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Negi, D. S., Datta, R. & Rusz, J. (2019). Defect driven spin state transition and the existence of half-metallicity in CoO. Journal of Physics: Condensed Matter, 31(11), Article ID 115602.
Open this publication in new window or tab >>Defect driven spin state transition and the existence of half-metallicity in CoO
2019 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 11, article id 115602Article in journal (Refereed) Published
Abstract [en]

We unveil the native defect induced high spin to low spin state transition in Co+3 and half- metallicity in CoO. First principles calculations unravel that, defect density holds a key role in dictating the spin-state transition in Co+3 ion in CoO, and introducing the half-metallicity. Charge transfer in the vicinity of vacancy plane favors the stabilization and coexistence of bivalent Co+2 and trivalent Co+3 ion in CoO. We propose that defect engineering could serve as a route to design the half metallicity in transition metal mono-oxides.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
half-metallicty, transition metal oxides, spin state transition
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-376709 (URN)10.1088/1361-648X/aafd11 (DOI)000456850300002 ()30625423 (PubMedID)
Funder
Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyCarl Tryggers foundation Swedish National Infrastructure for Computing (SNIC)
Available from: 2019-02-13 Created: 2019-02-13 Last updated: 2019-02-13Bibliographically approved
Negi, D. S., Spiegelberg, J., Muto, S., Thersleff, T., Ohtsuka, M., Schonstrom, L., . . . Rusz, J. (2019). Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope. Physical Review Letters, 122(3), Article ID 037201.
Open this publication in new window or tab >>Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope
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2019 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 3, article id 037201Article in journal (Refereed) Published
Abstract [en]

We propose a magnetic measurement method utilizing a patterned postsample aperture in a transmission electron microscope. While utilizing electron magnetic circular dichroism, the method circumvents previous needs to shape the electron probe to an electron vortex beam or astigmatic beam. The method can be implemented in standard scanning transmission electron microscopes by replacing the spectrometer entrance aperture with a specially shaped aperture, hereafter called a ventilator aperture. The proposed setup is expected to work across the whole range of beam sizes-from wide parallel beams down to atomic resolution magnetic spectrum imaging.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-376816 (URN)10.1103/PhysRevLett.122.037201 (DOI)000456782500011 ()
Funder
Swedish Research Council, 2016-05113Knut and Alice Wallenberg Foundation, 3DEM-NATUR 2012.0112Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyCarl Tryggers foundation
Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-02-19Bibliographically approved
Negi, D. S., Zeiger, P., Jones, L., Idrobo, J.-C., van Aken, P. A. & Rusz, J. (2019). Prospect for detecting magnetism of a single impurity atom using electron magnetic chiral dichroism. Physical Review B, 100(10), Article ID 104434.
Open this publication in new window or tab >>Prospect for detecting magnetism of a single impurity atom using electron magnetic chiral dichroism
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 10, article id 104434Article in journal (Refereed) Published
Abstract [en]

Dopants, even single atoms, can influence the electrical and magnetic properties of materials. Here we demonstrate the opportunity for detecting the magnetic response of an embedded magnetic impurity in a nonmagnetic host material. We combine a depth sectioning approach with electron magnetic circular dichroism in scanning transmission electron microscopy to compute the depth-resolved magnetic inelastic-scattering cross section of single Co impurity buried in the host crystal of GaAs. Our calculations suggest that the magnetic dichroic signal intensity is sensitive to the depth and lateral position of the electron probe relative to the magnetic impurity. Additionally, a more precise dichroic signal localization can be achieved via choosing higher-collection-angle (beta) apertures. Quantitative evaluation of the inelastic-scattering cross section and signal-to-noise ratio indicates that the magnetic signal from a single Co atom is on the verge of being detectable with today's state-of-the-art instrumentation.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-395834 (URN)10.1103/PhysRevB.100.104434 (DOI)000488252600004 ()
Funder
Swedish Research Council, 2017-04026Swedish National Infrastructure for Computing (SNIC)EU, Horizon 2020, 823717 ESTEEM3Göran Gustafsson Foundation for Research in Natural Sciences and Medicine
Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2019-10-25Bibliographically approved
Edstrom, A., Lubk, A. & Rusz, J. (2019). Quantum mechanical treatment of atomic-resolution differential phase contrast imaging of magnetic materials. Physical Review B, 99(17), Article ID 174428.
Open this publication in new window or tab >>Quantum mechanical treatment of atomic-resolution differential phase contrast imaging of magnetic materials
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 17, article id 174428Article in journal (Refereed) Published
Abstract [en]

Utilizing the Pauli equation based multislice method, introduced in a previous paper [A. Edstrom, A. Lubk, and J. Rusz, Phys. Rev. Lett. 116, 127203 (2016)], we study the atomic-resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center-of-mass pattern in a scanning transmission electron microscopy experiment carries information about both electric and magnetic fields. The momentum transfer remains curl free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-387586 (URN)10.1103/PhysRevB.99.174428 (DOI)000469324500002 ()
Funder
Swedish Research Council
Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-06-26Bibliographically approved
Wang, Z., Tavabi, A. H., Jin, L., Rusz, J., Tyutyunnikov, D., Jiang, H., . . . Zhong, X. (2018). Atomic scale imaging of magnetic circular dichroism by achromatic electron microscopy. Nature Materials, 17(3), 221-225
Open this publication in new window or tab >>Atomic scale imaging of magnetic circular dichroism by achromatic electron microscopy
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2018 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 17, no 3, p. 221-225Article in journal (Refereed) Published
Abstract [en]

In order to obtain a fundamental understanding of the interplay between charge, spin, orbital and lattice degrees of freedom in magnetic materials and to predict and control their physical properties1-3, experimental techniques are required that are capable of accessing local magnetic information with atomic-scale spatial resolution. Here, we show that a combination of electron energy-loss magnetic chiral dichroism(4) and chromatic-aberration-corrected transmission electron microscopy, which reduces the focal spread of inelastically scattered electrons by orders of magnitude when compared with the use of spherical aberration correction alone, can achieve atomic-scale imaging of magnetic circular dichroism and provide element-selective orbital and spin magnetic moments atomic plane by atomic plane. This unique capability, which we demonstrate for Sr2FeMoO6, opens the door to local atomic-level studies of spin configurations in a multitude of materials that exhibit different types of magnetic coupling, thereby contributing to a detailed understanding of the physical origins of magnetic properties of materials at the highest spatial resolution.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350057 (URN)10.1038/s41563-017-0010-4 (DOI)000426012000009 ()29403052 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 320832Swedish Research Council
Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-05-09Bibliographically approved
Spiegelberg, J., Song, D., Dunin-Borkowski, R., Zhu, J. & Rusz, J. (2018). Blind identification of magnetic signals in electron magnetic chiral dichroism using independent component analysis. Ultramicroscopy, 195, 129-135
Open this publication in new window or tab >>Blind identification of magnetic signals in electron magnetic chiral dichroism using independent component analysis
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2018 (English)In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 195, p. 129-135Article in journal (Refereed) Published
Abstract [en]

Electron magnetic chiral dichroism (EMCD) is a promising technique to investigate local magnetic structures in the electron microscope. However, recognition of the EMCD signal, or also finding optimal parameter settings for given materials and sample orientations typically requires extensive simulations to aid the experiment. Here, we discuss how modern data processing techniques, in particular independent component analysis, can be used to identify magnetic signals in an unsupervised manner from energy filtered transmission electron microscopy (EFTEM) images. On the background of the recent advent of 4D scanning transmission electron microscopy, we discuss how this data processing may enable simultaneous tracking of all three spatial components of the magnetic momenta for arbitrary materials and several sample orientations without the previous need of complementary simulations.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-348259 (URN)10.1016/j.ultramic.2018.08.021 (DOI)000450060100016 ()
Funder
Swedish Research CouncilEU, FP7, Seventh Framework Programme, 320832
Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2018-12-19Bibliographically approved
Gang, S.-g., Adam, R., Plötzing, M., von Witzleben, M., Weier, C., Parlak, U., . . . Oppeneer, P. M. (2018). Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation. Physical Review B, 97(6), Article ID 064412.
Open this publication in new window or tab >>Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 6, article id 064412Article in journal (Refereed) Published
Abstract [en]

We studied femtosecond spin dynamics in NixPd1-x magnetic thin films by optically pumping the system with infrared (1.55 eV) laser pulses and subsequently recording the reflectivity of extreme ultraviolet (XUV) pulses synchronized with the pump pulse train. XUV light in the energy range from 20 to 72 eV was produced by laser high-harmonic generation. The reflectivity of XUV radiation at characteristic resonant energies allowed separate detection of the spin dynamics in the elemental subsystems at the M-2,M-3 absorption edges of Ni (68.0 and 66.2 eV) and N-2,N-3 edges of Pd (55.7 and 50.9 eV). The measurements were performed in transversal magneto-optical Kerr effect geometry. In static measurements, we observed a magnetic signature of the Pd subsystem due to an induced magnetization. Calculated magneto-optical asymmetries based on density functional theory show close agreement with the measured results. Femtosecond spin dynamics measured at the Ni absorption edges indicates that increasing the Pd concentration, which causes a decrease in the Curie temperature T-C, results in a drop of the demagnetization time tau(M), contrary to the tau(M) similar to 1/T-C scaling expected for single-species materials. This observation is ascribed to the increase of the Pd-mediated spin-orbit coupling in the alloy.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-349353 (URN)10.1103/PhysRevB.97.064412 (DOI)000425492100005 ()
Funder
German Research Foundation (DFG), SCHN 353/17-1Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2015.0060Swedish National Infrastructure for Computing (SNIC)
Available from: 2018-05-02 Created: 2018-05-02 Last updated: 2018-05-02Bibliographically approved
Cedervall, J., Nonnet, E., Hedlund, D., Häggström, L., Ericsson, T., Werwinski, M., . . . Sahlberg, M. (2018). Influence of cobalt substitution on the magnetic properties of Fe5PB2. Inorganic Chemistry, 57(2), 777-784
Open this publication in new window or tab >>Influence of cobalt substitution on the magnetic properties of Fe5PB2
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2018 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 57, no 2, p. 777-784Article in journal (Refereed) Published
Abstract [en]

In this study the effects of cobalt substitutions in Fe5PB2 have been studied. An increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. At high cobalt concentrations, cobalt prefers to order at the M(2) position in the crystal structure. A tunable Curie transition like this shows some prerequisites for magnetic cooling applications.

The substitutional effects of cobalt in (Fe1–xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1–xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.

National Category
Inorganic Chemistry Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-331758 (URN)10.1021/acs.inorgchem.7b02663 (DOI)000422810900030 ()29298054 (PubMedID)
Funder
Swedish Research CouncilSwedish Energy AgencyEuropean Regional Development Fund (ERDF)
Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2018-10-19Bibliographically approved
Spiegelberg, J., Idrobo, J. C., Herklotz, A., Ward, T. Z., Zhou, W. & Rusz, J. (2018). Local low rank denoising for enhanced atomic resolution imaging. Ultramicroscopy, 187, 34-42
Open this publication in new window or tab >>Local low rank denoising for enhanced atomic resolution imaging
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2018 (English)In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 187, p. 34-42Article in journal (Refereed) Published
Abstract [en]

Atomic resolution imaging and spectroscopy suffers from inherently low signal to noise ratios often prohibiting the interpretation of single pixels or spectra. We introduce local low rank (LLR) denoising as tool for efficient noise removal in scanning transmission electron microscopy (STEM) images and electron energy-loss (EEL) spectrum images. LLR denoising utilizes tensor decomposition techniques, in particular the multilinear singular value decomposition (MLSVD), to achieve a denoising in a general setting largely independent of the signal features and data dimension, by assuming that the signal of interest is of low rank in segments of appropriately chosen size. When applied to STEM images of graphene, LLR denoising suppresses statistical noise while retaining fine image features such as scan row-wise distortions, possibly related to rippling of the graphene sheet and consequent motion of atoms. When applied to EEL spectra, LLR denoising reveals fine structures distinguishing different lattice sites in the spinel system CoFe2O4.

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-348246 (URN)10.1016/j.ultramic.2018.01.012 (DOI)000428131200005 ()
Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2018-06-04Bibliographically approved
Rodrigues, D. C. M., B Klautau, A., Edström, A., Rusz, J., Nordström, L., Pereiro, M., . . . Eriksson, O. (2018). Magnetic anisotropy in permalloy: hidden quantum mechanical features. Physical Review B, 97(22), Article ID 224402.
Open this publication in new window or tab >>Magnetic anisotropy in permalloy: hidden quantum mechanical features
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 22, article id 224402Article in journal (Refereed) Published
Abstract [en]

By means of relativistic, first principles calculations, we investigate the microscopic origin of the vanishingly low magnetic anisotropy of Permalloy, here proposed to be intrinsically related to the local symmetries of the alloy. It is shown that the local magnetic anisotropy of individual atoms in Permalloy can be several orders of magnitude larger than that of the bulk sample and 5–10 times larger than that of elemental Fe or Ni. We furthermore show that locally there are several easy axis directions that are favored, depending on local composition. The results are discussed in the context of perturbation theory, applying the relation between magnetic anisotropy and orbital moment. Permalloy keeps its pronounced soft ferromagnetic nature due to the exchange energy to be larger than the magnetocrystalline anisotropy. Our results shine light on the magnetic anisotropy of permalloy and of magnetic materials in general, and in addition enhance the understanding of pump-probe measurements and ultrafast magnetization dynamics.

Keywords
Permalloy, Orbital Anisotropy, Anisotropy Energy
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-319925 (URN)10.1103/PhysRevB.97.224402 (DOI)000434015300004 ()
Funder
Knut and Alice Wallenberg Foundation, 2012.0031Knut and Alice Wallenberg Foundation, 2013.0020Swedish Research CouncileSSENCE - An eScience CollaborationStandUp
Available from: 2017-04-11 Created: 2017-04-11 Last updated: 2018-09-14Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-0074-1349

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