167 research outputs found

    Intersublattice magnetocrystalline anisotropy using a realistic tight-binding method based on maximally localized Wannier functions

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    Using a realistic tight-binding Hamiltonian based on maximally localized Wannier functions, we investigate the two-ion magnetocrystalline anisotropy energy (MAE) in L10L1_0 transition metal compounds. MAE contributions from throughout the Brillouin zone are obtained using magnetic force theorem calculations with and without perturbation theory. The results from either method agree with one another, and both reflect features of the Fermi surface. The intra-sublattice and inter-sublattice contributions to MAE are evaluated using a Green's function method. We find that the sign of the inter-sublattice contribution varies among compounds, and that its amplitude may be significant, suggesting MAE can not be resolved accurately in a single-ion manner. The results are further validated by scaling spin-orbit-coupling strength in density functional theory. Overall, this realistic tight-binding method provide an effective approach to evaluate and analyze MAE while retaining the accuracy of corresponding first-principles methods.Comment: 11 pages, 7 figure

    Band-filling effect on magnetic anisotropy using a Green's function method

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    We use an analytical model to describe the magnetocrystalline anisotropy energy (MAE) in solids as a function of band filling. The MAE is evaluated in second-order perturbation theory, which makes it possible to decompose the MAE into a sum of transitions between occupied and unoccupied pairs. The model enables us to characterize the MAE as a sum of contributions from different, often competing terms. The nitridometalates Li2_{2}[(Li1−x_{1-x}Tx_{x})N], with TT=Mn, Fe, Co, Ni, provide a system where the model is very effective because atomic like orbital characters are preserved and the decomposition is fairly clean. Model results are also compared against MAE evaluated directly from first-principles calculations for this system. Good qualitative agreement is found

    Spin excitations in K2_{2}Fe4+x_{4+x}Se5_{5}: linear response approach

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    Using \emph{ab initio} linear response techniques we calculate spin wave spectra in K2_{2}Fe4+x_{4+x}Se5_{5}, and find it to be in excellent agreement with a recent experiment. The spectrum can be alternatively described rather well by localized spin Hamiltonian restricted to first and second nearest neighbor couplings. We confirm that exchange coupling between nearest neighbor Fe magnetic moments is strongly anisotropic, and show directly that in the ideal system this anisotropy has itinerant nature which can be imitated by introducing higher order terms in effective localized spin Hamiltonian (biquadratic coupling). In the real system, structural relaxation provides an additional source of the exchange anisotropy of approximately the same magnitude. The dependence of spin wave spectra on filling of Fe vacancy sites is also discussed

    Electron correlation effects on exchange interactions and spin excitations in 2D van der Waals materials

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    Despite serious effort, the nature of the magnetic interactions and the role of electron-correlation effects in magnetic two-dimensional (2D) van der Waals materials remain elusive. Using CrI3_3 as a model system, we show that the calculated electronic structure including nonlocal electron correlations yields spin excitations consistent with inelastic neutron scattering measurements. Remarkably, this approach identifies an unreported correlation-enhanced interlayer super-superexchange, which rotates the magnon Dirac lines off, and introduces a gap along, the high-symmetry Γ\Gamma-KK-MM path. This discovery provides a different perspective on the gap opening mechanism observed in CrI3_3, which was previously associated with spin-orbit coupling through the Dzyaloshinskii-Moriya interaction or Kitaev interaction. Our observation elucidates the critical role of electron correlations on the spin ordering and spin dynamics in magnetic van der Waals materials and demonstrates the necessity of explicit treatment of electron correlations in the broad family of 2D magnetic materials.Comment: 16 pages, 8 figure

    Intrinsic magnetic properties in R(Fe1−xCox)11TiZ(R=Yand Ce;Z=H,C,and N)

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    To guide improved properties coincident with reduction of critical materials in permanent magnets, we investigate via density functional theory (DFT) the intrinsic magnetic properties of a promising system, R(Fe1−xCox)11TiZ with R=Y, Ce and interstitial doping (Z=H,C,N). The magnetization M, Curie temperature TC, and magnetocrystalline anisotropy energy K calculated in local density approximation to DFT agree well with measurements. Site-resolved contributions to K reveal that all three Fe sublattices promote uniaxial anisotropy in YFe11Ti, while competing anisotropy contributions exist in YCo11Ti. As observed in experiments on R(Fe1−xCox)11Ti, we find a complex nonmonotonic dependence of K on Co content and show that anisotropy variations are a collective effect of MAE contributions from all sites and cannot be solely explained by preferential site occupancy. With interstitial doping, calculated TC enhancements are in the sequence of N\u3eC\u3eH, with volume and chemical effects contributing to the enhancement. The uniaxial anisotropy of R(Fe1−xCox)11TiZ generally decreases with C and N; although, for R=Ce, C doping is found to greatly enhance it for a small range of 0.

    Intrinsic magnetic properties in R(Fe1−xCox)11TiZ(R=Yand Ce;Z=H,C,and N)

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    To guide improved properties coincident with reduction of critical materials in permanent magnets, we investigate via density functional theory (DFT) the intrinsic magnetic properties of a promising system, R(Fe1−xCox)11TiZ with R=Y, Ce and interstitial doping (Z=H,C,N). The magnetization M, Curie temperature TC, and magnetocrystalline anisotropy energy K calculated in local density approximation to DFT agree well with measurements. Site-resolved contributions to K reveal that all three Fe sublattices promote uniaxial anisotropy in YFe11Ti, while competing anisotropy contributions exist in YCo11Ti. As observed in experiments on R(Fe1−xCox)11Ti, we find a complex nonmonotonic dependence of K on Co content and show that anisotropy variations are a collective effect of MAE contributions from all sites and cannot be solely explained by preferential site occupancy. With interstitial doping, calculated TC enhancements are in the sequence of N\u3eC\u3eH, with volume and chemical effects contributing to the enhancement. The uniaxial anisotropy of R(Fe1−xCox)11TiZ generally decreases with C and N; although, for R=Ce, C doping is found to greatly enhance it for a small range of 0.
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