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Publications (10 of 14) Show all publications
Huttmann, F., Rothenbach, N., Kraus, S., Ollefs, K., Arruda, L. M., Bernien, M., . . . Wende, H. (2019). Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator. Journal of Physical Chemistry Letters, 10(5), 911-917
Open this publication in new window or tab >>Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator
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2019 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 5, p. 911-917Article in journal (Refereed) Published
Abstract [en]

We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M-5,M-4 edge XMCD, orbital and spin magnetic moments of (-0.1 +/- 0.3)mu(B) and (+7.0 +/- 0.6)mu(B), respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M-5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-380459 (URN)10.1021/acs.jpclett.8b03711 (DOI)000461271700003 ()30717591 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020Knut and Alice Wallenberg Foundation, 2012.0031Swedish Foundation for Strategic Research
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-03-28Bibliographically approved
Hellsvik, J., Thonig, D., Modin, K., Iusan, D., Bergman, A., Eriksson, O., . . . Delin, A. (2019). General method for atomistic spin-lattice dynamics with first-principles accuracy. Physical Review B, 99(10), Article ID 104302.
Open this publication in new window or tab >>General method for atomistic spin-lattice dynamics with first-principles accuracy
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 10, article id 104302Article in journal (Refereed) Published
Abstract [en]

We present a computationally efficient and general first-principles based method for spin-lattice simulations for solids and clusters. The method is based on a coupling of atomistic spin dynamics and molecular dynamics simulations, expressed through a spin-lattice Hamiltonian, where the bilinear magnetic term is expanded up to second order in displacement. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and we observe good agreement with previous simulations. We also illustrate the coupled spin-lattice dynamics method on a more conceptual level, by exploring dissipation-free spin and lattice motion of small magnetic clusters (a dimer, trimer, and tetramer). The method discussed here opens the door for a quantitative description and understanding of the microscopic origin of many fundamental phenomena of contemporary interest, such as ultrafast demagnetization, magnetocalorics, and spincaloritronics.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-381080 (URN)10.1103/PhysRevB.99.104302 (DOI)000461953800003 ()
Funder
Swedish Research CouncilSwedish Energy AgencySwedish Foundation for Strategic Research Swedish Research Council, 2016-06955Knut and Alice Wallenberg Foundation
Available from: 2019-04-23 Created: 2019-04-23 Last updated: 2019-04-23Bibliographically approved
Pal, S., Jana, S., Govinda, S., Pal, B., Mukherjee, S., Keshavarz, S., . . . Sarma, D. D. (2019). Peculiar magnetic states in the double perovskite Nd2NiMnO6. Physical Review B, 100(4), Article ID 045122.
Open this publication in new window or tab >>Peculiar magnetic states in the double perovskite Nd2NiMnO6
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 4, article id 045122Article in journal (Refereed) Published
Abstract [en]

We present magnetic measurements on Nd2NiMnO6 which exhibits a well-known insulating paramagnetic state to an insulating ferromagnetic state transition when cooled below 200 K. Beyond this basic fact, there is a great deal of diversity in the reported magnetic properties and interpretation of specific anomalies observed in the magnetic data of this compound below the Curie temperature. We address specifically two anomalies discussed in the past, namely, a spin-glass like behavior observed in some samples near 100 K and a downturn in the magnetization with a lowering of the temperature below approximately 50 K. We show for the first time that the application of an increasing magnetic field can systematically change the low-temperature behavior to make the down-turn in the magnetization into an upturn. With the help of first principle calculations and extensive simulations along with our experimental observations, we provide a microscopic understanding of all magnetic properties observed in this interesting system to point out that the glassiness around 100 K is absent in well-ordered samples and that the low-temperature magnetic anomaly below 50 K is a consequence of a ferromagnetic coupling of the Nd spin moments with the spin of the Ni-Mn ordered sublattice without giving rise to any ordering of the Nd sublattice that remains paramagnetic, contrary to earlier claims. We explain this counter-intuitive interpretation of a ferromagnetic coupling of Nd spins with Ni-Mn spin giving rise to a decrease in the total magnetic moment by noting the less than half-filled 4f occupation of Nd that ensures orbital and spin moments of Nd to be opposite to each other due to the spin-orbit coupling. Since the ground state total magnetic moment of Nd has a contribution from the orbital moment, that is larger than the spin moment, the total moment of Nd is indeed pointing in a direction opposite to the direction of spin moments of the Ni-Mn sublattice as a consequence of the ferromagnetic exchange coupling between Nd and Ni-Mn spins.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-391944 (URN)10.1103/PhysRevB.100.045122 (DOI)000476687000002 ()
Funder
Swedish Research Council, P46561-1Swedish Research Council, 2016-04524Swedish Research Council, 2013-08316Swedish Foundation for Strategic Research Knut and Alice Wallenberg FoundationeSSENCE - An eScience CollaborationStandUp
Available from: 2019-08-29 Created: 2019-08-29 Last updated: 2019-08-29Bibliographically approved
Shaw, J. M., Delczeg-Czirjak, E. K., Edwards, E. R. J., Kvashnin, Y., Thonig, D., Schoen, M. A. W., . . . Nembach, H. T. (2018). Magnetic damping in sputter-deposited Co2MnGe Heusler compounds with A2, B2, and L2(1) orders: Experiment and theory. Physical Review B, 97(9), Article ID 094420.
Open this publication in new window or tab >>Magnetic damping in sputter-deposited Co2MnGe Heusler compounds with A2, B2, and L2(1) orders: Experiment and theory
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094420Article in journal (Refereed) Published
Abstract [en]

We show that very low values of the magnetic damping parameter can be achieved in sputter deposited polycrystalline films of Co2MnGe annealed at relatively low temperatures ranging from 240 degrees C to 400 degrees C. Damping values as low as 0.0014 are obtained with an intrinsic value of 0.0010 after spin-pumping contributions are considered. Of importance to most applications is the low value of inhomogeneous linewidth that yields measured linewidths of 1.8 and 5.1 mT at 10 and 40 GHz, respectively. The damping parameter monotonically decreases as the B2 order of the films increases. This trend is reproduced and explained by ab initio calculations of the electronic structure and damping parameter. Here, the damping parameter is calculated as the structure evolves from A2 to B2 to L2(1) orders. The largest decrease in the damping parameter occurs during the A2 to B2 transition as the half-metallic phase becomes established.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-351424 (URN)10.1103/PhysRevB.97.094420 (DOI)000427798500002 ()
Funder
Swedish Research Council, 2016-04524Swedish Research Council, 2016-06955Swedish Research Council, 2013-08316Knut and Alice Wallenberg Foundation, 2012.0031Knut and Alice Wallenberg Foundation, 2013.0030
Available from: 2018-06-01 Created: 2018-06-01 Last updated: 2018-06-01Bibliographically approved
Thonig, D., Kvashnin, Y., Eriksson, O. & Pereiro, M. (2018). Nonlocal Gilbert damping tensor within the torque-torque correlation model. Physical review materials, 2(1), Article ID 013801.
Open this publication in new window or tab >>Nonlocal Gilbert damping tensor within the torque-torque correlation model
2018 (English)In: Physical review materials, ISSN 2475-9953, Vol. 2, no 1, article id 013801Article in journal (Refereed) Published
Abstract [en]

An essential property of magnetic devices is the relaxation rate in magnetic switching, which depends strongly on the damping in the magnetization dynamics. It was recently measured that damping depends on the magnetic texture and, consequently, is a nonlocal quantity. The damping enters the Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping parameter a, which does not, in a straightforward formulation, account for nonlocality. Efforts were spent recently to obtain Gilbert damping from first principles for magnons of wave vector q. However, to the best of our knowledge, there is no report about real-space nonlocal Gilbert damping aij. Here, a torque-torque correlation model based on a tight-binding approach is applied to the bulk elemental itinerant magnets and it predicts significant off-site Gilbert damping contributions, which could be also negative. Supported by atomistic magnetization dynamics simulations, we reveal the importance of the nonlocal Gilbert damping in atomistic magnetization dynamics. This study gives a deeper understanding of the dynamics of the magnetic moments and dissipation processes in real magnetic materials. Ways of manipulating nonlocal damping are explored, either by temperature, materials doping, or strain.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-341502 (URN)10.1103/PhysRevMaterials.2.013801 (DOI)000419105000001 ()
Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2018-02-13Bibliographically approved
Salikhov, R., Reichel, L., Zingsem, B., Abrudan, R., Edström, A., Thonig, D., . . . Wiedwald, U. (2017). Enhanced spin–orbit coupling in tetragonally strained Fe–Co–B films. Journal of Physics: Condensed Matter, 9(27), Article ID 275802.
Open this publication in new window or tab >>Enhanced spin–orbit coupling in tetragonally strained Fe–Co–B films
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 9, no 27, article id 275802Article in journal (Refereed) Published
Abstract [en]

Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena-namely the magnetocrystalline anisotropy and the orbital moment-by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented.

Keywords
magnetism, spin-orbit, magnetic anisotropy, FRM, XMCD, DFT
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304518 (URN)10.1088/1361-648X/aa7498 (DOI)000403326600001 ()28530633 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 280670Swedish Research Council
Note

Title in list of papers in Thesis: Enhanced and Tunable Spin-Orbit Coupling in tetragonally Strained Fe-Co-B Films

Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2017-08-22Bibliographically approved
Thonig, D., Eriksson, O. & Pereiro, M. (2017). Magnetic moment of inertia within the torque-torque correlation model. Scientific Reports, 7, Article ID 931.
Open this publication in new window or tab >>Magnetic moment of inertia within the torque-torque correlation model
2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 931Article in journal (Refereed) Published
Abstract [en]

An essential property of magnetic devices is the relaxation rate in magnetic switching which strongly depends on the energy dissipation. This is described by the Landau-Lifshitz-Gilbert equation and the well known damping parameter, which has been shown to be reproduced from quantum mechanical calculations. Recently the importance of inertia phenomena have been discussed for magnetisation dynamics. This magnetic counterpart to the well-known inertia of Newtonian mechanics, represents a research field that so far has received only limited attention. We present and elaborate here on a theoretical model for calculating the magnetic moment of inertia based on the torque-torque correlation model. Particularly, the method has been applied to bulk itinerant magnets and we show that numerical values are comparable with recent experimental measurements. The theoretical analysis shows that even though the moment of inertia and damping are produced by the spin-orbit coupling, and the expression for them have common features, they are caused by very different electronic structure mechanisms. We propose ways to utilise this in order to tune the inertia experimentally, and to find materials with significant inertia dynamics.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-322156 (URN)10.1038/s41598-017-01081-z (DOI)000399534300002 ()28424483 (PubMedID)
Funder
Swedish Research CouncileSSENCE - An eScience Collaboration
Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2017-05-17Bibliographically approved
Chimata, R., Delczeg-Czirjak, E. K., Szilva, A., Cardias, R., Kvashnin, Y., Pereiro, M., . . . Eriksson, O. (2017). Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature. Physical review B, 95(21), Article ID 214417.
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
Fransson, J., Thonig, D., Bessarab, P. F., Bhattacharjee, S., Hellsvik, J. & Nordström, L. (2017). Microscopic theory for coupled atomistic magnetization and lattice dynamics. Physical Review Materials, 1(7), Article ID 074404.
Open this publication in new window or tab >>Microscopic theory for coupled atomistic magnetization and lattice dynamics
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2017 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 1, no 7, article id 074404Article in journal (Refereed) Published
Abstract [en]

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-339791 (URN)10.1103/PhysRevMaterials.1.074404 (DOI)000418772500005 ()
Funder
Swedish Research Council, 2016-06955Wenner-Gren FoundationsStiftelsen Olle Engkvist Byggmästare
Available from: 2018-02-23 Created: 2018-02-23 Last updated: 2018-02-23Bibliographically approved
Szilva, A., Thonig, D., Bessarab, P. F., Kvashnin, Y., Rodrigues, D. C. M., Cardias, R., . . . Eriksson, O. (2017). Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe. Physical Review B, 96(14), Article ID 144413.
Open this publication in new window or tab >>Theory of noncollinear interactions beyond Heisenberg exchange: Applications to bcc Fe
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 14, article id 144413Article in journal (Refereed) Published
Abstract [en]

We show for a simple noncollinear configuration of the atomistic spins (in particular, where one spin is rotated by a finite angle in a ferromagnetic background) that the pairwise energy variation computed in terms of multiple-scattering formalism cannot be fully mapped onto a bilinear Heisenberg spin model even in the absence of spin-orbit coupling. The non-Heisenberg terms induced by the spin-polarized host appear in leading orders in the expansion of the infinitesimal angle variations. However, an E-g - T-2g symmetry analysis based on the orbital decomposition of the exchange parameters in bcc Fe leads to the conclusion that the nearest-neighbor exchange parameters related to the T-2g orbitals are essentially Heisenberg-like: they do not depend on the spin configuration, and can, in this case, be mapped onto a Heisenberg spin model even in extreme noncollinear cases.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-340146 (URN)10.1103/PhysRevB.96.144413 (DOI)000412699400003 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2012.0031Knut and Alice Wallenberg Foundation, 2013.0020EU, FP7, Seventh Framework Programme, 600382
Available from: 2018-01-26 Created: 2018-01-26 Last updated: 2018-04-07Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-8007-5392

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