Publications by Year: 2016

2016
Zimmermann B, Long NH, Mavropoulos P, Blügel S, Mokrousov Y. Influence of complex disorder on skew-scattering Hall effects in L1(0)-ordered FePt alloy. PHYSICAL REVIEW B. 2016;94:060406.Abstract
We show by first-principles calculations that the skew-scattering anomalous Hall and spin Hall angles of L1(0)-ordered FePt drastically depend on different types of disorder. A different sign of the anomalous Hall angle is obtained when slightly deviating from the stoichiometric ratio towards the Fe-rich side as compared to the Pt-rich side. For stoichiometric samples, short-range ordering of defects has a profound effect on the Hall angles and can change them by a factor of 2 as compared to the case of uncorrelated disorder. This might explain the vast range of anomalous Hall angles measured in experiments, which undergo different preparation procedures and thus might differ in their crystallographic quality.
Geranton G, Zimmermann B, Long NH, Mavropoulos P, Blügel S, Freimuth F, Mokrousov Y. Spin-orbit torques and spin accumulation in FePt/Pt and Co/Cu thin films from first principles: The role of impurities. PHYSICAL REVIEW B. 2016;93:224420.Abstract
Using the Boltzmann formalism based on the first principles electronic structure and scattering rates, we investigate the current-induced spin accumulation and spin-orbit torques in FePt/Pt and Co/Cu bilayers in the presence of substitutional impurities. In FePt/Pt bilayers we consider the effect of intermixing of Fe and Pt atoms in the FePt layer and find a crucial dependence of spin accumulation and spin-orbit torques on the details of the distribution of these defects. In Co/Cu bilayers we predict that the magnitude and sign of the spin-orbit torque and spin accumulation depend very sensitively on the type of the impurities used to dope the Cu substrate. Moreover, simultaneously with impurity-driven scattering, we consider the effect of an additional constant quasiparticle broadening of the states at the Fermi surface to simulate phonon scattering at room temperature and discover that even a small broadening of the order of 25 meV can drastically influence the magnitude of the considered effects. We explain our findings based on the analysis of the complex interplay of several competing Fermi surface contributions to the spin accumulation and spin-orbit torques in these structurally and chemically nonuniform systems.
Long NH, Mavropoulos P, Zimmermann B, Blügel S, Mokrousov Y. Giant spin Nernst effect induced by resonant scattering at surfaces of metallic films. PHYSICAL REVIEW B. 2016;93:180406.Abstract
A concept realizing giant spin Nernst effect in nonmagnetic metallic films is introduced. It is based on the idea of engineering an asymmetric energy dependence of the longitudinal and transverse electrical conductivities, as well as a pronounced energy dependence of the spin Hall angle in the vicinity of the Fermi level by the resonant impurity states at the Fermi level. We employ an analytical model and demonstrate the emergence of a giant spin Nernst effect in Ag(111) films using ab initio calculations combined with the Boltzmann approach for transport properties arising from skew scattering off impurities.
Zimmermann B, Mavropoulos P, Long NH, Gerhorst C-R, Blügel S, Mokrousov Y. Fermi surfaces, spin-mixing parameter, and colossal anisotropy of spin relaxation in transition metals from ab initio theory. PHYSICAL REVIEW B. 2016;93:144403.Abstract
The Fermi surfaces and Elliott-Yafet spin-mixing parameter (EYP) of several elemental metals are studied by ab initio calculations. We focus first on the anisotropy of the EYP as a function of the direction of the spin-quantization axis {[}B. Zimmermann et al., Phys. Rev. Lett. 109, 236603 (2012)]. We analyze in detail the origin of the gigantic anisotropy in 5d hcp metals as compared to 5d cubic metals by band structure calculations and discuss the stability of our results against an applied magnetic field. We further present calculations of light (4d and 3d) hcp crystals, where we find a huge increase of the EYP anisotropy, reaching colossal values as large as 6000% in hcp Ti. We attribute these findings to the reduced strength of spin-orbit coupling, which promotes the anisotropic spin-flip hot loops at the Fermi surface. In order to conduct these investigations, we developed an adapted tetrahedron-based method for the precise calculation of Fermi surfaces of complicated shape and accurate Fermi-surface integrals within the full-potential relativistic Korringa-Kohn-Rostoker Green function method.
Long NH, Mavropoulos P, Bauer DSG, Zimmermann B, Mokrousov Y, Blügel S. Strong spin-orbit fields and Dyakonov-Perel spin dephasing in supported metallic films. PHYSICAL REVIEW B. 2016;94:180406.Abstract
Spin dephasing by the Dyakonov-Perel mechanism in metallic films deposited on insulating substrates is revealed, and quantitatively examined by means of density functional calculations combined with a kinetic equation. The surface-to-substrate asymmetry, probed by the metal wave functions in thin films, is found to produce strong spin-orbit fields and a fast Larmor precession, giving a dominant contribution to spin decay over the Elliott-Yafet spin relaxation up to a thickness of 70 nm. The spin dephasing is oscillatory in time with a rapid (subpicosecond) initial decay. However, parts of the Fermi surface act as spin traps, causing a persistent tail signal lasting 1000 times longer than the initial decay time. It is also found that the decay depends on the direction of the initial spin polarization, resulting in a spin-dephasing anisotropy of 200% in the examined cases.
Sessi P, Ruessmann P, Bathon T, Barla A, Kokh KA, Tereshchenko OE, Fauth K, Mahatha SK, Valbuena MA, Godey S, et al. Superparamagnetism-induced mesoscopic electron focusing in topological insulators. PHYSICAL REVIEW B. 2016;94:075137.Abstract
Recently it has been shown that surface magnetic doping of topological insulators induces backscattering of Dirac states which are usually protected by time-reversal symmetry {[}Sessi et al., Nat. Commun. 5, 5349 (2014)]. Here we report on quasiparticle interference measurements where, by improved Fermi level tuning, strongly focused interference patterns on surface Mn-doped Bi2Te3 could be directly observed by means of scanning tunneling microscopy at 4 K. Ab initio and model calculations reveal that their mesoscopic coherence relies on two prerequisites: (i) a hexagonal Fermi surface with large parallel segments (nesting) and (ii) magnetic dopants which couple to a high-spin state. Indeed, x-ray magnetic circular dichroism shows superparamagnetism even at very dilute Mn concentrations. Our findings provide evidence of strongly anisotropic Dirac-fermion-mediated interactions and demonstrate how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in topological insulators.