated: 2017-12-12Bibliographically approved
    2. Size-dependent transformation of energy structure in free tin clusters studied by photoelectron spectroscopy
    Open this publication in new window or tab >>Size-dependent transformation of energy structure in free tin clusters studied by photoelectron spectroscopy
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    2010 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 2, p. 023203-Article in journal (Refereed) Published
    Abstract [en]

    Free nanoscale tin clusters in the average size range of a few tens up to similar to 500 atoms are studied by means of valence and core-level photoelectron spectroscopy. A self-consistent picture arises from comparison of the work functions derived from the 4d core-level spectra, the experimental ionization energies extracted from the 5p valence spectra, and the expectations from the metallic sphere model for the clusters. The present studies demonstrate that the 4d core-level energy changes are a sensitive probe of the metal-to-semiconductor transition in the size range under investigation.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-137127 (URN)10.1103/PhysRevA.81.023203 (DOI)000275072500115 ()
    Available from: 2010-12-15 Created: 2010-12-15 Last updated: 2017-12-11Bibliographically approved
    3. Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy
    Open this publication in new window or tab >>Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy
    Show others...
    2011 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 9, p. 094511-Article in journal (Refereed) Published
    Abstract [en]

    Plasmons are investigated in free nanoscale Na, Mg, and K metal clusters using synchrotron radiation-based x-ray photoelectron spectroscopy. The core levels for which the response from bulk and surface atoms can be resolved are probed over an extended binding energy range to include the plasmon loss features. In all species the features due to fundamental plasmons are identified, and in Na and K also those due to either the first order plasmon overtones or sequential plasmon excitation are observed. These features are discussed in view of earlier results for planar macroscopic samples and free clusters of the same materials.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-157480 (URN)10.1063/1.3563751 (DOI)000288085300048 ()
    Available from: 2011-08-22 Created: 2011-08-22 Last updated: 2017-12-08Bibliographically approved
    4. The electronic structure of free aluminum clusters: Metallicity and plasmons
    Open this publication in new window or tab >>The electronic structure of free aluminum clusters: Metallicity and plasmons
    Show others...
    2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 20, p. 204504-Article in journal (Refereed) Published
    Abstract [en]

    The electronic structure of free aluminum clusters with similar to 3-4 nm radius has been investigated using synchrotron radiation-based photoelectron and Auger electron spectroscopy. A beam of free clusters has been produced using a gas-aggregation source. The 2p core level and the valence band have been probed. Photoelectron energy-loss features corresponding to both bulk and surface plasmon excitation following photoionization of the 2p level have been observed, and the excitation energies have been derived. In contrast to some expectations, the loss features have been detected at energies very close to those of the macroscopic solid. The results are discussed from the point of view of metallic properties in nanoparticles with a finite number of constituent atoms.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-178867 (URN)10.1063/1.4718362 (DOI)000304818400036 ()
    Available from: 2012-08-06 Created: 2012-08-02 Last updated: 2017-12-07Bibliographically approved
    5. Valence-band narrowing and metallic-screening disappearance in the metal-to-insulator transition in small Pb clusters—as seen by photoelectron spectroscopy
    Open this publication in new window or tab >>Valence-band narrowing and metallic-screening disappearance in the metal-to-insulator transition in small Pb clusters—as seen by photoelectron spectroscopy
    Show others...
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    A metal-to-insulator transition in small Pb clusters within the size range from a few tens of atoms up to ~100 atoms is investigated using synchrotron-based photoelectron spectroscopy. The electronic structure is seen to differ increasingly from that of the solid when the cluster size decreases. With the cluster size decrease the valence band narrows gradually -down to 25% of that of the solid-state at the smallest size of few tens of atoms. Simultaneously the Fermi-edge energy increases. The Pb 5d core level binding energy also increases and the spectral features broaden and change the shape. The changes observed for the valence and for the 5d energies and shapes are consistent with the picture of the loss of metallicity in Pb below the critical size of 20-30 atoms/cluster.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-183029 (URN)
    Available from: 2012-10-26 Created: 2012-10-21 Last updated: 2012-10-26
    6. Core-shell structure disclosed in self-assembled Cu-Ag nanoalloy particles—explanation for enhanced catalytic activity?
    Open this publication in new window or tab >>Core-shell structure disclosed in self-assembled Cu-Ag nanoalloy particles—explanation for enhanced catalytic activity?
    Show others...
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    Core-shell segregation of copper and silver in self-assembled, free nanoparticles is established by means of electron spectroscopy using different photon energies. The nanoparticles are formed from metal-atom mixed vapor created by sputtering of a bimetallic sample. Even at dominating copper concentration in the primary vapor the surface of the nanoparticles is composed of only silver. These conclusions are based on the analysis of the photon-energy-dependent changes of the Cu 3d and Ag 4d photoelectron spectra. The role of such segregation is discussed in connection to the suggested enhanced catalytic properties of Cu-Ag nanoparticles.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-183030 (URN)
    Available from: 2012-10-26 Created: 2012-10-21 Last updated: 2012-10-26
  • 35.
    Andersson, Y
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Rundqvist, S
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Beckman, O
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Lundgren, L
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Nordblad, P
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Properties of Fe2 P crystals prepared from a liquid copper medium1978In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 49, no 2, p. K153-K156Article in journal (Refereed)
  • 36. Andreeva, Marina A
    et al.
    Lindgren, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics III.
    Standing Waves and Reflectivity from an Ultrathin Layer2002In: JETP Letters, Vol. 76, no 12, p. 704-706Article in journal (Refereed)
  • 37.
    Anversa, Jonas
    et al.
    Univ Fed Santa Maria, Dept Fis, BR-97105900 Santa Maria, RS, Brazil.;Fac Meridional, Escola Engn Civil, BR-99070220 Passo Fundo, RS, Brazil..
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Piquini, Paulo
    Univ Fed Santa Maria, Dept Fis, BR-97105900 Santa Maria, RS, Brazil..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..
    High pressure driven superconducting critical temperature tuning in Sb2Se3 topological insulator2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 108, no 21, article id 212601Article in journal (Refereed)
    Abstract [en]

    In this letter, we are reporting the change of superconducting critical temperature in Sb2Se3 topological insulator under the influence of an external hydrostatic pressure based on first principles electronic structure calculations coupled with Migdal-Eliashberg model. Experimentally, it was shown previously that Sb2Se3 was undergoing through a transition to a superconducting phase when subjected to a compressive pressure. Our results show that the critical temperature increases up to 6.15K under the pressure unto 40GPa and, subsequently, drops down until 70 GPa. Throughout this pressure range, the system is preserving the initial Pnma symmetry without any structural transformation. Our results suggest that the possible relevant mechanism behind the superconductivity in Sb2Se3 is primarily the electron-phonon coupling. Published by AIP Publishing.

  • 38. Apell, S. P.
    et al.
    Hanson, G. W.
    Hägglund, Carl
    epartment of Chemical Engineering, Stanford University, USA.
    High optical absorption in grapheneManuscript (preprint) (Other academic)
  • 39.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB22015In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 92, no 5, article id 054516Article in journal (Refereed)
  • 40.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high-Tc superconductivity in FeSe/SrTiO32018In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Phys. Rev. B, Vol. 97, p. 060501(R)-Article in journal (Refereed)
  • 41.
    Aperis, Alexandros
    et al.
    Department of Physics, National Technical University of Athens, GR-15780 Athens, Greece.
    Varelogiannis, Georgios
    Littlewood, Peter B
    Magnetic-field-induced pattern of coexisting condensates in the superconducting state of CeCoIn52010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, P, Vol. 104, no 21, p. 216403-Article in journal (Refereed)
    Abstract [en]

    CeCoIn5 is an anomalous superconductor which exhibits a high-magnetic-field phase that consists of a modulated magnetic coupling together with persistent superconducting order. Here we use a generic microscopic model to argue that this state is a pattern of coexisting condensates: a d-wave singlet superconducting (SC) state, a staggered π-triplet SC state, and a spin density wave (SDW). Our microscopic picture allows a calculation of the phase diagram, and physical consequences including NMR. We interpret the appearance of the SDW order in the Q phase as being induced by odd-triplet pairing.

  • 42.
    Arapan, Sergiu
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Materials Science.
    Understanding Physical Reality via Virtual Experiments2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis I have studied some problems of condensed matter at high pressures and temperatures by means of numerical simulations based on Density Functional Theory (DFT).

    The stability of MgCO3 and CaCO3 carbonates at the Earth's mantle conditions may play an important role in the global carbon cycle through the subduction of the oceanic crust. By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transformation is characterized by the change of the sp-hybridization state of carbon atom and indicates a change to a new crystal-chemical regime. By performing ab initio Molecular Dynamics simulations we show the new phase to be stable at 250 GPa and 1000K. Thus, the formation of sp3 hybridized bonds in carbonates can explain the stability of MaCO3 and CaCO3 at pressures corresponding to the Earth's lower mantle conditions. We have also calculated phase transition sequence in CaCO3, SrCO3 and BaCO3, and have found that, despite the fact that these carbonates are isostructural and undergo the same type of aragonite to post-aragonite transition, their phase transformation sequences are different at high pressures.

    The continuous improvement of the high-pressure technique led to the discovery of new composite structures at high pressures and complex phases of many elements in the periodic table have been determined as composite host-guest incommensurate structures. We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells and exploiting the fact that the total energy of the system is a function of structural parameters. By applying this method to the Sc-II phase, we have determined the incommensurate ratio, lattice parameters and Wyckoff positions of Sc-II in excellent agreement with the available experimental data. Moreover, we predict the occurrence of an incommensurate high-pressure phase in Ca from first-principle calculations within this approach.

    The implementation of DFT in modern electronic structure calculation methods proved to be very successful in predicting the physical properties of a solid at low temperature. One can rigorously describe the thermodynamics of a crystal via the collective excitation of the ionic lattice, and the ab initio calculations give an accurate phonon spectra in the quasi-harmonic approximation. Recently an elegant method to calculate phonon spectra at finite temperature in a self-consistent way by using first principles methods has been developed. Within the framework of self-consistent ab initio lattice dynamics approach (SCAILD) it is possible to reproduce the observed stable phonon spectra of high-temperature bcc phase of Ti, Zr and Hf with a good accuracy. We show that this method gives also a good description of the thermodynamics of hcp and bcc phases of Ti, Zr and Hf at high temperatures, and we provide a procedure for the correct estimation of the hcp to bcc phase transition temperature.

  • 43.
    Arapan, Sergiu
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Korepov, Sveatoslav
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Liberman, Michael
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Johansson, Börje
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Conductance of a disordered double quantum wire in a magnetic field: Boundary roughness scattering2003In: PHYSICAL REVIEW B, Vol. 67, p. 115328-Article in journal (Refereed)
    Abstract [en]

    Boundary roughness scattering in disordered tunnel-coupled quantum wires in the presence of a magnetic

    field is considered. The low-temperature conductance as a function of applied magnetic field is calculated for

    different structure and disorder parameters using the method of the generalized S-matrix composition.We show

    that despite the fact that lateral wire width fluctuations of the size of few atomic layers may significantly shift

    the partial energy gap, the effect of the conductance enhancement at energies in the partial energy gap does

    take place in sufficiently strong magnetic fields and small correlation length of the disorder defined as the

    average distance between the neighboring discontinuities of the boundary profile. The last parameter is shown

    to be particularly important for the determination of the transport properties of the system. Remarkably, we find

    that in a wide range of system parameters the conductance decreases with the correlation length despite the

    decreasing number of boundary discontinuities.

  • 44.
    Arapan, Sergiu
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Korepov, Sveatoslav
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Liberman, Michael
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Johansson, Börje
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Effect of the boundary roughness on the conductance of double quantum wire in a magnetic field2003In: EUROPHYSICS LETTERS, Vol. 64, p. 239-245Article, review/survey (Other (popular scientific, debate etc.))
    Abstract [en]

    We investigate the effect of the boundary roughness scattering on the conductance of a disordered tunnel-coupled quantum wire in the presence of a magnetic field. It is shown that the average distance between the neighboring discontinuities of the boundary profile plays an important role in the transport properties of the system and the manifestation of the localization-delocalization effect. Present studies point out a new effect consisting in the

    decrease of the conductance with the increase of the disorder correlation length.

  • 45.
    Arapan, Sergiu
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Liberman, Michael
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Physics IV.
    Exciton levels and optical absorption in coupled double quantum well structures2005In: Journal of Luminescence, Vol. 112, no 1-4, p. 216-219Article in journal (Refereed)
    Abstract [en]

    We study exciton states in a coupled double quantum well (CDQW) semiconductor structure. Exciton levels and binding energies of direct and indirect excitons are calculated for a symmetric CDQW system with an applied electric field. The exciton states are obtained by solving the exciton effective-mass equation in the momentum space using the modified Gaussian quadrature method. Within this approach we perform realistic calculations of the exciton states by taking into account the coupling between different subband pairs and calculate optical-absorption coefficients. The calculated values of the exciton binding energy are in a good agreement with the experiment and the calculated absorption spectra qualitatively agree with the measured photoluminescence excitation spectra.

  • 46.
    Araujo, Carlos Moyses
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nagar, Sandeep
    Ramzan, Muhammad
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Shukla, R.
    Jayakumar, O. D.
    Tyagi, A. K.
    Liu, Yi-Sheng
    Chen, Jeng-Lung
    Glans, Per-Anders
    Chang, Chinglin
    Blomqvist, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lizárraga, Raquel
    Holmstrom, Erik
    Belova, Lyubov
    Guo, Jinghua
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden.
    Rao, K. V.
    Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites2014In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, p. 4686-Article in journal (Refereed)
    Abstract [en]

    We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

  • 47.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    de Almeida, J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    da Silva, A. Ferreira
    Univ Fed Bahia, Inst Fis, Salvador, BA, Brazil..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Insights in the electronic structure and redox reaction energy in LiFePO4 battery material from an accurate Tran-Blaha modified Becke Johnson potential2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 12, article id 125107Article in journal (Refereed)
    Abstract [en]

    The main goals of this paper are to investigate the accuracy of the Tran-Blaha modified Becke Johnson (TB-mBJ) potential to predict the electronic structure of lithium iron phosphate and the related redox reaction energy with the lithium deintercalation process. The computed electronic structures show that the TB-mBJ method is able to partially localize Fe-3d electrons in LiFePO4 and FePO4 which usually is a problem for the generalized gradient approximation (GGA) due to the self interaction error. The energy band gap is also improved by the TB-mBJ calculations in comparison with the GGA results. It turned out, however, that the redox reaction energy evaluated by the TB-mBJ technique is not in good agreement with the measured one. It is speculated that this disagreement in the computed redox energy and the experimental value is due to the lack of a formal expression to evaluate the exchange and correlation energy. Therefore, the TB-mBJ is an efficient method to improve the prediction of the electronic structures coming form the standard GGA functional in LiFePO4 and FePO4. However, it does not appear to have the same efficiency for evaluating the redox reaction energies for the investigated system.

  • 48.
    Araujo, Rafael B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    de Almeida, J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ferreira da Silva, A.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Lithium transport investigation in LixFeSiO4: A promising cathode material2013In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 173, p. 9-13Article in journal (Refereed)
    Abstract [en]

    In this paper we investigate lithium mobility in both Li2FeSiO4 and its   half-lithiated state LiFeSiO4 considering an orthorhombic crystal   structure. We find that the calculated activation energy of Li+ ions   hopping between adjacent equilibrium sites predicts two least hindered   diffusion pathways in both materials. One of them is along the [100]   direction characterizing an ionic diffusion in a straight line and the   other follows a zig-zag way between the Fe-Si-O layers. We also show   that diffusion of Li+ ions in the half-lithiated structure follows the   same behavior as in the lithiated structure. As a whole, the activation   energies for the investigated compounds present a greater value compared   with the activation energies in currently used materials such as   LiFePO4. The results were calculated in the framework of density   functional theory in conjunction with the climbing image nudged elastic   band method. The Hubbard term was added to the Kohn-Sham Hamiltonian to   overcome the delocalization problem of d electrons. Furthermore, the   diffusion coefficients were calculated for both structures considering   temperatures ranging from 300 to 700 K. (C) 2013 Elsevier Ltd. All   rights reserved.

  • 49.
    Arjmandi-Tash, Hadi
    et al.
    Leiden Univ, Leiden Inst Chem, Fac Sci, NL-2333 CC Leiden, Netherlands..
    Bellunato, Amedeo
    Leiden Univ, Leiden Inst Chem, Fac Sci, NL-2333 CC Leiden, Netherlands..
    Wen, Chenyu
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Olsthoorn, Rene C.
    Leiden Univ, Leiden Inst Chem, Fac Sci, NL-2333 CC Leiden, Netherlands..
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Zhang, Shi-Li
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Schneider, Gregory F.
    Leiden Univ, Leiden Inst Chem, Fac Sci, NL-2333 CC Leiden, Netherlands..
    Zero-Depth Interfacial Nanopore Capillaries2018In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 30, no 9, article id 1703602Article in journal (Refereed)
    Abstract [en]

    High-fidelity analysis of translocating biomolecules through nanopores demands shortening the nanocapillary length to a minimal value. Existing nanopores and capillaries, however, inherit a finite length from the parent membranes. Here, nanocapillaries of zero depth are formed by dissolving two superimposed and crossing metallic nanorods, molded in polymeric slabs. In an electrolyte, the interface shared by the crossing fluidic channels is mathematically of zero thickness and defines the narrowest constriction in the stream of ions through the nanopore device. This novel architecture provides the possibility to design nanopore fluidic channels, particularly with a robust 3D architecture maintaining the ultimate zero thickness geometry independently of the thickness of the fluidic channels. With orders of magnitude reduced biomolecule translocation speed, and lowered electronic and ionic noise compared to nanopores in 2D materials, the findings establish interfacial nanopores as a scalable platform for realizing nanofluidic systems, capable of single-molecule detection.

  • 50.
    Arnalds, Unnar B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Magnetic Order in Artificial Structures2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The topic of this thesis is the investigation of the magnetic properties of artificially created magnetic structures. Applying different characterization techniques, ranging from direct imaging methods to reciprocal space techniques, the properties of lithographically patterned arrays of magnetic thin film and multilayer elements are investigated by exploring their magnetic state, extending from the atomic scale up to collective ordering phenomena of nano-magnetic elements.

    Laterally patterned amorphous multilayer arrays of combined circular and ellipsoidal islands were investigated. The arrays contain a variety of length scales, ranging from their nanometer scale multilayer structure to their lateral periodicity in the micrometer range. The attributes of these arrays are explored using different techniques, applicable for addressing the magnetization at different length scales, including magneto-optical techniques, micromagnetic simulations and x-ray resonant magnetic scattering.

    Arrays of dipole interacting elongated magnetic elements composed of Pd(Fe) thin films were investigated. Pd(Fe) films have a low Curie temperature which can be tuned by the thickness of the Fe layer embedded in Pd. By this, the interaction and the shape anisotropy energies can be brought down to energy scales comparable to room temperature enabling the possibility of investigating the effect of thermal excitations on such arrays. The temperature dependent magnetization of an artificial square spin ice array was investigated by magneto-optical measurements demonstrating the possibility of observing an order-disorder transition in an artificial square spin ice system. The role of dipolar interactions and the possibility of achieving thermal ground state ordering was then further investigated by magnetically sensitive photoemission electron microscopy imaging of ring arrangements of elongated Pd(Fe) elements. The results reveal a high probability of achieving a thermal ground state ordering of the magnetization of the islands.

    List of papers
    1. Magnetic structure and diffracted magneto-optics of patterned amorphous multilayers
    Open this publication in new window or tab >>Magnetic structure and diffracted magneto-optics of patterned amorphous multilayers
    Show others...
    2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 14, p. 144434-1-144434-8, article id 144434Article in journal (Refereed) Published
    Abstract [en]

    We present magneto-optical Kerr effect measurements of patterned arrays of Co68Fe24Zr8 / Al2O3 amorphous multilayers. The multilayers were patterned in two dimensions into two different arrangements of circular and ellipsoidal islands. Magnetization loops were recorded in a longitudinal geometry using both the specularly reflected beam as well as diffracted beams scattered off the patterned films. The magnetization of the patterned structures is significantly different from the magnetization of a continuous multilayer owing to the lateral confinement of the pattern and the introduction of additional dipolar coupling between the layers at the edges of the islands. By investigating the magnetic response at the different diffraction orders from the two different configurations of islands we are able to observe the magnetization at different length scales and determine the magnetic response of the circular and ellipsoidal islands individually.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-133717 (URN)10.1103/PhysRevB.82.144434 (DOI)000283423000004 ()
    Funder
    Swedish Research CouncilKnut and Alice Wallenberg Foundation
    Available from: 2010-11-18 Created: 2010-11-15 Last updated: 2017-12-12Bibliographically approved
    2. X-ray resonant magnetic scattering from patterned multilayers
    Open this publication in new window or tab >>X-ray resonant magnetic scattering from patterned multilayers
    Show others...
    2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 6, p. 064426-Article in journal (Refereed) Published
    Abstract [en]

    We report on x-ray resonant magnetic scattering from laterally patterned arrays of amorphous Co68Fe24Zr8/Al2O3 multilayers. The arrays are composed of circular and ellipsoidal elements which display distinct individual magnetic responses enabling the investigation of the dependence of the observed magnetization on the scattering condition. We focus our attention to special points in reciprocal space, relating to the lateral and perpendicular structure of the samples, thereby revealing the magnetic structure of the multilayered arrays. This allows a comparison of the observed magnetization under different scattering conditions to magneto-optical measurements. The scattering data are supported by micromagnetic simulations which further enhance our understanding of the intricate charge and magnetic scattering from three dimensional patterns.

    National Category
    Atom and Molecular Physics and Optics
    Research subject
    Physics of Matter
    Identifiers
    urn:nbn:se:uu:diva-172373 (URN)10.1103/PhysRevB.86.064426 (DOI)000307719400003 ()
    Available from: 2012-04-09 Created: 2012-04-09 Last updated: 2017-12-07Bibliographically approved
    3. Melting artificial spin ice
    Open this publication in new window or tab >>Melting artificial spin ice