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  • 301.
    Cheruku, Pradeep
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
    Asymmetric Hydrogenations: Syntheses of Ligands and Expansion of Substrate Scope2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Asymmetric hydrogenation has emerged as a versatile methodology to obtain a wide range of chiral precursors. This thesis focused on the synthesis of new chiral ligands and the expansion of the substrate scope of asymmetric hydrogenations. Paper I described the synthesis and evaluation of N,P-ligands for the Ir-catalyzed hydrogenations of unfunctionalized olefins. The substrate scope of Ir-catalyzed asymmetric hydrogenations is limited to a narrow range of “test” olefins. The foremost focus of this thesis was to expand the substrate scope of Ir-catalyzed asymmetric hydrogenations. Papers II and III disclosed the potential of the N,P-ligated Ir complexes in hydrogenation of the enol phosphinates. This substrate class is attractive because the hydrogenated products are chiral alkylphosphinates that can be transformed into chiral alcohols and chiral phosphines without sacrificing enantiopurity. A wide range of enol phosphinates were hydrogenated to excellent conversions and enatioselectivities. The hydrogenation of purely alkyl-substituted enol phosphinates in very high conversions and ee values was emphasized in these studies. Paper IV described the investigation of unfunctionalized enamines as substrates in Ir-catalyzed hydrogenation studies. The hydrogenation results and structural limitations of the substrates are presented. Paper V described the asymmetric hydrogenation of diphenylvinylphosphine oxides, di- and trisubstituted vinyl phosphonates. The hydrogenation of diphenylvinylphosphine oxides gives direct access to protected chiral phosphines. The hydrogenated products of vinylphosphonates are highly synthetically useful in pharmaceutical and material chemistry. Hydrogenation of E/Z mixtures of carboxyethyl vinylphosphonates with perfect enantioselectivities was striking in these studies. In paper VI, we have reported the development of a new, highly enantioselective synthetic route to building blocks with CF3 at the chiral center. Several functionalized and unfunctionalized CF3-substituted olefins were hydrogenated with varied degree of success. This methedilogy is useful in the formation of chiral fluorine-containing molecules for a wide range of applications. Paper VII described the hydrogenation of imines using the phosphine-free Cp*Ru/diamine complexes. Chiral version of this reaction was also examined. Despite the modest results, this is the first study to use phosphine-free Cp*Ru/diamine complexes as catalysts for the reduction of C=N double bonds.

    List of papers
    1. Development of new thiazole-based iridium catalysts and their applications in the asymmetric hydrogenation of trisubstituted olefins
    Open this publication in new window or tab >>Development of new thiazole-based iridium catalysts and their applications in the asymmetric hydrogenation of trisubstituted olefins
    2008 (English)In: Organic and biomolecular chemistry, ISSN 1477-0520, E-ISSN 1477-0539, Vol. 6, no 2, p. 366-373Article in journal (Refereed) Published
    Abstract [en]

    New thiazole-based chiral N,P-ligands that are open-chain analogues of known cyclic thiazole ligands have been synthesized and evaluated in the iridium-catalyzed asymmetric hydrogenation of trisubstituted olefins. Chirality was introduced into the ligands through a highly diastereoselective alkylation using Oppolzer's camphorsultam as chiral auxiliary. In general, the new catalysts are as reactive and selective as their cyclic counterparts for the asymmetric hydrogenation of various trisubstituted olefins.

    National Category
    Organic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-97720 (URN)10.1039/b714744a (DOI)000252106700020 ()
    Available from: 2008-11-11 Created: 2008-11-11 Last updated: 2017-12-14Bibliographically approved
    2. Asymmetric Hydrogenation of Enol Phosphinates by Iridium Catalysts Having N, P Ligands
    Open this publication in new window or tab >>Asymmetric Hydrogenation of Enol Phosphinates by Iridium Catalysts Having N, P Ligands
    2007 (English)In: Organic Letters, ISSN 1523-7060, E-ISSN 1523-7052, Vol. 9, no 9, p. 1659-1661Article in journal (Refereed) Published
    Abstract [en]

    Enol phosphinates, which are structural analogues of enol acetates, have for the first time been employed as substrates for Ir-catalyzed asymmetric hydrogenation. A number of enol phosphinates have been synthesized and reduced successfully with up to and above 99% ee.

    National Category
    Chemical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97721 (URN)10.1021/ol070325l (DOI)
    Available from: 2008-11-11 Created: 2008-11-11 Last updated: 2017-12-14Bibliographically approved
    3. Asymmetric Hydrogenation of Di and Trisubstituted Enol Phosphinates with N,P-Ligated Iridium Complexes
    Open this publication in new window or tab >>Asymmetric Hydrogenation of Di and Trisubstituted Enol Phosphinates with N,P-Ligated Iridium Complexes
    2008 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 130, no 16, p. 5595-5599Article in journal (Refereed) Published
    Abstract [en]

    The iridium-catalyzed asymmetric hydrogenation of various di- and trisubstituted enol phosphinates has been studied. Excellent enantioselectivities (up to >99% ee) and full conversion were observed for a range of substrates with both aromatic and aliphatic side chains. Enol phosphinates are structural analogues of enol acetates, and the hydrogenated alkyl phosphinate products can easily be transformed into the corresponding alcohols with conservation of stereochemistry. We have also hydrogenated, in excellent ee, several purely alkyl-substituted enol phosphinates, producing chiral alcohols that are difficult to obtain highly enantioselectively from ketone hydrogenations.

    National Category
    Chemical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97722 (URN)10.1021/ja711372c (DOI)000255041400050 ()
    Available from: 2008-11-11 Created: 2008-11-11 Last updated: 2017-12-14Bibliographically approved
    4. Access to Chiral Tertiary Amines via Iridium Catalyzed Asymmetric Hydrogenation Of Enamines
    Open this publication in new window or tab >>Access to Chiral Tertiary Amines via Iridium Catalyzed Asymmetric Hydrogenation Of Enamines
    Show others...
    2008 (English)In: Tetrahedron Letters, Vol. 49, no 51, p. 7290-7293Article in journal (Refereed) Published
    Abstract [en]

    The asymmetric hydrogenation of N,N-dialkyl and N-alkyl-N-aryl enaminesto chiral tertiary amines was studied. All the N,P-ligated iridium complexes investigated were active catalysts for the reaction, but only those with bicycle-supported oxazoline-phosphine ligands gave reasonable stereoinduction. The best catalyst produced a range of chiral tertiary amines in up to 87% ee.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-97723 (URN)10.1016/j.tetlet.2008.10.035 (DOI)000261532000015 ()
    Available from: 2008-11-11 Created: 2008-11-11 Last updated: 2013-03-01Bibliographically approved
    5. Iridium-N,P-ligand-catalyzed enantioselective hydrogenation of diphenylvinylphosphine oxides and vinylphosphonates
    Open this publication in new window or tab >>Iridium-N,P-ligand-catalyzed enantioselective hydrogenation of diphenylvinylphosphine oxides and vinylphosphonates
    2009 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 131, no 23, p. 8285-8289Article in journal (Refereed) Published
    Abstract [en]

    Diphenylvinylphosphine oxides and di- and trisubstituted vinylphosphonates have been employed as substrates in iridium-catalyzed asymmetric hydrogenations. Complete conversions and excellent enantioselectivities (up to and above 99% ee) were observed for a range of substrates with both aromatic and aliphatic groups at the prochiral carbon. We have also hydrogenated electron-deficient carboxyethylvinylphosphonates with excellent stereoselectivity (up to and above 99% ee). The hydrogenated products of both classes of substrates are synthetically useful intermediates.

    National Category
    Chemical Sciences
    Identifiers
    urn:nbn:se:uu:diva-121865 (URN)10.1021/ja901437t (DOI)000267623100057 ()19462955 (PubMedID)
    Available from: 2010-03-31 Created: 2010-03-31 Last updated: 2017-12-12Bibliographically approved
    6. Highly Selective Iridium-Catalyzed Asymmetric Hydrogenation of Trifluoromethyl Olefins: A New Route to Trifluoromethyl- Bearing Stereocenters
    Open this publication in new window or tab >>Highly Selective Iridium-Catalyzed Asymmetric Hydrogenation of Trifluoromethyl Olefins: A New Route to Trifluoromethyl- Bearing Stereocenters
    Show others...
    2009 (English)In: Advanced Synthesis and Catalysis, ISSN 1615-4150, E-ISSN 1615-4169, Vol. 351, no 3, p. 375-378Article in journal (Refereed) Published
    Abstract [en]

    Fluorine-containing compounds are useful in many applications ranging from pharmaceuticals to ferroelectric crystals. We have developed a new, highly enantioselective synthetic route to trifluoromethyl-bearing stereocenters in up to 96% ee via asymmetric hydrogenation using N,P-ligated iridium catalysts. We also hydrogenated an isomeric mixture of olefins; this reaction gave the hydrogenation product highly enantioselectively (87% ee), and only the E isomer was present after the reaction had reached 56% conversion.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-97725 (URN)10.1002/adsc.200800645 (DOI)
    Available from: 2008-11-11 Created: 2008-11-11 Last updated: 2017-12-14Bibliographically approved
    7. Chemistry – An Asian Journal
    Open this publication in new window or tab >>Chemistry – An Asian Journal
    2008 In: Phosphine-Free RuCp*-Diamine Catalyzed Hydrogenation of Imi-nes, Vol. 3, p. 1390-1394Article in journal (Refereed) Published
    Identifiers
    urn:nbn:se:uu:diva-97726 (URN)
    Available from: 2008-11-11 Created: 2008-11-11Bibliographically approved
  • 302.
    Chico Carpio, Jonathan Philippe
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Magnetization dynamics on the nanoscale: From first principles to atomistic spin dynamics2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis first-principles methods, based on density functional theory, have been used to characterize a wide range of magnetic materials. Special emphasis has been put on pairwise magnetic interactions, such as Heisenberg exchange and Dzyaloshinskii-Moriya interactions, and also on in the Gilbert damping parameter. These parameters play a crucial role in determining the magnetization dynamics of the considered materials.

    Magnetic interaction parameters, has been calculated for several materials based on Co/Ni/Co heterostructures deposited on non-magnetic heavy metals where. The aim was to clarify how the composition of the underlayers affect the magnetic properties, in particular the Dzyaloshinskii-Moriya interactions. The DMI was found to be strongly dependent on the material of the underlayer, which is consistent with previous theoretical works. Such behaviour can be traced back to the change of the spin-orbit coupling with the material of the underlayer, as well as with the hybridization of the d- states of the magnetic system with the d- state of the non-magnetic substrate.

    First-principles calculations of the Gilbert damping parameter has been performed for several magnetic materials. Among them the full Heusler families, Co2FeZ, Co2MnZ with Z=(Al, Si, Ga, Ge). It was found that the first-principles methods, reproduce quite well the experimental trends, even though the obtained values are consistently smaller than the experimental measurements. A clear correlation between the Gilbert damping and the density of states at the Fermi energy was found, which is in agreement with previous works. In general as the density of states at the Fermi energy decreases, the damping decreases also.

    The parameters from first principles methods, have been used in conjunction with atomistic spin dynamics simulations, in order to study ultra-narrow domain walls. The domain wall motion of a monolayer of Fe on W(110) has been studied for a situation when the domain wall is driven via thermally generated spin waves from a thermal gradient. It was found that the ultra-narrow domain walls have an unexpected behaviour compared to wide domain walls in the continuum limit. This behaviour have been explained by the fact that for ultra-narrow domain walls the reflection of spin waves is not negligible.

    Furthermore, the dynamics of topologically protected structures, such as topological excitations in a kagome lattice and edge dislocations in FeGe has been studied. For the FeGe case, the description of the thermally driven dynamics of the edge dislocations, was found to be a possible explanation for the experimentally observed time dependence of the spiral wavelength. In the kagome lattice, it was also found that due to its topological properties, topological excitations can be created in it.

    List of papers
    1. Thermally driven domain-wall motion in Fe on W(110)
    Open this publication in new window or tab >>Thermally driven domain-wall motion in Fe on W(110)
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    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 1, p. 014434-Article in journal (Refereed) Published
    Abstract [en]

    It has recently been shown that domain walls (DWs) in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(110) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting this system to thermal gradients we observe a temperature-dependent movement of the domain wall. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. Our material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy, and the exchange interaction and shows that the larger DW velocities are found for materials with broader domain-wall width. The relatively slow DW motion of the Fe/W(110) system is hence primarily caused by its narrow domain-wall width, which results from its large magnetic anisotropy and soft exchange.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-231296 (URN)10.1103/PhysRevB.90.014434 (DOI)000339990000005 ()
    Available from: 2014-09-08 Created: 2014-09-07 Last updated: 2017-12-05
    2. Electronic structure and magnetic properties of L1(0) binary alloys
    Open this publication in new window or tab >>Electronic structure and magnetic properties of L1(0) binary alloys
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    2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 1, p. 014402-Article in journal (Refereed) Published
    Abstract [en]

    We present a systematic study of the magnetic properties of L1(0) binary alloys FeNi, CoNi, MnAl, and MnGa via two different density functional theory approaches. Our calculations show large magnetocrystalline anisotropies in the order 1 MJ/m(3) or higher for CoNi, MnAl, and MnGa, while FeNi shows a somewhat lower value in the range 0.48-0.77 MJ/m(3). Saturation magnetization values of 1.3 MA/m, 1.0 MA/m, 0.8 MA/m, and 0.9 MA/m are obtained for FeNi, CoNi, MnAl, and MnGa, respectively. Curie temperatures are evaluated via Monte Carlo simulations and show T-C = 916 K and T-C = 1130 K for FeNi and CoNi, respectively. For Mn-based compounds Mn-rich off-stoichiometric compositions are found to be important for the stability of a ferro- or ferrimagnetic ground state with T-C greater than 600 K. The effect of substitutional disorder is studied and found to decrease both magnetocrystalline anisotropies and Curie temperatures in FeNi and CoNi.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-229718 (URN)10.1103/PhysRevB.90.014402 (DOI)000338649700003 ()
    Available from: 2014-08-18 Created: 2014-08-12 Last updated: 2017-12-05
    3. Topological excitations in a kagome magnet
    Open this publication in new window or tab >>Topological excitations in a kagome magnet
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    2014 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 4815-Article in journal (Refereed) Published
    Abstract [en]

    Chirality-that is, left or right handedness-is present in many scientific areas, and particularly in condensed matter physics. Inversion symmetry breaking relates chirality with skyrmions, which are protected field configurations with particle-like and topological properties. Here we show that a kagome magnet, with Heisenberg and Dzyaloshinskii-Moriya interactions, causes non-trivial topological and chiral magnetic properties. We also find that under special circumstances, skyrmions emerge as excitations, having stability even at room temperature. Chiral magnonic edge states of a kagome magnet offer, in addition, a promising way to create, control and manipulate skyrmions. This has potential for applications in spintronics, that is, for information storage or as logic devices. Collisions between these particle-like excitations are found to be elastic at very low temperature in the skyrmion-skyrmion channel, albeit without mass-conservation. Skyrmion-antiskyrmion collisions are found to be more complex, where annihilation and creation of these objects have a distinct non-local nature.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-236091 (URN)10.1038/ncomms5815 (DOI)000342929000005 ()
    Available from: 2014-11-14 Created: 2014-11-12 Last updated: 2017-12-05
    4. Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films
    Open this publication in new window or tab >>Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films
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    2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 21, article id 214424Article in journal (Refereed) Published
    Abstract [en]

    The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization M-S, Gilbert damping alpha, and exchange stiffness A. M-S and A are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for M-S and alpha. The calculated A shows the same trend as the experimental data but has a larger magnitude. In addition to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-274280 (URN)10.1103/PhysRevB.92.214424 (DOI)000366499500004 ()
    Funder
    Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologySwedish Research CouncilSwedish Energy AgencyKnut and Alice Wallenberg FoundationCarl Tryggers foundation
    Available from: 2016-02-25 Created: 2016-01-20 Last updated: 2017-11-30
    5. First principle studies of Co/Ni/Co heterostructures on heavy metal substrates
    Open this publication in new window or tab >>First principle studies of Co/Ni/Co heterostructures on heavy metal substrates
    (English)Manuscript (preprint) (Other (popular science, discussion, etc.))
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-287426 (URN)
    Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-06-15
    6. Helical spiral states in the MnZSn half-Heuslers with Z=(Tc, Ru, Rh, Os, Ir, Pt).A first principles study
    Open this publication in new window or tab >>Helical spiral states in the MnZSn half-Heuslers with Z=(Tc, Ru, Rh, Os, Ir, Pt).A first principles study
    (English)Manuscript (preprint) (Other (popular science, discussion, etc.))
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-287424 (URN)
    Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-06-15
    7. Relativistic effects in domain wall dynamics in magnetic heterostrcutures
    Open this publication in new window or tab >>Relativistic effects in domain wall dynamics in magnetic heterostrcutures
    (English)Manuscript (preprint) (Other (popular science, discussion, etc.))
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-287428 (URN)
    Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-06-15
  • 303.
    Chimata, Raghuveer
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Magnetization dynamics of complex magnetic materials by atomistic spin dynamics simulations2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present.

    In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials.

    The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. The very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials.

    List of papers
    1. Microscopic Model for Ultrafast Remagnetization Dynamics
    Open this publication in new window or tab >>Microscopic Model for Ultrafast Remagnetization Dynamics
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    2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 15, p. 157201-Article in journal (Refereed) Published
    Abstract [en]

    In this Letter, we provide a microscopic model for the ultrafast remagnetization of atomic moments already quenched above the Stoner-Curie temperature by a strong laser fluence. Combining first-principles density functional theory, atomistic spin dynamics utilizing the Landau-Lifshitz-Gilbert equation, and a three-temperature model, we analyze the temporal evolution of atomic moments as well as the macroscopic magnetization of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to picoseconds. Our simulations show a variety of complex temporal behavior of the magnetic properties resulting from an interplay between electron, spin, and lattice subsystems, which causes an intricate time evolution of the atomic moment, where longitudinal and transversal fluctuations result in a macrospin moment that evolves highly nonmonotonically.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-185192 (URN)10.1103/PhysRevLett.109.157201 (DOI)000309590300042 ()
    Available from: 2012-11-22 Created: 2012-11-21 Last updated: 2017-12-07Bibliographically approved
    2. All-thermal switching of amorphous Gd-Fe alloys: Analysis of structural properties and magnetization dynamics
    Open this publication in new window or tab >>All-thermal switching of amorphous Gd-Fe alloys: Analysis of structural properties and magnetization dynamics
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    2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 9, article id 094411Article in journal (Refereed) Published
    Abstract [en]

    In recent years there has been an intense interest in understanding the microscopic mechanism of thermally induced magnetization switching driven by a femtosecond laser pulse. Most of the effort has been dedicated to periodic crystalline structures while the amorphous counterparts have been less studied. By using a multiscale approach, i.e., first-principles density functional theory combined with atomistic spin dynamics, we report here on the very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale level. Both structural and dynamical properties of Gd-Fe alloys reported in this work are in good agreement with previous experiments. We calculated the dynamic behavior of homogeneous and inhomogeneous amorphous Gd-Fe alloys and their response under the influence of a femtosecond laser pulse. In the homogeneous sample, the Fe sublattice switches its magnetization before the Gd one. However, the temporal sequence of the switching of the two sublattices is reversed in the inhomogeneous sample. We propose a possible explanation based on a mechanism driven by a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration, modeled by an antiferromagnetic second-neighbor exchange interaction between Gd atoms in the Gd-rich region. We also report on the influence of laser fluence and damping effects in the all-thermal switching.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-263429 (URN)10.1103/PhysRevB.92.094411 (DOI)000360884700002 ()
    Available from: 2015-10-07 Created: 2015-09-30 Last updated: 2017-12-01Bibliographically approved
    3. Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
    Open this publication in new window or tab >>Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
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    2017 (English)In: Physical review B, ISSN 2469-9950, Vol. 95, no 21, article id 214417Article in journal (Refereed) Published
    Abstract [en]

    Temperature-dependent magnetic experiments such as pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g., as reflected by the temperature dependence of the exchange interactions. In this paper, we demonstrate from first-principles theory that the magnetic excitation spectra in Co in fcc, bcc, and hcp structures are nearly identical in a wide range of noncollinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration-independent effective model. We have used such a Hamiltonian, together with ab initio calculated damping parameters, to investigate the magnon dispersion relationship as well as ultrafast magnetization dynamics of Co and Co-rich CoMn alloys.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-308321 (URN)10.1103/PhysRevB.95.214417 (DOI)000404015400003 ()
    Funder
    Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031StandUp
    Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2018-04-07Bibliographically approved
    4. Ultrafast magnetization dynamics in pure and doped Heusler alloys
    Open this publication in new window or tab >>Ultrafast magnetization dynamics in pure and doped Heusler alloys
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    (English)Manuscript (preprint) (Other academic)
    National Category
    Condensed Matter Physics
    Research subject
    Physics with spec. in Atomic, Molecular and Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-308103 (URN)
    Available from: 2016-11-24 Created: 2016-11-23 Last updated: 2016-11-24
    5. Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe
    Open this publication in new window or tab >>Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe
    2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 22, article id 224417Article in journal (Refereed) Published
    Abstract [en]

    In this paper, we present a systematic study of the effects of Fe doping on the electronic and magnetic structures of spinel CoCr2O4 by ab initio density functional theory and atomistic spin dynamics calculations. Our calculated magnetic structure for pristine CoCr2O4 correctly reproduces the experimental one with a q-vector of (0.67, 0.67,0.0), establishing the accuracy of the calculated interatomic exchange interactions. We show that the noncollinear spin structure with a nonzero q-vector in the spinel structure is driven towards collinearity by Fe doping by a complex interplay between interatomic exchange interactions. In the inverse spinel structure with 100% Fe doping, a collinear antiferromagnetic order develops along with a half-metallic electronic structure, which evolves due to the chemical disorder between Fe and Co in the B sites described by the coherent potential approximation. This is a comprehensive theoretical study to understand the evolution of magnetic and electronic properties of multiferroic CoCr2O4 doped with Fe.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-274283 (URN)10.1103/PhysRevB.92.224417 (DOI)000366500100006 ()
    Funder
    Carl Tryggers foundation Swedish Research CouncilEU, European Research CouncilSwedish National Infrastructure for Computing (SNIC)
    Available from: 2016-02-03 Created: 2016-01-20 Last updated: 2017-11-30Bibliographically approved
    6. Temperature dependence of dielectric behavior in Ca3CoMnO6: insights from density functional theory and atomistic spin dynamics simulations
    Open this publication in new window or tab >>Temperature dependence of dielectric behavior in Ca3CoMnO6: insights from density functional theory and atomistic spin dynamics simulations
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    (English)Manuscript (preprint) (Other academic)
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-308302 (URN)
    Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2016-11-24
  • 304.
    Chmiel, Hannah Elisa
    Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
    The role of sediments in the carbon cycle of boreal lakes2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Inland waters are active sites of carbon (C) processing and emitters of carbon dioxide (CO2) and methane (CH4) to the atmosphere. In the boreal zone, where surface waters receive large quantities of organic carbon (OC) from surrounding forests and wetlands, lakes and streams act as strong sources of these greenhouse gases. Lake sediments provide the only long-term sink of C in boreal inland waters, through burial of OC. However, mineralization of OC counteracts the efficiency of lake sediments in removing C from the short-term C cycle. In this context, this thesis provides a better insight into the dual role of boreal lake sediments as C source and C sink.

    The presented work is based on empirical assessments of OC burial and OC mineralization rates in boreal lakes. The temporal variability of OC burial and the stability of the buried OC was assessed on both centennial and millennial timescales. The quantitative importance of sediment OC burial and mineralization in comparison both to other C fluxes within the lake, and to C fluxes within the tributary stream network, was quantified. By simulating the effect of climate change on water temperature, we also gauged the potential future efficiency of lake sediments in storing C.

    The results demonstrate that OC mineralization in sediments dominates three-fold over OC burial when observed at a whole-basin and annual scale. The contribution of sediment OC mineralization to annual C emission from the assessed study lake was, however, found to be small (16%), when compared to OC mineralization in the water column (37%) and catchment import of C (47%). Furthermore, C emission from headwater streams was found to dominate greatly over the lake C emission, mainly triggered by the higher gas transfer velocity of streams compared to lakes.

    On a long-term (Holocene) scale, the continuous OC burial flux results in a large amount of C stored in sediments. The temporal variability of this OC accumulation was found to vary across lakes, with, however, time-dependent patterns: On a millennial scale, smaller lakes exhibited a higher variability than larger lakes of the study area. For the last century, similar variability and a trend to increased OC accumulation was found for most study lakes, irrespective of their size. Analysis of lignin phenols in the accumulated OC did not indicated post-depositional degradation, independent of the age of the sediment OC, implying that sediments are a very stable sink for land-derived OC in boreal lakes.

    Simulation of warming water temperatures in boreal lakes resulted in declines of the OC burial efficiency BE (OCBE; OC burial/OCdeposition) up to 16%, depending, however, on basin morphometry. Predicted declines in OCBE were higher for the more shallow lake compared to the deeper lake.

    In conclusion, this thesis illustrates that sediments play, despite a small quantitative impact on aquatic C cycling, an important role as a very stable C sink in boreal lakes. However, the efficiency of this C sink is likely to be reduced in the future.

    List of papers
    1. Uncoupled organic matter burial and quality in boreal lake sediments over the Holocene
    Open this publication in new window or tab >>Uncoupled organic matter burial and quality in boreal lake sediments over the Holocene
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    2015 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 120, no 9, p. 1751-1763Article in journal (Refereed) Published
    Abstract [en]

    Boreal lake sediments are important sites of organic carbon (OC) storage, which have accumulated substantial amounts of OC over the Holocene epoch; the temporal evolution and the strength of this Holocene carbon (C) sink is, however, not well constrained. In this study we investigated the temporal record of carbon mass accumulation rates (CMARs), and assessed qualitative changes of terrestrially derived OC in the sediment profiles of seven Swedish boreal lakes, in order to evaluate the variability of boreal lake sediments as a C sink over time.

    CMARs were resolved on a short-term (centennial) and long-term (i.e. over millennia of the Holocene) time scale, using radioactive lead (210 Pb) and carbon (14C) isotope dating. Sources and degradation state of terrestrially derived OC were identified and characterized by molecular analyses of lignin phenols.

    We found that CMARs varied substantially on both short-term and long-term scales, and that the variability was mostly attributed to sedimentation rates and uncoupled from the OC content in the sediment profiles. The lignin phenol analyses revealed that woody material from gymnosperms was a dominant and constant OC source to the sediments over the Holocene. Furthermore, lignin-based degradation indices, such as acid-to-aldehyde ratios, indicated that post-depositional degradation in the sediments was very limited on longer time scales, implying that terrestrial OC is stabilized in the sediments on a permanent basis.

    Keywords
    organic carbon, accumulation rates, radiocarbon dating, lead dating, lignin phenols, carbon sink
    National Category
    Environmental Sciences
    Research subject
    Biology with specialization in Limnology
    Identifiers
    urn:nbn:se:uu:diva-260660 (URN)10.1002/2015JG002987 (DOI)000363332900004 ()
    Funder
    Swedish Research Council Formas
    Available from: 2015-09-07 Created: 2015-08-21 Last updated: 2018-05-21Bibliographically approved
    2. The role of sediments in the carbon budget of a small boreal lake
    Open this publication in new window or tab >>The role of sediments in the carbon budget of a small boreal lake
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    2016 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 61, no 5, p. 1814-1825Article in journal (Refereed) Published
    Abstract [en]

    We investigated the role of lake sediments as carbon (C) source and sink in the annual C budget of a small (0.07 km2), shallow (mean depth 3.4 m), and humic lake (mean DOC concentration 17 mg L-1) in boreal Sweden. Organic carbon (OC) burial and mineralization in sediments were quantified from 210Pb-dated sediment and laboratory sediment incubation experiments, respectively, and upscaled to the entire basin and to one whole year, by using sediment thickness derived sub-bottom profiling, basin morphometry, and water column monitoring data of temperature and oxygen concentration. Furthermore, catchment C import, open water metabolism, photochemical mineralization as well as carbon dioxide (CO2) and methane (CH4) emissions to the atmosphere, were quantified to relate sediment processes to other lake C fluxes. We found that on a whole-basin and annual scale, sediment OC mineralization was three times larger than OC burial, and contributed about 16% to the annual CO2 emission from the lake to the atmosphere. Remaining contributions to the CO2 emission were attributed to water column metabolism (31%), photochemical mineralization (6%), and catchment imports via inlet streams and inflow of shallow groundwater (47%). We conclude that on an annual and whole-basin scale 1) sediment OC mineralization dominated over OC burial, 2) water column OC mineralization contributed more than sediments to lake CO2 emission, and 3) catchment import of C to the lake was greater than lake-internal C cycling. 

    Keywords
    mineralization, burial, carbon dioxide, methane, emission, photomineralization
    National Category
    Environmental Sciences
    Research subject
    Biology with specialization in Limnology
    Identifiers
    urn:nbn:se:uu:diva-260666 (URN)10.1002/lno.10336 (DOI)000383621800019 ()
    Funder
    EU, European Research CouncilSwedish Research Council FormasSwedish Research Council
    Available from: 2015-09-07 Created: 2015-08-21 Last updated: 2018-05-21Bibliographically approved
    3. Carbon dioxide evasion from headwater systems strongly contributes to the total export of carbon from a small boreal lake catchment
    Open this publication in new window or tab >>Carbon dioxide evasion from headwater systems strongly contributes to the total export of carbon from a small boreal lake catchment
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    2015 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 120, no 1, p. 13-28Article in journal (Refereed) Published
    Abstract [en]

    Inland waters are hotspots for carbon (C) cycling and therefore important for landscape C budgets. Small streams and lakes are particularly important; however, quantifying C fluxes is difficult and has rarely been done for the entire aquatic continuum, composed of connected streams and lakes within the same catchment. We investigated carbon dioxide (CO2) evasion and fluvial fluxes of dissolved inorganic carbon and dissolved organic carbon (DIC and DOC) in stream and lake systems within the 2.3km(2) catchment of a small boreal lake. Our results show pronounced spatial and temporal variability in C fluxes even at a small spatial scale. C loss from the catchment through CO2 evasion from headwaters for the total open water-sampling period was 9.7g C m(-2) catchment, dominating the total catchment C loss (including CO2 evasion, DIC, and DOC export from the lake, which were 2.7, 0.2, and 5.2g C m(-2) catchment, respectively). Aquatic CO2 evasion was dominated by headwater streams that occupy similar to 0.1% of the catchment but contributed 65% to the total aquatic CO2 evasion from the catchment. The importance of streams was mainly an effect of the higher gas transfer velocities than compared to lakes (median, 67 and 2.2cmh(-1), respectively). Accurately estimating the contribution of C fluxes from headwater streams, particularly the temporal and spatial dynamics in their gas transfer velocity, is key to landscape-scale C budgets. This study demonstrates that CO2 evasion from headwaters can be the major pathway of C loss from boreal catchments, even at a small spatial scale.

    National Category
    Climate Research
    Identifiers
    urn:nbn:se:uu:diva-241585 (URN)10.1002/2014JG002706 (DOI)000349899200002 ()
    Available from: 2015-01-13 Created: 2015-01-13 Last updated: 2018-05-21Bibliographically approved
    4. Decreased efficiency of sediment carbon burial in boreal lakes at warming lake water temperatures
    Open this publication in new window or tab >>Decreased efficiency of sediment carbon burial in boreal lakes at warming lake water temperatures