| 1. |
- Arnalds, Unnar B., et al.
(författare)
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A new look on the two-dimensional Ising model : thermal artificial spins
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18
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Tidskriftsartikel (refereegranskat)abstract
- We present a direct experimental investigation of the thermal ordering in an artificial analogue of an asymmetric two-dimensional Ising system composed of a rectangular array of nano-fabricated magnetostatically interacting islands. During fabrication and below a critical thickness of the magnetic material the islands are thermally fluctuating and thus the system is able to explore its phase space. Above the critical thickness the islands freeze-in resulting in an arrested thermalized state for the array. Determining the magnetic state we demonstrate a genuine artificial two-dimensional Ising system which can be analyzed in the context of nearest neighbor interactions.
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| 2. |
- Bondarenko, Nina, et al.
(författare)
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Static and dynamical properties of spin-polarons in La-doped CaMnO3
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The static and dynamic properties of spin-polarons in La-doped CaMnO3 are explored theoretically,by means of an eective low energy Hamiltonian. All parameters from the Hamiltoniainare evaluated from rst principles theory, without adjustable parameters. We compare dierentgeometries, like spin-polarons in bulk, surface and as single two-dimensional layers. The low energyHamiltonian is used to investigate the temperature stability of the spin-polaron, as well as theresponse to an external applied electric eld. Technically this involves atomistic spin-dynamics simulationsin combination with kinetic Monte Carlo simulations. Where a comparison can be made,our simulations exhibit an excellent agreement with available experimental data and previous theory.Remarkably, we nd that excellent control of the mobility of spin-polarons in this material canbe achieved, and that the critical parameters deciding this is the temperature and strength of theapplied electrical eld. We outline dierent technological implications of our ndings.
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| 3. |
- Borlenghi, Simone, et al.
(författare)
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Energy and magnetization transport in nonequilibrium macrospin systems
- 2015
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 92:1
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Tidskriftsartikel (refereegranskat)abstract
- We investigate numerically the magnetization dynamics of an array of nanodisks interacting through the magnetodipolar coupling. In the presence of a temperature gradient, the chain reaches a nonequilibrium steady state where energy and magnetization currents propagate. This effect can be described as the flow of energy and particle currents in an off-equilibrium discrete nonlinear Schrodinger (DNLS) equation. This model makes transparent the transport properties of the system and allows for a precise definition of temperature and chemical potential for a precessing spin. The present study proposes a setup for the spin-Seebeck effect, and shows that its qualitative features can be captured by a general oscillator-chain model.
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| 4. |
- Chico Carpio, Jonathan Philippe, 1987-
(författare)
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Magnetization dynamics on the nanoscale : From first principles to atomistic spin dynamics
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 5. |
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| 6. |
- Chico, Jonathan, et al.
(författare)
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First-principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 93:21
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Tidskriftsartikel (refereegranskat)abstract
- Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e., the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families Co(2)MnZ, Co(2)FeZ, and Mn(2)VZ with Z = (Al, Si, Ga, Ge) is performed. A key aspect is the determination of the Gilbert damping from first-principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic Co2FeSi and the ferrimagnetic Mn2VAl. Correlation effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard U and the local spin density approximation plus dynamical mean field theory approximation, which allows one to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.
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| 7. |
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| 8. |
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| 9. |
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| 10. |
- Chimata, Raghuveer, et al.
(författare)
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Laser heated ferromagnetic simulations
- 2015
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Ingår i: ULTRAFAST MAGNETISM I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 76-78
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Konferensbidrag (refereegranskat)abstract
- In this work, we show a model of ferromagnetic material heated by a laser pulse. The laser creates a pattern of circles on the ferromagnetic materials with hot regions heated up to 3000K and cold regions at 100K and 400K. In our model the Landau-Lifshitz-Gilbert equation for a macrospin and spin temperature is passed through stochastic field. We show that the difference of magnon dispersion in the cold regions of the material.
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| 11. |
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| 12. |
- Dürrenfeld, Philipp, et al.
(författare)
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Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films
- 2015
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:21
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Dussaux, A., et al.
(författare)
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Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe.
- 2016
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Chiral magnetic interactions induce complex spin textures including helical and conical spin spirals, as well as particle-like objects such as magnetic skyrmions and merons. These spin textures are the basis for innovative device paradigms and give rise to exotic topological phenomena, thus being of interest for both applied and fundamental sciences. Present key questions address the dynamics of the spin system and emergent topological defects. Here we analyse the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining magnetic force microscopy, single-spin magnetometry and Landau-Lifschitz-Gilbert simulations we show that the nanoscale dynamics are governed by the depinning and subsequent motion of magnetic edge dislocations. The motion of these topologically stable objects triggers perturbations that can propagate over mesoscopic length scales. The observation of stochastic instabilities in the micromagnetic structure provides insight to the spatio-temporal dynamics of itinerant helimagnets and topological defects, and discloses open challenges regarding their technological usage.
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| 14. |
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| 15. |
- Edström, Alexander, et al.
(författare)
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Electronic structure and magnetic properties of L1(0) binary alloys
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:1, s. 014402-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 16. |
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| 17. |
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| 18. |
- Nystrand, Thomas, et al.
(författare)
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Coarse-Graining Approach to Atomistic SpinDynamics
- 2015
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Ingår i: ULTRAFAST MAGNETISM I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 162-165
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Konferensbidrag (refereegranskat)abstract
- We introduce a coarse-graining approach to study the movement of a Domain Wall (DW) under the influence of a spin polarized current, in the framework of atomistic spin dynamics. An increase in performance of up to 35% is obtained. We show the dependence of the method on both exchange range and temperature effects.
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| 19. |
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| 20. |
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| 21. |
- Pan, Fan, 1987-, et al.
(författare)
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Extended spin model in atomistic simulations of alloys
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:18
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Tidskriftsartikel (refereegranskat)abstract
- An extended atomistic spin model allowing for studies of the finite-temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parametrization from a first-principles basis, interpolating from both the low-temperature ferromagnetic and the high-temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.
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| 22. |
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| 23. |
- Pan, Fan, et al.
(författare)
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Systematic study of magnetodynamic properties at finite temperatures in doped permalloy from first-principles calculations
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 94:21
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Tidskriftsartikel (refereegranskat)abstract
- By means of first-principles calculations, we have systematically investigated how the magnetodynamic properties Gilbert damping, magnetization, and exchange stiffness are affected when permalloy (Py) (Fe0.19Ni0.81) is doped with 4d or 5d transition metal impurities. We find that the trends in the Gilbert damping can be understood from relatively few basic parameters such as the density of states at the Fermi level, the spin-orbit coupling, and the impurity concentration. The temperature dependence of the Gilbert damping is found to be very weak which we relate to the lack of intraband transitions in alloys. Doping with 4d elements has no major impact on the studied Gilbert damping, apart from diluting the host. However, the 5d elements have a profound effect on the damping and allow it to be tuned over a large interval while maintaining the magnetization and exchange stiffness. As regards the spin stiffness, doping with early transition metals results in considerable softening, whereas late transition metals have a minor impact. Our result agree well with earlier calculations where available. In comparison to experiments, the computed Gilbert damping appears slightly underestimated, whereas the spin stiffness shows a general good agreement.
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| 24. |
- Pereiro, Manuel, et al.
(författare)
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Topological excitations in a kagome magnet
- 2014
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5
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Tidskriftsartikel (refereegranskat)
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