# Publications

2020

Kosma A, Ruessmann P, Blügel S, Mavropoulos P. Strong spin-orbit torque effect on magnetic defects due to topological surface state electrons in Bi2Te3. PHYSICAL REVIEW B. 2020;102:144424.Abstract

We investigate the spin-orbit torque exerted on the magnetic moments of the transition-metal impurities Cr, Mn, Fe, and Co, embedded in the surface of the topological insulator Bi2Te3, in response to an electric field and a consequent electrical current flow in the surface. The multiple scattering problem of electrons off impurity atoms is solved by first-principles calculations within the full-potential relativistic Korringa-Kohn-Rostoker (KKR) Green function method, while the spin-orbit torque calculations are carried out by combining the KKR method with the semiclassical Boltzmann transport equation. We analyze the correlation of the spin-orbit torque to the spin accumulation and spin flux in the impurities and unveil the effect of resonant scattering. In addition, we relate the torque to the resistivity and Joule heat production. We predict that the Mn/Bi2Te3 is optimal among the studied systems.

Ruessmann P, Mavropoulos P, Blügel S. Ab Initio Theory of Fourier-Transformed Quasiparticle Interference Maps and Application to the Topological Insulator Bi2Te3. PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS. 2020.Abstract

The quasiparticle interference (QPI) technique is a powerful tool that allows to uncover the structure and properties of electronic structure of a material combined with scattering properties of defects at surfaces. Recently, this technique has been pivotal in proving the unique properties of the surface state of topological insulators which manifests itself in the absence of backscattering. Herein, a Green function-based formalism is derived for the ab initio computation of Fourier-transformed QPI images. The efficiency of the new implementation is shown at the examples of QPI that forms around magnetic and nonmagnetic defects at the Bi2Te3 surface. This method allows a deepened understanding of the scattering properties of topologically protected electrons off defects and is a useful tool in the study of quantum materials in the future.

2019

Ruessmann P, Mavropoulos P, Blügel S. Lifetime and surface-to-bulk scattering off vacancies of the topological surface state in the three-dimensional strong topological insulators Bi2Te3 and Bi2Se3. JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS. 2019;128:258-264.Abstract

We analyze the finite lifetimes of the topologically protected electrons in the surface state of Bi2Te3 and Bi2Se3 due to elastic scattering off surface vacancies and as a function of energy. The scattering rates are decomposed into surface-to-surface and surface-to-bulk contributions, giving us new fundamental insights into the scattering properties of the topological surface states (TSS). If the number of possible final bulk states is much larger than the number of final surface states, then the surface-to-bulk contribution is of importance, otherwise the surface-to-surface contribution dominates. Additionally, we find defect resonances that have a significant impact on the scattering properties of the TSS. They can strongly change the lifetime of the surface state to vary between tens of fs to ps at surface defect concentrations of 1 at%. We also see that the effective scattering angle shows a strong dependence on the Fermi surface warping. Our results compare fairly well with available experiments.

Popescu V, Kratzer P, Entel P, Heiliger C, Czerner M, Tauber K, Toepler F, Herschbach C, Fedorov DV, Gradhand M, et al. Spin caloric transport from density-functional theory. JOURNAL OF PHYSICS D-APPLIED PHYSICS. 2019;52:073001.Abstract

Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes the three most frequently employed approaches to the problem, namely the Kubo, Boltzmann, and Landauer-Buttiker methods. These are show cased in specific examples that span, on the one hand, a wide range of materials, such as bulk metallic alloys, nano-structured metallic and tunnel junctions, or magnetic overlayers on heavy metals, and, on the other hand, a wide range of effects, such as the spin-Seebeck, magneto-Seebeck, and spin-Nernst effects, spin disorder, and the thermal spin-transfer and thermal spin-orbit torques.

Grytsiuk S, Hoffmann M, Hanke J-P, Mavropoulos P, Mokrousov Y, Bihlmayer G, Blügel S. Ab initio analysis of magnetic properties of the prototype B20 chiral magnet FeGe. PHYSICAL REVIEW B. 2019;100:214406.Abstract

FeGe in the B20 phase is an experimentally well-studied prototypical chiral magnet exhibiting helical spirals, skyrmion lattices, and individual skyrmions with a robust length of 70 nm. While the helical spiral ground state can be verified by first-principles calculations based on density functional theory, this feature size could not be reproduced even approximately. To develop a coherent picture of the discrepancy between experiment and theory, we investigate in this work the magnetic properties of FeGe from first principles using different electronic-structure methods. We study atomistic as well as micromagnetic parameters describing exchange and Dzyaloshinskii-Moriya interactions, and discuss their subtle dependence on computational, structural, and correlation parameters. In particular, we quantify how these magnetic properties are affected by changes of the lattice parameter, different atomic arrangements, exchange and correlation effects, finite Fermi-function broadening, and momentum-space sampling. In addition, we use the obtained atomistic parameters to determine the corresponding Curie temperature, which agrees well with experiments. Our results indicate that the well-known and well-accepted relation between the micromagnetic parameters and the period of the helical structure is not valid for FeGe. This calls for new experiments exploring the relation by measuring independently the spin stiffness, the spiralization, and the period of the helical spin spiral.

2018

Zanolli Z, Niu C, Bihlmayer G, Mokrousov Y, Mavropoulos P, Verstraete MJ, Blügel S. Hybrid quantum anomalous Hall effect at graphene-oxide interfaces. PHYSICAL REVIEW B. 2018;98:155404.Abstract

Interfaces are ubiquitous in materials science, and in devices in particular. As device dimensions are constantly shrinking, understanding the physical properties emerging at interfaces is crucial to exploit them for applications, here for spintronics. Using first-principles techniques and Monte Carlo simulations, we investigate the mutual magnetic interaction at the interface between graphene and an antiferromagnetic semiconductor BaMnO3. We find that graphene deeply affects the magnetic state of the substrate, down to several layers below the interface, by inducing an overall magnetic softening, and switching the in-plane magnetic ordering from antiferromagnetic to ferromagnetic. The graphene-BaMnO3 system presents a Rashba gap 300 times larger than in pristine graphene, leading to a flavor of quantum anomalous Hall effect (QAHE), a hybrid QAHE, characterized by the coexistence of metallic and topological insulating states. These findings could be exploited to fabricate devices that use graphene to control the magnetic configuration of a substrate.

Weber AP, Ruessmann P, Xu N, Muff S, Fanciulli M, Magrez A, Bugnon P, Berger H, Plumb NC, Shi M, et al. Spin-Resolved Electronic Response to the Phase Transition in MoTe2. PHYSICAL REVIEW LETTERS. 2018;121:156401.Abstract

The semimetal MoTe2 is studied by spin- and angle-resolved photoemission spectroscopy across the centrosymmetry-breaking structural transition temperature of the bulk. A three-dimensional spin- texture is observed in the bulk Fermi surface in the low temperature, noncentrosymmetric phase that is consistent with first-principles calculations. The spin texture and two types of surface Fermi arc are not completely suppressed above the bulk transition temperature. The lifetimes of quasiparticles forming the Fermi arcs depend on thermal history and lengthen considerably upon cooling toward the bulk structural transition. The results indicate that a new form of polar instability exists near the surface when the bulk is largely in a centrosymmetric phase.

Ruessmann P, Weber AP, Glott F, Xu N, Fanciulli M, Muff S, Magrez A, Bugnon P, Berger H, Bode M, et al. Universal scattering response across the type-II Weyl semimetal phase diagram. PHYSICAL REVIEW B. 2018;97:075106.Abstract

The discovery of Weyl semimetals represents a significant advance in topological band theory. They paradigmatically enlarged the classification of topological materials to gapless systems while simultaneously providing experimental evidence for the long-sought Weyl fermions. Beyond fundamental relevance, their high mobility, strong magnetoresistance, and the possible existence of even more exotic effects, such as the chiral anomaly, make Weyl semimetals a promising platform to develop radically new technology. Fully exploiting their potential requires going beyond the mere identification of materials and calls for a detailed characterization of their functional response, which is severely complicated by the coexistence of surface-and bulk-derived topologically protected quasiparticles, i.e., Fermi arcs and Weyl points, respectively. Here, we focus on the type-II Weyl semimetal class in which we find a stoichiometry-dependent phase transition from a trivial to a nontrivial regime. By exploring the two extreme cases of the phase diagram, we demonstrate the existence of a universal response of both surface and bulk states to perturbations. We show that quasiparticle interference patterns originate from scattering events among surface arcs. Analysis reveals that topologically nontrivial contributions are strongly suppressed by spin texture. We also show that scattering at localized impurities can generate defect-induced quasiparticles sitting close to the Weyl point energy. These give rise to strong peaks in the local density of states, which lift the Weyl node, significantly altering the pristine low-energy spectrum. Remarkably, by comparing the WTe2 and the MoTe2 cases we found that scattering response and topological transition are not directly linked. Visualizing the existence of a universal microscopic response to scattering has important consequences for understanding the unusual transport properties of this class of materials. Overall, our observations provide a unifying picture of the type-II Weyl phase diagram.

Ruessmann P, Mahatha SK, Sessi P, Valbuena MA, Bathon T, Fauth K, Godey S, Mugarza A, Kokh KA, Tereshchenko OE, et al. Towards microscopic control of the magnetic exchange coupling at the surface of a topological insulator. JOURNAL OF PHYSICS-MATERIALS. 2018;1:015002.Abstract

Magnetically doped topological insulators may produce novel states of electronic matter, where for instance the quantum anomalous Hall effect state can be realized. Pivotal to this goal is a microscopic control over the magnetic state, defined by the local electronic structure of the dopants and their interactions. We report on the magnetic coupling among Mn or Co atoms adsorbed on the surface of the topological insulator Bi2Te3. Our findings uncover the mechanisms of the exchange coupling between magnetic atoms coupled to the topological surface state in strong topological insulators. The combination of x-ray magnetic circular dichroism and ab initio calculations reveals that the sign of the magnetic coupling at short adatom-adatom distances is opposite for Mn with respect to Co. For both elements, the magnetic exchange reverses its sign at a critical distance between magnetic adatoms, as a result of the interplay between superexchange, double exchange and Ruderman-Kittel-Kasuya-Yoshida interactions.

2017

Geranton G, Zimmermann B, Long NH, Mavropoulos P, Blügel S, Freimuth F, Mokrousov Y. Nonlocal fieldlike spin-orbit torques in Rashba systems: Ab initio study of a Ag2Bi-terminated Ag(111) film grown on a ferromagnetic Fe(110) substrate. PHYSICAL REVIEW B. 2017;95:134449.Abstract

We investigate from first principles the fieldlike spin-orbit torques (SOTs) in a Ag2Bi-terminated Ag(111) film grown on ferromagnetic Fe(110). We find that a large part of the SOT arises from the spin-orbit interaction (SOI) in the Ag2Bi layer far away from the Fe layers. These results clearly hint at a long-range spin transfer in the direction perpendicular to the film that does not originate in the spin Hall effect. In order to bring evidence of the nonlocal character of the computed SOT, we show that the torque acting on the Fe layers can be engineered by the introduction of Bi vacancies in the Ag2Bi layer. Overall, we find a drastic dependence of the SOT on the disorder type, which we explain by a complex interplay of different contributions to the SOT in the Brillouin zone.

2016

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.

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.

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.

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.

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.

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.

2015

Jakobsson A, Mavropoulos P, Sasioglu E, Blügel S, Lezaic M, Sanyal B, Galanakis I. First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors. PHYSICAL REVIEW B. 2015;91:174439.Abstract

Employing first-principles electronic-structure calculations in conjunction with the frozen-magnon method, we calculate exchange interactions, spin-wave dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin gapless semiconductor (SGS) compounds Mn2CoAl, Ti2MnAl, Cr2ZnSi, Ti2CoSi, and Ti2VAs. We find that their magnetic behavior is similar to the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice exchange interactions play an essential role in the formation of the magnetic ground state and in determining the Curie temperature T-c. All compounds, except Ti2CoSi, possess a ferrimagnetic ground state. Due to the finite energy gap in one spin channel, the exchange interactions decay sharply with the distance, and hence magnetism of these SGSs can be described considering only nearest-and next-nearest-neighbor exchange interactions. The calculated spin-wave dispersion curves are typical for ferrimagnets and ferromagnets. The spin-wave stiffness constants turn out to be larger than those of the elementary 3d ferromagnets. Calculated exchange parameters are used as input to determine the temperature dependence of the magnetization and Tc of the SGSs. We find that the Tc of all compounds is much above the room temperature. The calculated magnetization curve for Mn2CoAl as well as the Curie temperature are in very good agreement with available experimental data. This study is expected to pave the way for a deeper understanding of the magnetic properties of the inverse-Heusler-based SGSs and enhance the interest in these materials for application in spintronic and magnetoelectronic devices.

Gayles J, Freimuth F, Schena T, Lani G, Mavropoulos P, Duine RA, Blügel S, Sinova J, Mokrousov Y. Dzyaloshinskii-Moriya Interaction and Hall Effects in the Skyrmion Phase of Mn1-xFexGe. PHYSICAL REVIEW LETTERS. 2015;115:036602.Abstract

We carry out density functional theory calculations which demonstrate that the electron dynamics in the Skyrmion phase of Fe-rich Mn1-xFexGe alloys is governed by Berry phase physics. We observe that the magnitude of the Dzyaloshinskii-Moriya interaction directly related to the mixed space-momentum Berry phases, changes sign and magnitude with concentration x in direct correlation with the data of Shibata et al. {[}Nat. Nanotechnol. 8, 723 (2013)]. The computed anomalous and topological Hall effects in FeGe are also in good agreement with available experiments. We further develop a simple tight-binding model able to explain these findings. Finally, we show that the adiabatic Berry phase picture is violated in the Mn-rich limit of the alloys.

Kovacik R, Mavropoulos P, Blügel S. Spin transport and spin-caloric effects in (Cr,Zn) Te half-metallic nanostructures: Effect of spin disorder at elevated temperatures from first principles. PHYSICAL REVIEW B. 2015;91:014421.Abstract

An important contribution to the thermoelectric and spin-caloric transport properties in magnetic materials at elevated temperatures is the formation of a spin-disordered state due to local moment fluctuations. This effect has not been largely investigated so far. We focus on various magnetic nanostructures of CrTe in the form of thin layers or nanowires embedded in ZnTe matrix, motivated by the miniaturization of spintronics devices and by recent suggestions that magnetic nanostructures can lead to extraordinary thermoelectric effects due to quantum confinement. The electronic structure of the studied systems is calculated within the multiple scattering screened Korringa-Kohn-Rostoker Green function (KKR-GF) framework. The Monte Carlo method is used to simulate the magnetization in the temperature induced spin disorder. The transport properties are evaluated from the transmission probability obtained using the Baranger-Stone approach within the KKR-GF framework. We find qualitative and quantitative changes in the thermoelectric and spin-caloric coefficients when spin disorder is included in the calculation. Furthermore, we show that substitutional impurities in CrTe nanowires could considerably enhance the Seebeck coefficient and the thermoelectric figure of merit.

2014

Bouhassoune M, Zimmermann B, Mavropoulos P, Wortmann D, Dederichs PH, Blügel S, Lounis S. Quantum well states and amplified spin-dependent Friedel oscillations in thin films. NATURE COMMUNICATIONS. 2014;5:5558.Abstract

Electrons mediate many of the interactions between atoms in a solid. Their propagation in a material determines its thermal, electrical, optical, magnetic and transport properties. Therefore, the constant energy contours characterizing the electrons, in particular the Fermi surface, have a prime impact on the behaviour of materials. If anisotropic, the contours induce strong directional dependence at the nanoscale in the Friedel oscillations surrounding impurities. Here we report on giant anisotropic charge density oscillations focused along specific directions with strong spin-filtering after scattering at an oxygen impurity embedded in the surface of a ferromagnetic thin film of Fe grown on W(001). Utilizing density functional theory, we demonstrate that by changing the thickness of the Fe films, we control quantum well states confined to two dimensions that manifest as multiple flat energy contours, impinging and tuning the strength of the induced charge oscillations which allow to detect the oxygen impurity at large distances (approximate to 50 nm).

Fukushima T, Katayama-Yoshida H, Sato K, Bihlmayer G, Mavropoulos P, Bauer DSG, Zeller R, Dederichs PH. Hubbard U calculations for gap states in dilute magnetic semiconductors. JOURNAL OF PHYSICS-CONDENSED MATTER. 2014;26:274202.Abstract

On the basis of constrained density functional theory, we present ab initio calculations for the Hubbard U parameter of transition metal impurities in dilute magnetic semiconductors, choosing Mn in GaN as an example. The calculations are performed by two methods: (i) the Korringa-Kohn-Rostoker (KKR) Green function method for a single Mn impurity in GaN and (ii) the full-potential linearized augmented plane-wave (FLAPW) method for a large supercell of GaN with a single Mn impurity in each cell. By changing the occupancy of the majority t(2) gap state of Mn, we determine the U parameter either from the total energy differences E(N + 1) and E(N - 1) of the (N +/- 1) -electron excited states with respect to the ground state energy E(N), or by using the single-particle energies for n(0) +/- 1/2 occupancies around the charge-neutral occupancy n(0) (Janak's transition state model). The two methods give nearly identical results. Moreover the values calculated by the supercell method agree quite well with the Green function values. We point out an important difference between the `global' U parameter calculated using Janak's theorem and the `local' U of the Hubbard model.

Long NH, Mavropoulos P, Zimmermann B, Bauer DSG, Blügel S, Mokrousov Y. Spin relaxation and spin Hall transport in 5d transition-metal ultrathin films. PHYSICAL REVIEW B. 2014;90:064406.Abstract

The spin relaxation induced by the Elliott-Yafet mechanism and the extrinsic spin Hall conductivity due to the skew scattering are investigated in 5d transition-metal ultrathin films with self-adatom impurities as scatterers. The values of the Elliott-Yafet parameter and of the spin-flip relaxation rate reveal a correlation with each other that is in agreement with the Elliott approximation. At 10-layer thickness, the spin-flip relaxation time in 5d transition-metal films is quantitatively reported about few hundred nanoseconds at atomic percent. This time scale is one and two orders of magnitude shorter than the values in Au and Cu thin films, respectively. The anisotropy effect of the Elliott-Yafet parameter and of the spin-flip relaxation rate with respect to the direction of the spin-quantization axis in relation to the crystallographic axes is also analyzed. We find that the anisotropy of the spin-flip relaxation rate is enhanced due to the Rashba surface states on the Fermi surface, reaching values as high as 97% in 10-layer Hf(0001) film or 71% in 10-layer W(110) film. Finally, the spin Hall conductivity as well as the spin Hall angle due to the skew scattering off self-adatom impurities are calculated using the Boltzmann approach. Our calculations employ a relativistic version of the first-principles full-potential Korringa-Kohn-Rostoker Green function method.

Kovacik R, Mavropoulos P, Wortmann D, Blügel S. Spin-caloric transport properties of cobalt nanostructures: Spin disorder effects from first principles. PHYSICAL REVIEW B. 2014;89:134417.Abstract

The fundamental aspects of spin-dependent transport processes and their interplay with temperature gradients, as given by the spin Seebeck coefficient, are still largely unexplored and a multitude of contributing factors must be considered. We used density functional theory together with a Monte-Carlo-based statistical method to simulate simple nanostructures, such as Co nanowires and films embedded in a Cu host or in vacuum, and investigated the influence of spin disorder scattering on electron transport at elevated temperatures. While we show that the spin-dependent scattering of electrons due to temperature-induced disorder of the local magnetic moments contributes significantly to the resistance, thermoelectric, and spin-caloric transport coefficients, we also conclude that the actual magnitude of these effects cannot be predicted, quantitatively or qualitatively, without such detailed calculations.

Zimmermann B, Chadova K, Koedderitzsch D, Blügel S, Ebert H, Fedorov DV, Long NH, Mavropoulos P, Mertig I, Mokrousov Y, et al. Skew scattering in dilute ferromagnetic alloys. PHYSICAL REVIEW B. 2014;90:220403.Abstract

The challenging problem of skew scattering for Hall effects in dilute ferromagnetic alloys, with intertwined effects of spin-orbit coupling, magnetism, and impurity scattering, is studied here from first principles. Our main aim is to identify chemical trends and work out simple rules for large skew scattering in terms of the impurity and host states at the Fermi surface, with particular emphasis on the interplay of the spin and anomalous Hall effects in one and the same system. The predicted trends are benchmarked by referring to three different ab initio methods based on different approximations with respect to the electronic structure and transport properties.

2013

Jakobsson A, Sasioglu E, Mavropoulos P, Lezaic M, Sanyal B, Bihlmayer G, Blügel S. Tuning the Curie temperature of FeCo compounds by tetragonal distortion. APPLIED PHYSICS LETTERS. 2013;103:102404.Abstract

Combining density-functional theory calculations with a classical Monte Carlo method, we show that for B2-type FeCo compounds, tetragonal distortion gives rise to a strong reduction of the Curie temperature T-C. The T-C monotonically decreases from 1575K (for c/a = 1) to 940K (for c/a = root 2). We find that the nearest neighbor Fe-Co exchange interaction is sufficient to explain the c/a behavior of the T-C. Combination of high magnetocrystalline anisotropy energy with a moderate TC value suggests tetragonal FeCo grown on the Rh substrate with c/a = 1.24 to be a promising material for heat-assisted magnetic recording applications. (C) 2013 AIP Publishing LLC.

Lezaic M, Mavropoulos P, Bihlmayer G, Blügel S. Exchange interactions and local-moment fluctuation corrections in ferromagnets at finite temperatures based on noncollinear density-functional calculations. PHYSICAL REVIEW B. 2013;88:134403.Abstract

We explore the derivation of interatomic exchange interactions in ferromagnets within density-functional theory (DFT) and the mapping of DFT results onto a spin Hamiltonian. We delve into the problem of systems comprising atoms with strong spontaneous moments together with atoms with weak induced moments. All moments are considered as degrees of freedom, with the strong moments thermally fluctuating only in angle and the weak moments thermally fluctuating in angle and magnitude. We argue that a quadratic dependence of the energy on the weak local moments magnitude, which is a good approximation in many cases, allows for an elimination of the weak-moment degrees of freedom from the thermodynamic expressions in favor of a renormalization of the Heisenberg interactions among the strong moments. We show that the renormalization is valid at all temperatures accounting for the thermal fluctuations and resulting in temperature-independent renormalized interactions. These are shown to be the ones directly derived from total-energy DFT calculations by constraining the strong-moment directions, as is done, e. g., in spin-spiral methods. We furthermore prove that within this framework the thermodynamics of the weak-moment subsystem, and in particular all correlation functions, can be derived as polynomials of the correlation functions of the strong-moment subsystem with coefficients that depend on the spin susceptibility and that can be calculated within DFT. These conclusions are rigorous under certain physical assumptions on the measure in the magnetic phase space. We implement the scheme in the full-potential linearized augmented plane wave method using the concept of spin-spiral states, considering applicable symmetry relations and the use of the magnetic force theorem. Our analytical results are corroborated by numerical calculations employing DFT and a Monte Carlo method.

Long NH, Mavropoulos P, Heers S, Zimmermann B, Mokrousov Y, Blügel S. Spin-flip hot spots in ultrathin films of monovalent metals: Enhancement and anisotropy of the Elliott-Yafet parameter. PHYSICAL REVIEW B. 2013;88:144408.Abstract

In contrast to the long-known fact that spin-flip hot spots, i.e., special k points on the Fermi surface showing a high spin-mixing parameter, do not occur in the bulk of monovalent (noble and alkali) metals, we found them on the surface Brillouin-zone boundary of ultrathin films of these metals. Density-functional calculations within the Korringa-Kohn-Rostoker Green function method for ultrathin (001) oriented Cu, Ag, and Au films of 10-layer thickness show that the region around the hot spots can have a substantial contribution, e. g., 52% in Au(001), to the integrated spin-mixing parameter, that could lead to a significant enhancement of the spin-relaxation rate or spin-Hall angle in thin films. Owing to the appearance of spin-flip hot spots, a large anisotropy of the Elliott-Yafet parameter {[}50% for Au(001)] is also found in these systems. The findings are important for spintronics applications in which noble metals are frequently used and in which the dimensionality of the sample is reduced.

Long NH, Mavropoulos P, Zimmermann B, Heers S, Bauer DSG, Blügel S, Mokrousov Y. Spin relaxation and the Elliott-Yafet parameter in W(001) ultrathin films: Surface states, anisotropy, and oscillation effects. PHYSICAL REVIEW B. 2013;87:224420.Abstract

Using first-principles methods based on density-functional theory, we investigate the spin relaxation in W(001) ultrathin films. Within the framework of the Elliott-Yafet theory, we calculate the spin mixing of the Bloch states and we explicitly consider spin-flip scattering off self-adatoms. At small film thicknesses, we find an oscillatory behavior of the spin-mixing parameter and relaxation rate as a function of the film thickness, which we trace back to surface-state properties. We also analyze the Rashba effect experienced by the surface states and discuss its influence on the spin relaxation. Finally, we calculate the anisotropy of the spin-relaxation rate with respect to the polarization direction of the excited spin population relative to the crystallographic axes of the film. We find that the spin-relaxation rate can increase by as much as 27% when the spin polarization is directed out of plane, compared to the case when it is in plane. Our calculations are based on the multiple-scattering formalism of the Korringa-Kohn-Rostoker Green-function method.

Gupta K, Mahadevan P, Mavropoulos P, Lezaic M. Orbital-Ordering-Induced Ferroelectricity in SrCrO3. PHYSICAL REVIEW LETTERS. 2013;111:077601.Abstract

Using density functional theory calculations, ultrathin films of SrVO3(d(1)) and SrCrO3(d(2)) on SrTiO3 substrates have been studied as possible multiferroics. Although both are metallic in the bulk limit, they are found to be insulating as a result of orbital ordering driven by lattice distortions at the ultrathin limit. While the distortions in SrVO3 have a first-order Jahn-Teller origin, those in SrCrO3 are ferroelectric in nature. This route to ferroelectricity results in polarizations comparable with conventional ferroelectrics.

Mokrousov Y, Zhang H, Freimuth F, Zimmermann B, Long NH, Weischenberg J, Souza I, Mavropoulos P, Blügel S. Anisotropy of spin relaxation and transverse transport in metals. JOURNAL OF PHYSICS-CONDENSED MATTER. 2013;25:163201.Abstract

Using first-principles methods we explore the anisotropy of the spin relaxation and transverse transport properties in bulk metals with respect to the real-space direction of the spin-quantization axis in paramagnets or of the spontaneous magnetization in ferromagnets. Owing to the presence of the spin-orbit coupling the orbital and spin character of the Bloch states depends sensitively on the orientation of the spins relative to the crystal axes. This leads to drastic changes in quantities which rely on interband mixing induced by the spin-orbit interaction. The anisotropy is particularly striking for quantities which exhibit spiky and irregular distributions in the Brillouin zone, such as the spin-mixing parameter or the Berry curvature of the electronic states. We demonstrate this for three cases: (i) the Elliott-Yafet spin-relaxation mechanism in paramagnets with structural inversion symmetry; (ii) the intrinsic anomalous Hall effect in ferromagnets; and (iii) the spin Hall effect in paramagnets. We discuss the consequences of the pronounced anisotropic behavior displayed by these properties for spin-polarized transport applications.

2012

Heers S, Mavropoulos P, Lounis S, Zeller R, Blügel S. Lifetime reduction of surface states at Cu, Ag, and Au(111) caused by impurity scattering. PHYSICAL REVIEW B. 2012;86:125444.Abstract

We present density-functional results on the lifetime of the (111) surface state of the noble metals. We consider scattering on the Fermi surface caused by impurity atoms belonging to the 3d and 4sp series. The results are analyzed with respect to film thickness and with respect to separation of scattering into bulk or into surface states. While for impurities in the surface layer the overall trends are similar to the long-known bulk-state scattering, for adatom-induced scattering we find a surprising behavior with respect to the adatom atomic number. A plateau emerges in the scattering rate of the 3d adatoms, instead of a peak characteristic of the d resonance. Additionally, the scattering rate of 4sp adatoms changes in a zigzag pattern, contrary to a smooth parabolic increase following Linde's rule that is observed in bulk. We interpret these results in terms of the weaker charge screening and of interference effects induced by the lowering of symmetry at the surface.

Zimmermann B, Mavropoulos P, Heers S, Long NH, Blügel S, Mokrousov Y. Anisotropy of Spin Relaxation in Metals. PHYSICAL REVIEW LETTERS. 2012;109:236603.Abstract

The concept of anisotropy of spin relaxation in nonmagnetic metals with respect to the spin direction of the injected electrons relative to the crystal orientation is introduced. The effect is related to an anisotropy of the Elliott-Yafet parameter, arising from a modulation of the decomposition of the spin-orbit Hamiltonian into spin-conserving and spin-flip terms as the spin quantization axis is varied. This anisotropy, reaching gigantic values for uniaxial transition metals (e.g., 830% for hcp Hf) as density-functional calculations show, is related to extended ``spin-flip hot areas'' on the Fermi surface created by the proximity of extended sheets of the surface, or ``spin-flip hot loops'' at the Brillouin zone boundary, and has no theoretical upper limit. Possible ways of measuring the effect as well as consequences in application are briefly outlined.

2011

Slipukhina I, Mavropoulos P, Blügel S, Lezaic M. Ferromagnetic Spin Coupling of 2p Impurities in Band Insulators Stabilized by an Intersite Coulomb Interaction: Nitrogen-Doped MgO. PHYSICAL REVIEW LETTERS. 2011;107:137203.Abstract

For a nitrogen dimer in insulating MgO, a ferromagnetic coupling between spin-polarized 2p holes is revealed by calculations based on the density functional theory amended by an on-site Coulomb interaction and corroborated by the Hubbard model. It is shown that the ferromagnetic coupling is facilitated by a T-shaped orbital arrangement of the 2p holes, which is in its turn controlled by an intersite Coulomb interaction due to the directionality of the p orbitals. We thus conjecture that this interaction is an important ingredient of ferromagnetism in band insulators with 2p dopants.

Bauer DSG, Mavropoulos P, Lounis S, Blügel S. Thermally activated magnetization reversal in monatomic magnetic chains on surfaces studied by classical atomistic spin-dynamics simulations. JOURNAL OF PHYSICS-CONDENSED MATTER. 2011;23:394204.Abstract

We analyse the spontaneous magnetization reversal of supported monatomic chains of finite length due to thermal fluctuations via atomistic spin-dynamics simulations. Our approach is based on the integration of the Landau-Lifshitz equation of motion of a classical spin Hamiltonian in the presence of stochastic forces. The associated magnetization lifetime is found to obey an Arrhenius law with an activation barrier equal to the domain wall energy in the chain. For chains longer than one domain wall width, the reversal is initiated by nucleation of a reversed magnetization domain primarily at the chain edge followed by a subsequent propagation of the domain wall to the other edge in a random-walk fashion. This results in a linear dependence of the lifetime on the chain length, if the magnetization correlation length is not exceeded. We studied chains of uniaxial and triaxial anisotropy and found that a triaxial anisotropy leads to a reduction of the magnetization lifetime due to a higher reversal attempt rate, even though the activation barrier is not changed.

Sandratskii LM, Mavropoulos P. Magnetic excitations and femtomagnetism of FeRh: A first-principles study. PHYSICAL REVIEW B. 2011;83:174408.Abstract

The paper is partly motivated by recent pump-probe experiments with ultrashort laser pulses on antiferromagnetic FeRh that have shown the generation of magnetization within a subpicosecond time scale. On the other hand, the physical mechanism of the thermal antiferromagnetic-ferromagnetic (AFM-FM) phase transition in FeRh, known for many decades, remains a topic of controversial discussions. The selection of the magnetic degrees of freedom as well as the treatment of the magnetic excited states differ strongly in recent models by different authors. We report a density functional theory (DFT) investigation of FeRh. For the study of excited states, DFT calculations with constraints imposed on the directions and values of the atomic moments are employed. We show that the formation of the Rh moment as a consequence of the AFM-FM phase transition cannot be described within the Stoner picture. Instead, an implicit spin splitting of the Rh states takes place in the AFM phase, resulting in the intra-atomic spin polarization of the Rh atoms. This property is a consequence of the strong hybridization between Rh and Fe states. The Fe-Rh hybridization is an important factor in the physics of FeRh. We demonstrate that the ferromagnetic Fe-Rh exchange interaction is robust with respect to the crystal volume variation, whereas the antiferromagnetic Fe-Fe exchange interaction is strongly volume dependent. These different volume dependencies of the competing exchange interactions lead to their strong compensation at certain crystal volume. We perform Monte Carlo simulations and show that the calculated thermodynamics depends on the way the magnetic degrees of freedom are selected. We argue that the excited states resulting from the variation of the value of the Rh moment treated as degree of freedom are important for both the equilibrium thermodynamics of FeRh and the femtomagnetic phenomena in this system. We also study the spin mixing caused by spin-orbit coupling. The obtained value of the Elliott-Yafet spin-mixing parameter is comparable with earlier calculations for the ferromagnetic 3d metals. We draw the conclusion that the Elliott-Yafet mechanism of the angular-momentum transfer between electrons and lattice plays an important role in the femtomagnetic properties of FeRh.

Lezaic M, Mavropoulos P, Blügel S, Ebert H. Complex magnetic behavior and high spin polarization in Fe3-xMnxSi alloys. PHYSICAL REVIEW B. 2011;83:094434.Abstract

Fe3Si is a ferromagnetic material with possible applications in magnetic tunnel junctions. When doped with Mn, the material shows a complex magnetic behavior, as suggested by older experiments. We employed the Korringa-Kohn-Rostoker Green-function method within density-functional theory in order to study the alloy Fe3-xMnxSi, with 0 <= x <= 1. Chemical disorder is described within the coherent potential approximation. In agreement with experiment, we find that the Mn atoms align ferromagnetically to the Fe atoms, and that the magnetization and Curie temperature drop with increasing Mn concentration x. The calculated spin polarization P at the Fermi level varies strongly with x, from P = -0.3 at x = 0 (ordered Fe3Si) through P = 0 at x = 0.28, to P = +1 for x > 0.75; i.e., at high Mn concentrations the system is half metallic. We discuss the origin of the trends of magnetic moments, exchange interactions, Curie temperature, and the spin polarization.

Meier F, Lounis S, Wiebe J, Zhou L, Heers S, Mavropoulos P, Dederichs PH, Blügel S, Wiesendanger R. Spin polarization of platinum (111) induced by the proximity to cobalt nanostripes. PHYSICAL REVIEW B. 2011;83:075407.Abstract

We measured a spin polarization above a Pt(111) surface in the vicinity of a Co nanostripe by spin-polarized scanning tunneling spectroscopy. The spin polarization exponentially decays away from the Pt-Co interface and is detectable at distances larger than 1 nm. By performing self-consistent ab initio calculations of the electronic structure for a related model system we reveal the interplay between the induced magnetic moments within the Pt surface and the spin-resolved electronic density of states above the surface.

2010

Mavropoulos P, Lounis S, Blügel S. Exchange coupling in transition-metal nanoclusters on Cu(001) and Cu(111) surfaces. PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS. 2010;247:1187-1196.Abstract

We present results of density-functional calculations on the magnetic properties of Cr, Mn, Fe and Co nanoclusters (1-9 atoms large) supported on Cu(001) and Cu(111). The inter-atomic exchange coupling is found to depend on competing mechanisms, namely ferromagnetic double exchange and antiferromagnetic kinetic exchange. Hybridization-induced broadening of the resonances is shown to be important for the coupling strength. The cluster shape is found to affect the coupling via a mechanism that comprises the different orientation of the atomic d-orbitals and the strength of nearest-neighbour hopping. Especially in Fe clusters, a correlation of binding energy and exchange coupling is also revealed. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2009

Costi TA, Bergqvist L, Weichselbaum A, von Delft J, Micklitz T, Rosch A, Mavropoulos P, Dederichs PH, Mallet F, Saminadayar L, et al. Kondo Decoherence: Finding the Right Spin Model for Iron Impurities in Gold and Silver. PHYSICAL REVIEW LETTERS. 2009;102:056802.Abstract

We exploit the decoherence of electrons due to magnetic impurities, studied via weak localization, to resolve a long-standing question concerning the classic Kondo systems of Fe impurities in the noble metals gold and silver: which Kondo-type model yields a realistic description of the relevant multiple bands, spin, and orbital degrees of freedom? Previous studies suggest a fully screened spin S Kondo model, but the value of S remained ambiguous. We perform density functional theory calculations that suggest S = 3/2. We also compare previous and new measurements of both the resistivity and decoherence rate in quasi-one-dimensional wires to numerical renormalization group predictions for S = 1/2, 1, and 3/2, finding excellent agreement for S = 3/2.

Mavropoulos P, Lezaic M, Blügel S. Ferromagnetism in nitrogen-doped MgO: Density-functional calculations. PHYSICAL REVIEW B. 2009;80:184403.Abstract

The magnetic state of nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be spin polarized with a moment of 1 mu(B) per nitrogen atom and to interact ferromagnetically via the double-exchange mechanism in the full concentration range. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The disorder is described within the coherent-potential approximation, with the exchange interactions harvested by the method of infinitesimal rotations. The Curie temperature T-C, calculated within the random-phase approximation, increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single nitrogen atoms, also interstitial nitrogen atoms, dimers and trimers, and their structural relaxations are discussed with respect to the magnetic state. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase in T-C is difficult to achieve, requiring a particular attention to the choice of chemistry.

Slipukhina I, Arras E, Mavropoulos P, Pochet P. Simulation of the enhanced Curie temperature in Mn5Ge3Cx compounds. APPLIED PHYSICS LETTERS. 2009;94:192505.Abstract

Mn5Ge3Cx films with x >= 0.5 were experimentally shown to exhibit a strongly enhanced Curie temperature T-C compared to Mn5Ge3. In this letter we present the results of our first principles calculations within Green's function approach, focusing on the effect of carbon doping on the electronic and magnetic properties of the Mn5Ge3. The calculated exchange coupling constants revealed an enhancement of the ferromagnetic Mn-Mn interactions mediated by carbon. The essentially increased T-C in Mn5Ge3C is well reproduced in our Monte Carlo simulations and together with the decrease of the total magnetization is found to be predominantly of an electronic nature.

2008

Mavropoulos P. Spin injection from Fe into Si(001): Ab initio calculations and role of the Si complex band structure. PHYSICAL REVIEW B. 2008;78:054446.Abstract

We study the possibility of spin injection from Fe into Si(001), using the Schottky barrier at the Fe/Si contact as tunneling barrier. Our calculations are based on density-functional theory for the description of the electronic structure and on a Landauer-Buttiker approach for the current. The current-carrying states correspond to the six conduction-band minima (pockets) of Si, which, when projected on the (001) surface Brillouin zone (SBZ), form five conductance hot spots: one at the SBZ center and four symmetric satellites. The satellites yield a current polarization of about 50%, while the SBZ center can, under very low gate voltage, yield up to almost 100%, showing a zero-gate anomaly. This extremely high polarization is traced back to the symmetry mismatch of the minority-spin Fe wave functions to the conduction-band wave functions of Si at the SBZ center. The tunneling current is determined by the complex band structure of Si in the {[}001] direction, which shows qualitative differences compared to that of direct-gap semiconductors. Depending on the Fermi level position and Schottky barrier thickness, the complex band structure can cause the contribution of the satellites to be orders of magnitude higher or lower than the central contribution. Thus, by appropriate tuning of the interface properties, there is a possibility to cut off the satellite contribution and to reach high injection efficiency. Also, we find that a moderate strain of 0.5% along the {[}001] direction is sufficient to lift the degeneracy of the pockets so that only states at the zone center can carry current.

Lounis S, Reif M, Mavropoulos P, Glaser L, Dederichs PH, Martins M, Blügel S, Wurth W. Non-collinear magnetism of Cr nanostructures on Fe-3ML/Cu(001): First principles and experimental investigations. EPL. 2008;81:47004.Abstract

A combined experimental, using the X-ray magnetic circular dichroism, and theoretical investigation, using the full-potential Korringa-Kohn-Rostoker (KKR) Green function method, is carried out to study the spin structure of small magnetic Cr adatom clusters on the surface of 3 monolayers of fcc Fe deposited on Cu(001). The exchange interaction between the different Cr adatoms as well as between the Cr atoms and the Fe atoms is of antiferromagnetic nature and of comparable magnitude, leading due to frustration to complex non-collinear magnetic configurations. The presence of non-collinear magnetic coupling obtained by ab initio calculations is confirmed by the experimental results. Copyright (C) EPLA, 2008.

2007

Lounis S, Mavropoulos P, Zeller R, Dederichs PH, Blügel S. Noncollinear magnetism of Cr and Mn nanoclusters on Ni(111): Changing the magnetic configuration atom by atom. PHYSICAL REVIEW B. 2007;75:174436.Abstract

The Korringa-Kohn-Rostoker Green-function method for noncollinear magnetic structures was applied on Mn and Cr nanoclusters deposited on the Ni(111) surface. We consider various dimers, trimers, and tetramers. We obtain collinear and noncollinear magnetic solutions, brought about by the competition of antiferromagnetic interactions. It is found that the triangular geometry of the Ni(111) substrate, together with the intracluster antiferromagnetic interactions, is the main cause of the noncollinear states, which are secondarily affected by the cluster-substrate exchange interactions. The stabilization energy of the noncollinear, compared to the collinear, states is calculated to be typically of the order of 100 meV/atom, while multiple local-energy minima are found, corresponding to different noncollinear states, differing typically by 1-10 meV/atom. Open structures exhibit sizable total moments, while compact clusters tend to have very small total moments, resulting from the complex frustration mechanisms in these systems.

Lezaic M, Mavropoulos P, Blügel S. First-principles prediction of high Curie temperature for ferromagnetic bcc-Co and bcc-FeCo alloys and its relevance to tunneling magnetoresistance. APPLIED PHYSICS LETTERS. 2007;90:082504.Abstract

{The authors determine from first principles the Curie temperature T-C for bulk Co in the hcp, fcc, bcc, and body-centered-tetragonal (bct) phases, for FeCo alloys, and for bcc and bct Fe. For bcc Co

Mavropoulos P, Galanakis I. A review of the electronic and magnetic properties of tetrahedrally bonded half-metallic ferromagnets. JOURNAL OF PHYSICS-CONDENSED MATTER. 2007;19:315221.Abstract

The emergence of the field of spintronics has brought half-metallic ferromagnets to the centre of scientific research. A lot of interest was focused on newly created transition-metal pnictides (such as CrAs) and chalcogenides (such as CrTe) in the metastable zinc-blende lattice structure. These compounds were found to have the advantage of high Curie temperatures in addition to their structural similarity to semiconductors. Significant theoretical activity has been devoted to the study of the electronic and magnetic properties of these compounds in an effort to achieve a better control of their experimental behaviour in realistic applications. This paper is devoted to an overview of the studies of these compounds, with emphasis on theoretical results, covering their bulk properties (electronic structure, magnetism, stability of the zinc-blende phase, stability of ferromagnetism) as well as low-dimensional structures (surfaces, interfaces, nanodots and transition-metal delta-doped semiconductors) and phenomena that can possibly destroy the half-metallic property, like structural distortions or defects.

Galanakis I, Mavropoulos P. Spin-polarization and electronic properties of half-metallic Heusler alloys calculated from first principles. JOURNAL OF PHYSICS-CONDENSED MATTER. 2007;19:315213.Abstract

Half-metallic Heusler alloys are amongst the most promising materials for future magneto-electronic applications. We review some recent results on the electronic properties of these compounds. The origin of the gap in these half-metallic alloys and its connection to the magnetic properties are well understood. Changing the lattice parameter slightly shifts the Fermi level. Spin-orbit coupling induces states within the gap but the alloys keep a very high degree of spin polarization at the Fermi level. Small degrees of doping and disorder as well as defects with low formation energy have little effect on the properties of the gap, while temperature effects can lead to a quick loss of half-metallicity. Finally, we discuss two special issues: the case of quaternary Heusler alloys and the half-metallic ferrimagnets.

2006

Lounis S, Mavropoulos P, Dederichs PH, Blügel S. Surface-state scattering by adatoms on noble metals: Ab initio calculations using the Korringa-Kohn-Rostoker Green function method. PHYSICAL REVIEW B. 2006;73:195421.Abstract

When surface-state electrons scatter at perturbations, such as magnetic or nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized at the adatom site, can develop below the bottom of the surface-state band for both spin channels. In the case of adatoms, these states have been found very recently in scanning tunneling spectroscopy experiments for the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried out a systematic theoretical investigation of the electronic structure of these surface states in the presence of magnetic and nonmagnetic atoms on Cu(111). We found that Ca and all 3d adatoms lead to a split-off state at the bottom of the surface band which is, however, not seen for the sp elements Ga and Ge. The situation is completely reversed if the impurities are embedded in the surface: Ga and Ge are able to produce a split-off state whereas the 3d impurities are not. The resonance arises from the s state of the impurities and is explained in terms of strength and the interaction nature (attraction or repulsion) of the perturbing potential.

Galanakis I, Mavropoulos P, Dederichs PH. Electronic structure and Slater-Pauling behaviour in half-metallic Heusler alloys calculated from first principles. JOURNAL OF PHYSICS D-APPLIED PHYSICS. 2006;39:765-775.Abstract

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to their high Curie temperatures and their structural similarity to binary semiconductors. In this review we present all overview of the basic electronic and magnetic properties of both Heusler families: the so-called half-Heusler alloys like NiMnSb and the full-Heusler alloys like Co2MnGe. Ab initio results suggest that both the electronic and magnetic properties ill these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment M-t scales linearly with the number of the valence electrons Z(t), such that M-t = Z(t) - 24 for the full-Heusler and M-t = Z(t) - 18 for the half-Heusler alloys, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.

Lezaic M, Mavropoulos P, Bihlmayer G, Blugel S. Scanning tunnelling microscopy of surfaces of half-metals: an ab-initio study on NiMnSb(001). JOURNAL OF PHYSICS D-APPLIED PHYSICS. 2006;39:797-802.Abstract

We present a first-principles Study of the unreconstructed (001) surfaces of the half-metallic ferromagnet NiMnSb. Both terminations (MnSb and Ni) are considered. We find that half-metallicity is lost at the surfaces. After a discussion of the geometric relaxations and the spin-polarized surface band structure, we focus on topography images which are expected to be found with spin-polarized scanning tunnelling microscopy. For the MnSb-terrninated surface we find that only the Sb atoms are visible, reflecting a geometric buckling caused by relaxations. For the Ni-terminated surface we find a strong contrast between the images of forward and reverse tip-sample-bias of 0.5 eV, as well as a stripe-like image for reverse bias. We interpret these findings in terms of highly directional Surface states which are formed in the spin-down gap region.

Mavropoulos P, Lounis S, Zeller R, Blugel S. Fe clusters on Ni and Cu: size and shape dependence of the spin moment. APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING. 2006;82:103-107.Abstract

We present ab-initio calculations of the electronic structure of small Fe clusters (1-9 atoms) on Ni( 001), Ni( 111), Cu( 001) and Cu( 111) surfaces. Our focus is on the spin moments and their dependence on cluster size and shape. We derive a simple quantitative rule that relates the moment of each Fe atom linearly to its coordination number. Thus, for an arbitrary Fe cluster the spin moment of the cluster and of the individual Fe atoms can be readily found if the positions of the atoms are known.

Moras P, Ferrari L, Spezzani C, Gardonio S, Lezaic M, Mavropoulos P, Blügel S, Carbone C. Probing quasiparticle states bound by disparate periodic potentials. PHYSICAL REVIEW LETTERS. 2006;97:206802.Abstract

Thin films of Ag(111) with two-dimensional crystallinity of large lateral coherence grow on Ge(111), free of in-plane registry with the underlying substrate. Ag s-p electrons forming two-dimensional quantum well states scatter coherently at the buried interface potential, resulting in an unexpected set of new quasiparticle states, as observed by angle-resolved photoemission. These new features originate from interactions among Ag quantum well bands, gaining a momentum equivalent to a reciprocal vector of the substrate lattice.

Chioncel L, Mavropoulos P, Lezaic M, Blügel S, Arrigoni E, Katsnelson MI, Lichtenstein AI. Half-metallic ferromagnetism induced by dynamic electron correlations in VAs. PHYSICAL REVIEW LETTERS. 2006;96:197203.Abstract

The electronic structure of the VAs compound in the zinc-blende structure is investigated using a combined density-functional and dynamical mean-field theory approach. Contrary to predictions of a ferromagnetic semiconducting ground state obtained by density-functional calculations, dynamical correlations induce a closing of the gap and produce a half-metallic ferromagnetic state. These results emphasize the importance of dynamic correlations in materials suitable for spintronics.

Lezaic M, Mavropoulos P, Enkovaara J, Bihlmayer G, Blügel S. Thermal collapse of spin polarization in half-metallic ferromagnets. PHYSICAL REVIEW LETTERS. 2006;97:026404.Abstract

We propose two novel approaches to study the temperature dependence of the magnetization and the spin polarization at the Fermi level in magnetic compounds, and apply them to half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab initio calculations. The thermal properties are studied by mapping ab initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.

2005

Dederichs PH, Galanakis I, Mavropoulos P. Half-metallic alloys: electronic structure, magnetism and spin polarization. JOURNAL OF ELECTRON MICROSCOPY. 2005;54:I52-I55.Abstract

Using the state-of-the-art screened Korringa-Kohn-Rostoker Green function method we study the electronic and magnetic properties of NiMnSb and similar Heusler alloys. We show that all these compounds are half-metals, e.g. the minority-spin band is semiconducting and the Fermi level falls within this gap resulting in 100% spin polarization at the Fermi level. The total spin moment M-t shows the so-called Slater-Pauling behaviour and scales with the total valence charge Z(t) following the rule M-t = Z(t) - 18 for half and M-t = Z(t) - 24 for full Heusler alloys. These rules are connected to the origin of the gap. Finally we show that the inclusion of the spin-orbit interaction in our calculations kills the half-metallic gap but the spin-polarization at the Fermi level can be still very high, similar to 99% for NiMnSb, but much lower for a half-metallic compound like zinc-blende MnBi (77%).

Mavropoulos P, Lezaic M, Blugel S. Half-metallic ferromagnets for magnetic tunnel junctions by ab initio calculations. PHYSICAL REVIEW B. 2005;72:174428.Abstract

Using theoretical arguments, we show that, in order to exploit half-metallic ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to eliminate interface states at the Fermi level within the half-metallic gap; contrary to this, no such problem arises in giant magnetoresistance elements. Moreover, based on an a priori understanding of the electronic structure, we propose an antiferromagnetically coupled TMR element based on half-metallic zinc-blende chalcogenides, in which interface states are eliminated, as a paradigm of materials design from first principles. Our conclusions are supported by ab initio calculations.

Lounis S, Mavropoulos P, Dederichs PH, Blugel S. Noncollinear Korringa-Kohn-Rostoker Green function method: Application to 3d nanostructures on Ni(001). PHYSICAL REVIEW B. 2005;72:224437.Abstract

Magnetic nanostructures on nonmagnetic or magnetic substrates have attracted strong attention due to the development of interesting experimental methods with atomic resolution. Motivated by this progress we have extended the full-potential Korringa-Kohn-Rostoker Green-function method to treat noncollinear magnetic nanostructures on surfaces. We focus on magnetic 3d impurity nanoclusters, sitting as adatoms on or in the first surface layer on Ni(001), and investigate the size and orientation of the local moments and, moreover, the stabilization of noncollinear magnetic solutions. While clusters of Fe, Co, Ni atoms are magnetically collinear, noncollinear magnetic coupling is expected for Cr and Mn clusters on surfaces of elemental ferromagnets. The origin of frustration is the competition of the antiferromagnetic exchange coupling among the Cr or Mn atoms with the antiferromagnetic (for Cr) or ferromagnetic (for Mn) exchange coupling between the impurities and the substrate. We find that Cr and Mn first-neighboring dimers and a Mn trimer on Ni(001) show noncollinear behavior nearly degenerate with the most stable collinear configuration. Increasing the distance between the dimer atoms leads to a collinear behavior, similar to the one of the single impurities. Finally, we compare some of the noncollinear ab initio results to those obtained within a classical Heisenberg model, where the exchange constants are fitted to total energies of the collinear states; the agreement is surprisingly good.

2004

Mavropoulos P, Papanikolaou N, Dederichs PH. Korringa-Kohn-Rostoker Green-function formalism for ballistic transport. PHYSICAL REVIEW B. 2004;69:125104.Abstract

We develop a method for the calculation of ballistic transport from first principles. The multiple scattering screened Korringa-Kohn-Rostoker (KKR) method is combined with a Green-function formulation of the Landauer approach for the ballistic transport. We obtain an efficient O(N) algorithm for the calculation of ballistic conductance through a scattering region connected to semi-infinite crystalline leads. In particular we generalize the results of Baranger and Stone in the case of Bloch wave boundary conditions and, we discuss relevant properties of the S matrix. We consider the implications on the application of the formalism in conjunction with a cellular multiple scattering description of the electronic structure; and demonstrate the convergence properties concerning the angular momentum expansions.

Mavropoulos P, Sato K, Zeller R, Dederichs PH, Popescu V, Ebert H. Effect of the spin-orbit interaction on the band gap of half metals. PHYSICAL REVIEW B. 2004;69:054424.Abstract

The spin-orbit interaction can cause a nonvanishing density of states (DOS) within the minority-spin band gap of half metals around the Fermi level. We examine the magnitude of the effect in Heusler alloys, zinc-blende half metals, and diluted magnetic semiconductors, using first-principles calculations. We find that the ratio of spin-down to spin-up DOS at the Fermi level can range from below 1% (e.g., 0.5% for NiMnSb) over several percents {[}4.2% for (Ga,Mn)As] to 13% for MnBi. This gives a spin polarization P at E-F ranging from above 99% for NiMnSb to Papproximate to92% for (Ga,Mn)As and to Papproximate to77% for MnBi.

Mavropoulos P, Galanakis I, Dederichs PH. Multilayers of zinc-blende half-metals with semiconductors. JOURNAL OF PHYSICS-CONDENSED MATTER. 2004;16:4261-4272.Abstract

We report on first-principles calculations for multilayers of zinc-blende half-metallic ferromagnets CrAs and CrSb with III-V and II-VI semiconductors, in the {[}001] orientation. We examine the ideal and tetragonalized structures, as well as the case of an intermixed interface. We find that, as a rule, half-metallicity can be conserved throughout the heterostructures, provided that the character of the local coordination and bonding is not disturbed. We describe a mechanism operative at the interfaces with semiconductors that can also give a non-integer spin moment per interface transition atom, and derive a simple rule for evaluating it.

Wunnicke O, Mavropoulos P, Zeller R, Dederichs PH. Ballistic spin injection from Fe into ZnSe(001), (111), and (110), and into GaAs(001). JOURNAL OF PHYSICS-CONDENSED MATTER. 2004;16:4643-4659.Abstract

We present first-principles calculations of ballistic spin injection in Fe/GaAs and Fe/ZnSe junctions with orientations (00 1), (111), and (I 10). We find that the symmetry mismatch of the Fe minority spin states with the semiconductor conduction states can lead to extremely high spin polarization of the current through the (001) interface for hot and thermal injection processes. Such a symmetry mismatch does not exist for the (I 11) and (I 10) interfaces, where smaller spin injection efficiencies are found. The presence of interface states at the Fermi energy is found to lower the current spin polarization.

Mavropoulos P, Galanakis I, Popescu V, Dederichs PH. The influence of spin-orbit coupling on the band gap of Heusler alloys. JOURNAL OF PHYSICS-CONDENSED MATTER. 2004;16:S5759-S5762.Abstract

The band gap of half-metallic ferromagnets can be affected by the spin-orbit coupling, which introduces there a small, but non-vanishing, density of states. We study this effect in the case of Heusler alloys. We find that, as a rule, the spin polarization in the middle of the gap decreases for compounds of heavier elements.

2003

Galanakis I, Mavropoulos P. Zinc-blende compounds of transition elements with N, P, As, Sb, S, Se, and Te as half-metallic systems. PHYSICAL REVIEW B. 2003;67:104417.Abstract

We report systematic first-principles calculations for ordered zinc-blende compounds of the transition metal elements V, Cr, and Mn with the sp elements N, P, As, Sb, S, Se, and Te, motivated by a recent fabrication of zinc-blende CrAs, CrSb, and MnAs. They show a ferromagnetic half-metallic behavior for a wide range of lattice constants. We discuss the origin and trends of half-metallicity, present the calculated equilibrium lattice constants, and examine the half-metallic behavior of their transition element terminated (001) surfaces.

Wunnicke O, Mavropoulos P, Dederichs PH. Spin injection: Interface resistance in Fe/semiconductor junctions calculated from first principles. JOURNAL OF SUPERCONDUCTIVITY. 2003;16:171-173.Abstract

We calculate the current spin polarization and the interface resistance of Fe/GaAs and Fe/ZnSe (001) spin injection junctions from first principles, including also the possibility of a Schottky barrier. From our results of interface resistance we estimate the barrier thickness needed for efficient spin injection if the process is nonballistic.

Mavropoulos P. Coordination dependence of hyperfine fields of 5sp impurities on Ni surfaces. JOURNAL OF PHYSICS-CONDENSED MATTER. 2003;15:8115-8122.Abstract

We present first-principles calculations of the magnetic hyperfine fields H-hf of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the dependence of H-hf on the coordination number by placing the impurity in the surfaces, on top of them at the adatom positions, and in the bulk. We find a strong coordination dependence of H-hf, different and characteristic for each impurity. The behaviour is explained in terms of the on-site s-p hybridization as the symmetry is reduced at the surface. Our results are in agreement with recent experimental findings.

Wunnicke O, Mavropoulos P, Dederichs PH. Spin injection in ballistic Fe-ZnSe and Fe-GaAs(001) junctions. PHASE TRANSITIONS. 2003;76:511-521.Abstract

We present ab initio calculations for spin injection in Fe-ZnSe and Fe-GaAs(001) systems, with and without detection by a second Fe lead. We consider the case of hot injection, as well as the presence of a tunneling barrier at the interface. Our calculations are valid in the ballistic regime. We find that these systems can be very efficient spin filters, leading to current spin polarizations and magnetoresistance ratios very close to the ideal 100%.

2002

Wunnicke O, Mavropoulos P, Zeller R, Dederichs PH, Grundler D. Ballistic spin injection from Fe(001) into ZnSe and GaAs. PHYSICAL REVIEW B. 2002;65:241306.Abstract

We consider the spin injection from Fe into ZnSe and GaAs in the ballistic limit. By means of the ab initio screened Korringa-Kohn-Rostoker method we calculate the ground-state properties of epitaxial Fe\textbackslash{}ZnSe(001) and Fe\textbackslash{}GaAs(001) heterostructures. Three injection processes are considered: injection of hot electrons and injection of ``thermal{''} electrons with and without an interface barrier. The calculation of the conductance by the Landauer formula shows that these interfaces act like a nearly ideal spin filter, with spin polarization as high as 99%. This can be traced back to the symmetry of the band structure of Fe for normal incidence.

Mavropoulos P, Wunnicke O, Dederichs PH. Ballistic spin injection and detection in Fe/semiconductor/Fe junctions. PHYSICAL REVIEW B. 2002;66:024416.Abstract

We present ab initio calculations of the spin-dependent electronic transport in Fe/GaAs/Fe and Fe/ZnSe/Fe (001) junctions simulating the situation of a spin-injection experiment. We follow a ballistic Landauer-Buttiker approach for the calculation of the spin-dependent dc conductance in the linear-response regime, in the limit of zero temperature. We show that the bulk band structure of the leads and of the semiconductor, and even more the electronic structure of a clean and abrupt interface, are responsible for a current polarization and a magnetoresistance ratio of almost the ideal 100%, if the transport is ballistic. In particular, we study the significance of the transmission resonances caused by the presence of two interfaces.

Freyss M, Mavropoulos P, Papanikolaou N, Bellini V, Zeller R, Dederichs PH. Tunneling transport in Ferromagnet/Insulator/Ferromagnet junctions. PHASE TRANSITIONS. 2002;75:159-166.Abstract

We study the spin-dependent transport in expitaxial Ferromagnet/Insulator/Ferromagnet junctions. Firstly we show that the tunneling through the insulator can be described by the complex band structure of the insulator in the gap region, i.e. by the metal-induced gap states. Since the imaginary part of the Bloch vector describes the decay of the wave function, we calculate the spectrum of the decay parameters K for several semiconductors. For large thicknesses the state with the smallest K-value dominates the current. In the second part we present the results of ground state calculation for Fe/ZnSe/Fe(001) and related junctions. We obtain a rather localized charge transfer from the interface Fe layer to the neighbouring semiconductor layer, which is largest for the low-valent termination. Moreover we find that the local moments at the interface depend sensitively on the lattice parameter chosen. Finally, we show that in the minority band at E(F) an Fe interface state exists, which deeply penetrates into the barrier.

Dederichs PH, Mavropoulos P, Wunnicke O, Papanikolaou N, Bellini V, Zeller R, Drchal V, Kudrnovsky J. Importance of complex band structure and resonant states for tunneling. JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS. 2002;240:108-113.Abstract

The paper aims at understanding the tunneling process in epitaxial magnetic tunnel junctions. Firstly, we stress the importance of the complex band structure of the insulator for the tunneling of the metal electrons. For large insulator thicknesses the tunneling current is carried by very few states, i.e., those states in the gap of the semiconductor having the smallest imaginary component of the k-vector. In the case of GaAs, ZnSe and MgO these are Delta(1)-states at the Gamma-point. Secondly, we discuss the role of resonant interface states for tunneling. Based on simple model calculations and ab initio results we demonstrate that for symmetrical barriers the minority conductance can be dominated in an intermediate thickness range by few `hot spots' in the surface Brillouin zone, arising from resonant interface states. In these hot spots full transmission can still be obtained, when all other states are already strongly attenuated, so that the usual exponential decay can be considerably delayed. (C) 2002 Elsevier Science B.V. All rights reserved.

2000

Mavropoulos P, Papanikolaou N, Dederichs PH. Complex band structure and tunneling through ferromagnet/insulator/ferromagnet junctions. PHYSICAL REVIEW LETTERS. 2000;85:1088-1091.Abstract

We investigate the importance of metal-induced gap states for the tunneling of metal electrons through epitaxial insulator films. By introducing an imaginary part kappa to the wave vector in order to describe the decay of the wave function in the insulator, we obtain the complex band structure in the gap region. The spectrum of the decay parameters kappa is calculated for the semiconductors Si, Ge, GaAs, and ZnSe. in most cases, for large enough film thicknesses the tunneling is dominated by states of normal incidence on the interface. Possible exceptions are considered. Based on our conclusions, we discuss the spin-dependent tunneling in Fe/semiconductor/Fe (001) junctions.

1998

Mavropoulos P, Stefanou N, Nonas B, Zeller R, Dederichs PH. Hyperfine fields of probe atoms on the (001) surface of Ni. PHILOSOPHICAL MAGAZINE B-PHYSICS OF CONDENSED MATTER STATISTICAL MECHANICS ELECTRONIC OPTICAL AND MAGNETIC PROPERTIES. 1998;78:435-440.Abstract

We present first-principles calculations of the electronic structure and hyperfine fields of 3d and 4sp impurities on the (001) surface of Ni. The calculations are based on the local-spin-density-functional theory and employ a Korringa-Kohn-Rostoker Green's function method for impurities at surfaces. The systematic behaviour obtained for the hyperfine fields of the 4sp adatoms or impurities in the first surface layer is completely different from that found in the bulk. Instead of a single maximum with a very large hyperfine-field value at about the end of an sp series, the adatoms exhibit two maxima with a pronounced minimum in between. This behaviour can be traced back to the reduced coordination number of the adatoms which leads to a much smaller relative splitting of the bonding and antibonding peaks, and to the lower symmetry at the surface which results in an on-site s-p(z) hybridization. The hyperfine fields found for the 3d impurities at the surface are determined basically by the ferromagnetic or antiferromagnetic coupling of the local impurity moment to the substrate magnetization and are therefore more or less similar to those for bulk impurities.

Mavropoulos P, Papanikolaou N, Stefanou N, Apostolopoulos G, Boukos N, Papastaikoudis C. Low-field Hall coefficient of Al-4d dilute alloys: The role of the anisotropic impurity scattering. SOLID STATE COMMUNICATIONS. 1998;106:405-408.Abstract

The role of the anisotropic impurity scattering in the determination of the low-field Hall coefficient of Al-4d dilute alloys is investigated by means of systematic theoretical calculations, as well as experimental measurements for Al-Zr and AI-Mo. The theoretical results, obtained without using any adjustable parameter, are in excellent agreement with the experimental data and a consistent interpretation of the systematic variation of the low-field Hall coefficient for aluminium-based dilute alloys with transition-metal impurities is given. (C) 1998 Elsevier Science Ltd. All rights reserved.

Mavropoulos P, Stefanou N, Papanikolaou N. Magnetic impurity states in simple metals: A study of the spin-polarization energy. PHYSICAL REVIEW B. 1998;58:1096-1099.Abstract

We report a systematic study of the spin-polarization energy of 3d impurities in monovalent simple-metal hosts, by means of self-consistent, local-spin-density-functional, impurity-in-jellium calculations, and propose a phenomenological model for the interpretation of our results. {[}S0163-1829(98)03424-9].

Mavropoulos P, Stefanou N, Nonas B, Zeller R, Dederichs PH. Hyperfine fields of sp impurities on Ni and Fe surfaces. PHYSICAL REVIEW LETTERS. 1998;81:1505-1508.Abstract

We present first-principles calculations of the electronic structure and hyperfine fields of 4sp impurities un the (001) surfaces of Ni and Fe. The calculations are based on the local-spin-density functional theory and employ a Green's function method for impurities at surfaces. The systematic behavior obtained for the hyperfine fields of adatoms or impurities in the first surface layer is completely different from that found in the bulk, mainly due to the reduction of the symmetry and the coordination number at the surface. Our results explain the surprisingly small hyperfine-field values measured for Se adatoms and provide challenging predictions to be confirmed by future experiments.

1997

Mavropoulos P, Stefanou N. Low-field galvanomagnetic properties of aluminium-based dilute alloys. JOURNAL OF PHYSICS-CONDENSED MATTER. 1997;9:8997-9006.Abstract

We report a systematic study of low-field galvanomagnetic properties of aluminium-based dilute alloys with 3d and 4sp impurities. The low-field magnetoresistivity tensor is determined by exactly solving the linearized Boltzmann equation for the anisotropic vector mean free path, without using any adjustable parameter. Our method of calculation is based on the on-Fermi-sphere approximation which allows us to combine the full anisotropy of the host Fermi surface, obtained by the four-orthogonal-plane-wave method, with the impurity scattering phase shifts, evaluated by self-consistent local-density-functional impurity-in-jellium calculations. Our results for the Hall coefficient and the magnetoresistance are in good agreement with the experimental data.

1995

Mavropoulos P, Papanikolaou N, Stefanou N. LOW-TEMPERATURE THERMOPOWER OF AL-BASED DILUTE ALLOYS. JOURNAL OF PHYSICS-CONDENSED MATTER. 1995;7:4665-4671.Abstract

We report systematic calculations of the low-temperature diffusion thermopower of Al-based dilute alloys with 3d and 4sp impurities, by solving self-consistently the linearized Boltzmann equation. The impurity scattering is described by the phase shifts obtained from self-consistent local-density-functional impurity-in-jellium calculations. Moreover, the influence of the full anisotropy of the Al Fermi surface on the scattering process is taken into account within the on-Fermi-sphere approximation. Our results explain successfully the experimentally measured variations in the thermoelectric power, except for Mn impurities. In this case, the presence of a narrow many-body resonance at the Fermi level in the localized-spin-fluctuations regime seems to be responsible for the large negative value of thermopower observed.