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The electronic and thermal transport properties of the Earth’s core are crucial for many geophysical models such as the geodynamo model of the Earth’s magnetic field. We show by first-principles modeling and methods of statistical physics that the spin disorder, not considered in previous studies, gives an essential contribution to the electrical resistivity at the Earth’s core conditions. The origin of this spin-disorder resistivity (SDR) consists in the existence of fluctuating local moments that are stabilized at high temperatures by the magnetic entropy even at pressures at which the ground state of iron is non-magnetic. It turns out that the contributions of various scattering mechanisms (alloy disorder, phonon scattering, spin disorder, and electron-electron correlations) are comparable, but not additive. Here we report results for iron and iron-rich alloys (Fe-O, Fe-Si, Fe-S) that can be present in the Earth’s core. Special attention is paid to alloys with two magnetic elements (Fe-Ni).

We present an ab initio theory of the spin-wave stiffness tensor for ordered and disordered itinerant ferromagnets with pair exchange interactions derived from a method of infinitesimal spin rotations. The resulting formula bears an explicit form of a linear-response coefficient which involves one-particle Green's functions and effective velocity operators encountered in a recent theory of electron transport. Application of this approach to ideal metal crystals yields more reliable values of the spin stiffness than traditional ill-convergent real-space lattice summations. The formalism can also be combined with the coherent potential approximation for an effective-medium treatment of random alloys, which leads naturally to an inclusion of disorder-induced vertex corrections to the spin stiffness. The calculated concentration dependence of the spin-wave stiffness of random fcc Ni-Fe alloys can be ascribed to a variation of the reciprocal value of alloy magnetization. Calculations for random iron-rich bcc Fe-Al alloys reveal that their spin-wave stiffness is strongly reduced owing to the atomic ordering; this effect takes place due to weakly coupled local magnetic moments of Fe atoms surrounded by a reduced number of Fe nearest neighbors., Comment: 10 pages, 4 figures, 2 tables; text slightly extended; accepted in Phys. Rev. B

We present an ab initio theory of the Gilbert damping in ferromagnetic alloys with substitutional disorder. The theory is based on nonlocal torques that are represented by nonrandom, site-off-diagonal, and spin-independent matrices, which simplifies the configuration averaging. The formalism is developed for the relativistic tight-binding linear muffin-tin orbital (TB-LMTO) method and the coherent potential approximation (CPA). The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on random local torques. The theory is illustrated by calculations for various random transition metal alloys: FeNi, FeCo, Heusler alloys, permalloy with impurities, Fe with vacancies, and stoichiometric FePt and CoPt alloys with a varying degree of L1 0 atomic long-range order. Results are in a reasonable agreement with other calculations and accessible experimental data.

We present an extension of the relativistic electron transport theory for the standard (charge) conductivity tensor of random alloys within the tight-binding linear muffin-tin orbital method to the so-called spin-dependent conductivity tensor, which describes the Kubo linear response of spin currents to external electric fields. The approach is based on effective charge- and spin-current operators, that correspond to intersite electron transport and that are nonrandom, which simplifies the configuration averaging by means of the coherent potential approximation. Special attention is paid to the Fermi sea term of the spin-dependent conductivity tensor, which contains a nonzero incoherent part, in contrast to the standard conductivity tensor. The developed formalism is applied to the spin Hall effect in binary random nonmagnetic alloys, both on a model level and for Pt-based alloys with an fcc structure. We show that the spin Hall conductivity consists of three contributions (one intrinsic and two extrinsic) which exhibit different concentration dependences in the dilute limit of an alloy. Results for selected Pt alloys (Pt-Re, Pt-Ta) lead to the spin Hall angles around 0.2; these sizable values are obtained for compositions that belong to thermodynamically equilibrium phases. These alloys can thus be considered as an alternative to other systems for efficient charge to spin conversion, which are often metastable crystalline or amorphous alloys., Comment: 15 pages, 6 figures, Supplemental Material included

Pressure variations of the Curie temperature of the 5f ferromagnet β-UH3 were studied using the Mo-alloyed hydride (UH3)0.82Mo0.18, which is stable in air and has very similar TC and magnetization per U atom. By means of ac magnetic susceptibility a linear decrease of TC was observed for pressures up to 3.2 GPa. The coefficient dTC/dp = −2.05 K/GPa gives dlnTC/dp = 1/T*dTC/dp ≈ −0.011 GPa−1. This value is smaller than expected for a 5f-band ferromagnet with relatively short U-U distances and suggests that UH3 may be more localized than expected. Among AnX compounds, similar dependence was found e.g. for US. Revisiting existing data on lattice elasticity for β-UH3, bulk modulus B ≈ 100 GPa can be assumed, leading to dlnTC/dlnV = 1.1. Experimental data are confronted with results of GGA + U electronic structure calculations. Plausible values of direct Coulomb U and Hund’s exchange J are deduced. The lattice compression was found to reduce predominantly the orbital moments.

The electronic and galvanomagnetic properties of the equiatomic quaternary Heusler alloy CoMnFeSi, which was recently synthesized and which crystallizes in the cubic LiMgPdSb structure, are studied from first principles. We concentrate on two problems: (i) the origin of the alloy disorder, which was found to exist in this alloy, and (ii) the evaluation of basic galvanomagnetic properties such as the dc conductivity ${\ensuremath{\sigma}}_{\mathrm{tot}}$ and the anomalous Hall conductivity(AHC) for swap defects, accompanied by a comparison of the calculated results with a recent experiment. The origin of the alloy disorder is investigated by estimating corresponding formation energies of possible swap defects which preserve the sample stoichiometry. We discuss various defects with respect to the sample half metallicity. The most favorable swap types are those for which the calculated ${\ensuremath{\sigma}}_{\mathrm{tot}}$ and AHC agree reasonably with the experiment. On the basis of calculated transport quantities ${\ensuremath{\sigma}}_{\mathrm{tot}}$ (AHC), the two most favorable swap types can be identified, namely, (i) the Co-Fe swaps, which preserve the half metallicity and have the lowest formation energy, and (ii) the Co-Mn swaps, which violate the half metallicity but have larger formation energy.

Antiferromagnets are magnetically ordered materials which exhibit no net moment and thus are insensitive to magnetic fields. Antiferromagnetic spintronics aims to take advantage of this insensitivity for enhanced stability, while at the same time active manipulation up to the natural THz dynamic speeds of antiferromagnets is possible, thus combining exceptional storage density and ultra-fast switching. However, the active manipulation and read-out of the N\'eel vector (staggered moment) orientation is challenging. Recent predictions have opened up a path based on a new spin-orbit torque, which couples directly to the N\'eel order parameter. This N\'eel spin-orbit torque was first experimentally demonstrated in a pioneering work using semimetallic CuMnAs. Here we demonstrate for Mn$_2$Au, a good conductor with a high ordering temperature suitable for applications, reliable and reproducible switching using current pulses and readout by magnetoresistance measurements. The symmetry of the torques agrees with theoretical predictions and a large read-out magnetoresistance effect of more than $\simeq 6$~$\%$ is reproduced by ab initio transport calculations., Comment: 5 pages, 4 figures

We study the relativistic effects on electron transport in spin-polarized metals and random alloys on ab initio level using the fully relativistic tight-binding linear muffin-tin-orbital (TB-LMTO) method. We employ a Kubo linear-response approach adapted to disordered multisublattice systems in which the chemical disorder is described in terms of the coherent potential approximation (CPA). The CPA vertex corrections are included. We calculate both the Fermi surface and Fermi sea terms of the full conductivity tensor. We find that in cubic ferromagnetic 3d transition metals (Fe, Co, Ni) and their random binary alloys (Ni-Fe, Fe-Si) the Fermi sea term in the anomalous Hall conductivity is small in comparison with the Fermi surface term, however, in more complicated structures, such as hexagonal Co and selected Co-based Heusler alloys, it becomes important. We find an overall good agreement between the theory and experimental data.

The electronic and thermal transport properties of the Earth's core are crucial for many geophysical models such as the geodynamo model of the Earth's magnetic field and of its reversals. Here we show, by considering bcc-iron and iron-rich iron-silicon alloy as a representative of the Earth's core composition and applying the first-principles modeling that the spin disorder at the Earth's core conditions provides an essential contribution, of order 20~$\mu\Omega$\,cm, to the electrical resistivity. This value is comparable in magnitude with the electron-phonon and with the recently estimated electron-electron scattering contributions. The origin of the spin-disorder resistivity (SDR) consists in the existence of fluctuating local moments that are stabilized at high temperatures by the magnetic entropy even at pressures at which the ground state of iron is non-magnetic. We find that electron-phonon and SDR contributions are not additive at high temperatures. We thus observe a large violation of the Matthiessen rule, not common in conventional metallic alloys at ambient conditions., Comment: 4 figures

The spin-wave stiffness D of ferromagnetic systems represents an important micromagnetic parameter closely related to exchange interactions between local magnetic moments.

Physical properties induced by exchange interactions (Curie temperature and spin stiffness) and spin-orbit coupling (anomalous Hall effect, anisotropic magnetoresistance, and Gilbert damping) in the diluted (Ga,Mn)As ferromagnetic semiconductor are studied from first principles. Recently developed Kubo-Bastin transport theory and nonlocal torque operator formulation of the Gilbert damping as formulated in the tight-binding linear muffin-tin orbital method are used. The first-principles Liechtenstein mapping is employed to construct an effective Heisenberg Hamiltonian and to estimate Curie temperature and spin stiffness in the real-space random-phase approximation. Good agreement of calculated physical quantities with experiments on well-annealed samples containing only a small amount of compensating defects is obtained.

We present a first-principles study of the electronic, magnetic, and transport properties of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ doped with Mn atoms in substitutional $({\mathrm{Mn}}_{\mathrm{Bi}})$ and interstitial van der Waals gap positions $({\mathrm{Mn}}_{i})$, which act as acceptors and donors, respectively. The effect of native ${\mathrm{Bi}}_{\mathrm{Te}}$- and ${\mathrm{Te}}_{\mathrm{Bi}}$-antisite defects and their influence on calculated electronic transport properties is also investigated. We have studied four models representing typical cases, namely, (i) ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ with and without native defects, (ii) ${\mathrm{Mn}}_{\mathrm{Bi}}$ defects with and without native defects, (iii) the same, but for ${\mathrm{Mn}}_{i}$ defects, and (iv) the combined presence of ${\mathrm{Mn}}_{\mathrm{Bi}}$ and ${\mathrm{Mn}}_{i}$. It has been found that lattice relaxations around ${\mathrm{Mn}}_{\mathrm{Bi}}$ defects play an important role for both magnetic and transport properties. The resistivity is strongly influenced by the amount of carriers, their type, and by the relative positions of the Mn-impurity energy levels and the Fermi energy. Our results suggest strategies to tune bulk resistivities and also clarify the location of Mn atoms in samples. Calculations indicate that at least two of the considered defects have to be present simultaneously in order to explain the experimental observations, and the role of interstitials may be more important than expected.

The theoretical formulation and numerical evaluation of the vertex corrections in multiorbital techniques of theories of electronic properties of random alloys are analyzed. It is shown that current approaches to static transport properties within the so-called conserving approximations lead to the inversion of a singular matrix as a direct consequence of the Ward identity relating the vertex corrections to one-particle self-energies. We propose a simple removal of the singularity for quantities (operators) with vanishing average values for electron states at the Fermi energy, such as the velocity or the spin torque; the proposed scheme is worked out in details in the self-consistent Born approximation and the coherent potential approximation. Applications involve calculations of the residual resistivity for various random alloys, including spin-polarized and relativistic systems, treated on an ab initio level, with particular attention paid to the role of different symmetries (inversion of space and time)., Comment: 7 pages, 3 figures, Section II slightly extended, accepted in Physical Review B

Based on the first-principles electronic-structure calculations, we construct the effective magnetic Hamiltonians that can be used to search for the ground state of solids and nanostructures. Moreover, using the methods of statistical mechanics, one can study the magnetic properties at T>0 such as the magnetic structure as a function of temperature, magnon spectra, the Curie/Neel temperature, etc. The present approach is highly flexible and it makes possible to study magnetic structure of complex systems with the accuracy of ab initio methods. As an illustration, we show how the magnetic structure of a magnetic monolayer (Fe) on a non-magnetic substrate (Ir(001)) can be determined.

The transport properties of Pd-rich PdFe alloys are investigated as a function of the atomic and magnetic orders on the L21 (Cu3Au) lattice. The residual resistivity, anisotropic magnetoresistance (AMR), and anomalous Hall effect are calculated from first-principles as a function of the atomic order. The calculations are based on the relativistic generalization of the transport Kubo–Greenwood approach as formulated in the framework of the first principles TB-LMTO method and Coherent Potential Approximation (CPA). The effect of the thermal magnetic disorder on the resistivity and AMR has been also studied using the disordered local moment approach based on the CPA. Our results are in an overall good agreement with available experimental data.

International audience; This review summarizes recent first-principles investigations of the electronic structure and magnetism of dilute magnetic semiconductors (DMSs), which are interesting for applications in spintronics. Details of the electronic structure of transition-metal-doped III-V and II-VI semiconductors are described, especially how the electronic structure couples to the magnetic properties of an impurity. In addition, the underlying mechanism of the ferromagnetism in DMSs is investigated from the electronic structure point of view in order to establish a unified picture that explains the chemical trend of the magnetism in DMSs. Recent efforts to fabricate high-TC DMSs require accurate materials design and reliable TC predictions for the DMSs. In this connection, a hybrid method (ab initio calculations of effective exchange interactions coupled to Monte Carlo simulations for the thermal properties) is discussed as a practical method for calculating the Curie temperature of DMSs. The calculated ordering temperatures for various DMS systems are discussed, and the usefulness of the method is demonstrated. Moreover, in order to include all the complexity in the fabrication process of DMSs into advanced materials design, spinodal decomposition in DMSs is simulated and we try to assess the effect of inhomogeneity in them. Finally, recent works on first-principles theory of transport properties of DMSs are reviewed. The discussion is mainly based on electronic structure theory within the local-density approximation to density-functional theory.

We develop an ab initio material-specific theory of current induced magnetization switching. The formulation is based on non-equilibrium Green's functions (NEGF) formalism applied to the so-called spin-mixing conductance, i.e., the linear response of the spin torque exerted on a magnetic layer to a spin accumulation in an adjacent non-magnetic layer. The numerical implementation employs the local spin density approximation (LSDA) and the linear muffin-tin orbital (LMTO) method. Possible substitutional disorder is treated using the coherent potential approximation (CPA) that allows for evaluation of the coherent (ballistic) and incoherent (diffusive) parts of the spin-mixing conductance. The developed scheme is applied to a few selected magnetic layers including systems containing diluted magnetic semiconductors and Heusler alloys. Their halfmetallic character leads to enhanced spin-mixing conductances with respect to charge conductances.

Effective exchange interactions in bulk ferromagnets as well as in magnetic overlayers on Cu(001) covered by a Cu-cap layer of varying thickness were determined from first principles by mapping of corresponding total energies onto the effective Heisenberg model in the framework of the adiabatic approximation and magnetic force theorem. The effective Heisenberg model is then used to determine spin-wave stiffness constants and Curie temperatures evaluated in the framework of the random-phase approximation. Calculations are in a fair agreement with available experimental data for bulk ferromagnets and reproduce an oscillatory Curie temperature of magnetic overlayers as a function of Cu-cap thickness in a qualitative agreement with recent experiments.

The effect of the long-range order (LRO) on the longitudinal $({\ensuremath{\rho}}_{xx},{\ensuremath{\rho}}_{zz})$ and anomalous Hall $({\ensuremath{\rho}}_{xy})$ resistivities as well as on the anisotropic magnetoresistance (AMR) in partially ordered $L{1}_{0}\phantom{\rule{0.28em}{0ex}}\mathrm{FePt}$ alloys is studied from first principles. The linear-response theory as formulated in the framework of the relativistic tight-binding linear muffin-tin orbital method which includes both the Fermi-surface and Fermi-sea terms is used. The effect of disorder is treated by means of the coherent-potential approximation. The main result is a weak dependence of the anomalous Hall conductivity ${\ensuremath{\sigma}}_{xy}$ on the LRO, which is, however, compatible with resistivities ${\ensuremath{\rho}}_{xx}$ and ${\ensuremath{\rho}}_{xy}$, which both depend strongly on the disorder present in the system. The resistivity and the AMR are predicted to increase with increasing degree of the LRO. We also investigate the effect of spin fluctuations on studied quantities using a simple model of the spin disorder. We have found good agreement between the theory and the recent experiment.

Spin-transfer torques in layered structures are studied with particular attention to their direction. Their direction is in spin valves determined mainly by the properties of the free layer and its interfaces. Situations which lead to a significant component of the torque in the direction perpendicular to both magnetizations present in the spin valve (out-of-plane torque) are described and analyzed. This component may arise from the transmitted spin current, as is demonstrated for a Cu | Ni | Cu ( 0 0 1 ) based system, or from the reflected spin current, which is found to be particularly high for systems with Cr | Co 2 MnSi interface. Ab initio calculations were performed for these systems employing the local spin density approximation (LSDA) and the linear muffin-tin orbital (LMTO) method. Information about the direction of the torque was obtained from the spin-mixing conductance.

The temperature dependence of interlayer exchange coupling ~IEC! is studied theoretically within the asymptotic theory with preasymptotic corrections included, and by employing an ab initio approach based on the Green function formulation of the IEC. In this paper an efficient method for calculating integrals involving the Fermi-Dirac distribution by representing the occurring integrands by a sum of complex exponentials is discussed. In particular a method which allows us to extract separately the temperature-dependence of the short-period ~SPO! and long-period ~LPO! oscillations in the case of Co/Cu/Co~001! trilayers from computed values is suggested. Furthermore, a detailed discussion of predictions of asymptotic theory is given. It is found that in the limit of a large spacer thickness N ab initio calculations confirm the results of asymptotic theories for SPO for a variety of trilayer geometries, namely, that the oscillation amplitudes depend on the temperature T as cNT/sinh(cNT). On the other hand, for the case of the LPO this simple form does not apply. We explain this behavior by large preasymptotic corrections necessary for the LPO. The combined effect of the temperature and the disorder in the spacer is also discussed. @S0163-1829~99!03234-8#

In this work we extend our recently published ab initio study [J. Rusz, L. Bergqvist, J. Kudrnovský, I. Turek, Phys. Rev. B 73 (2006) 214412] of finite temperature magnetic properties of Ni 1 + x MnSb Heusler alloy family ( 0 ⩽ x ⩽ 1 ) with particular attention to the role of electronic correlations beyond the local density approximation. Exchange interactions are calculated using the magnetic force theorem on the base of zero-temperature electronic structure obtained within the density functional theory. The resulting effective classical Heisenberg hamiltonian is treated by methods of statistical physics. Our results are in improved agreement with experiment compared to bare local density approximation results, particularly for alloys with lower nickel content.

We present a formulation of the so-called Fermi sea contribution to the conductivity tensor of spin-polarized random alloys within the fully relativistic tight-binding linear muffin-tin-orbital (TB-LMTO) method and the coherent potential approximation (CPA). We show that the configuration averaging of this contribution leads to the CPA-vertex corrections that are solely due to the energy dependence of the average single-particle propagators. Moreover, we prove that this contribution is indispensable for the invariance of the anomalous Hall conductivities with respect to the particular LMTO representation used in numerical implementation. Ab initio calculations for cubic ferromagnetic 3d transition metals (Fe, Co, Ni) and their random binary alloys (Ni-Fe, Fe-Si) indicate that the Fermi sea term is small against the dominating Fermi surface term. However, for more complicated structures and systems, such as hexagonal cobalt and selected ordered and disordered Co-based Heusler alloys, the Fermi sea term plays a significant role in the quantitative theory of the anomalous Hall effect., 13 pages, accepted in Phys. Rev. B

Using ab initio band-structure calculations and employing the disordered local moment formalism we study the 4fmoment induced magnetic splitting of the valence band in pure hcp Gd metal at zero and critical temperature Tc .W e find that the local exchange splitting of the valence band persists above Tc, which is in strong disagreement with the conventional Stoner picture. Our results favor the strong coupling scenario of the interaction between the localized and itinerant part of the Gd electronic system. Our analysis provides a theoretical background for the discussion of recent photoemission experiments. r 2005 Elsevier B.V. All rights reserved.

We estimate the anomalous Hall effect of selected Heusler alloys from first principles. An emphasis is put on the effect of the native disorder which is often present in the stoichiometric samples. Such disorder can strongly influence both the magnetic and transport properties of these alloys. We employ a recently developed fully relativistic Kubo-St\ifmmode \check{r}\else \v{r}\fi{}eda approach adapted to disordered multisublattice systems in which the chemical disorder is described in terms of the coherent potential approximation. As case studies we choose half-metallic Heusler alloys Co${}_{2}$CrAl and Co${}_{2}$MnAl, as well as the spin gapless semiconductor alloy Mn${}_{2}$CoAl for which experimental and theoretical studies appeared recently. We demonstrate that proper inclusion of disorder significantly improves agreement between the experiment and theory.

We have extended the validity of the correlation between the surface 3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d transition metal series and to the neighboring elements Sr and Ag via accurate first-principles calculations. We find that the correlation is quasilinear and robust with respect to the differencies both between initial and final-state calculations of the SCLS's and two distinct measures of the SDBS's. We show that despite the complex spatial dependence of the surface potential shift (SPS) and the location of the 3d and 4d orbitals in different regions of space, the correlation exists because the sampling of the SPS by the 3d and 4d orbitals remains similar. We show further that the sign change of the SCLS's across the transition series does indeed arise from the d band-narrowing mechanism previously proposed. However, while in the heavier transition metals the predicted increase of d electrons in the surface layer relative to the bulk arises primarily from transfers from s and p states to d states within the surface layer, in the lighter transition metals the predicted decrease of surface d electrons arises primarily from flow out into the vacuum., RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.B

P. Bruno,1 J. Kudrnovský,2,3 V. Drchal,2,3 and I. Turek4 1Institut d’Electronique Fondamentale, CNRS URA 22, Bâtiment 220, Universite Paris-Sud, F-91405 Orsay, France 2Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-180 40 Praha 8, Czech Republic 3Institute for Technical Electrochemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Vienna, Austria 4Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-616 62 Brno, Czech Republic (Received 6 February 1996)

We describe the applications of rst-principles calculations to the analysis of transport and magnetic propertiesof rare-earth materials. The rst application is a detailed calculation of the spin-disorder resistivity of heavyrare-earth metals in the Gd-Tm series. The crystallographic anisotropy of the spin-disorder resistivity agreeswell with experiment, but its magnitude is signi cantly underestimated. Possible origins of this discrepancy arediscussed. In the second part we analyze the exchange interaction in Gd-doped EuO using a magnetostructuralcluster expansion based on the ab initio total energies. The calculated Curie temperature has a broad maximumextending over 10-20% Gd concentration and reaching approximately 150 K. 1. INTRODUCTION Rare-earth materials have long been a subject of extensive fundamental research. 1{3 The key feature controllingtheir physical properties is the existence of partially lled, strongly correlated and well localized 4 f electronshells, which interact with the itinerant (valence and/or conduction) electrons through on-site exchange inter-action. With some notable exceptions, the hybridization of 4 f electrons is usually weak. The on-site exchangeinteraction gives rise to indirect (RKKY) exchange

Electrical resistivity of heavy rare-earth metals has a dominant contribution from thermal spin-disorder scattering. Here this spin-disorder resistivity is calculated for the Gd-Tm series of metals in the paramagnetic state. Calculations are performed within the tight-binding linear muffin-tin orbital method using two complementary methods: (1) averaging of the Landauer-B¨ uttiker conductance of a supercell over random noncollinear spin-disorder configurations, and (2) linear response calculations with the spin-disordered state described in the coherent potential approximation. The agreement between these two methods is found to be excellent. The spin-disorder resistivity in the series follows an almost universal dependence on the exchange splitting. While the crystallographic anisotropy of the spin-disorder resistivity agrees well with experiment, its magnitude is significantly underestimated. These results suggest that the classical picture of slowly rotating self-consistent local moments is inadequate for rare-earth metals. A simple quantum correction improves agreement with experiment but does not fully account for the discrepancy, suggesting that more complicated scattering mechanisms may be important.

The electronic structure and band magnetism of disordered ${\mathrm{Fe}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$ and ${\mathrm{Ni}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cu}}_{\mathit{x}}$ bulk alloys and of corresponding disordered overlayers on an fcc Cu(001) substrate were studied by means of the first-principles tight-binding linear muffin-tin orbital coherent potential approximation method. For the Fe-Co system, we found that the composition dependence of the averaged and the local Fe magnetic moments is strongly affected by the dimensionality of the system while the local Co moments remain nearly constant. In the bulk bcc alloys, a nonmonotonic behavior of the averaged magnetization occurs due to the transition from weak to strong ferromagnetism. The averaged magnetization of the random overlayers depends linearly on the composition while the local Fe and Co moments are constant since the strong ferromagnetism is stabilized by the substrate throughout the whole composition range. For the Ni-Cu system, a transition from ferromagnetism to paramagnetism is found both in the bulk fcc alloys and in the overylayers. The ferromagnetism of the overlayers is reduced in comparison to the bulk systems of the same composition. The concentration trends of local densities of states, local numbers of electrons, and work functions are discussed.

We present a theory of voltage-induced spin-transfer torques in ballistic non-collinear spin valves. The torkance on one ferromagnetic layer is expressed in terms of scattering coefficients of the whole spin valve, in analogy to the Landauer conductance formula. The theory is applied to Co/Cu/Ni(001)-based systems where long-range oscillations of the Ni-torkance as a function of Ni thickness are predicted. The oscillations represent a novel quantum size effect due to the non-collinear magnetic structure. The oscillatory behavior of the torkance contrasts a thickness-independent trend of the conductance., Comment: Version 3: 6 pages, 3 figures. Corrected version that passed the peer-review process

International audience; The paper reviews theoretical backgrounds of an approximate treatment of relativistic effects within the tight-binding (TB) linear muffin-tin orbital (LMTO) method for ab initio electronic structure calculations of solids. The formalism employs two-component spinors and an orbital basis set of the scalar-relativistic approximation. The developed theory is applied to selected metallic systems and its results are compared to those of a fully relativistic TB-LMTO method.

We present a first-principles study of the physical properties of the disordered ${\mathrm{Ni}}_{2\ensuremath{-}x}\mathrm{Mn}\mathrm{Sb}$ alloys which form a continuous series connecting two typical members of the Heusler alloy family: namely, the half-metallic semi-Heusler alloy NiMnSb $(x=1)$ and related metallic Heusler alloy ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sb}$ $(x=0)$. Magnetic moments, exchange interactions, magnon spectra, and Curie temperatures at ambient and elevated pressures are determined and compared with available experimental data. The spin-spin correlation functions at the critical temperature are also calculated. The pair exchange interactions and corresponding classical Heisenberg Hamiltonian are derived from self-consistent electronic structure calculations using a magnetic force theorem. Heusler alloys NiMnSb and ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sb}$ exhibit strikingly different asymptotic behavior of exchange interactions with the distance between magnetic atoms. The Curie temperatures are estimated using multisublattice versions of the mean-field approximation, random-phase approximation, and Monte Carlo simulations. The robustness of the results with respect to the effect of correlations beyond the local density approximation, the selected reference state used for mapping to the Heisenberg Hamiltonian, and the applied mapping procedure are also discussed.

International audience; The contribution reviews an ab initio two-step procedure to determine exchange interactions, spin-wave spectra, and thermodynamic properties of itinerant magnets. In the first step, the selfconsistent electronic structure of a system is calculated for a collinear spin structure at zero temperature. In the second step, parameters of an effective classical Heisenberg Hamiltonian are determined using the magnetic force theorem and the one-electron Green functions. The Heisenberg Hamiltonian and methods of statistical physics are employed in subsequent evaluation of magnon dispersion laws, spin-wave stiffness constants, and Curie/Néel temperatures. Applicability of the developed scheme is illustrated by selected properties of various systems such as transition and rare-earth metals, disordered alloys including diluted magnetic semiconductors, ultrathin films, and surfaces. A comparison to other ab initio approaches is presented as well.

We propose a theory that combines first-principles evaluations of inter-atomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations. Exchange interactions are determined using the magnetic force theorem and the one-electron Green functions. The magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that; have a strong directional dependence. We show that; critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, arc reproduced with a good accuracy only when the magnetic atoms arc randomly positioned on the Ga (Zn) sites, whereas in ordered structure of the magnetic atoms results in critical temperatures that are too high. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical ternperatures should be very sensitive to the details of the sample preparation, in agreement with observations.

We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short- ...

We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors. We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.

Effective pair exchange interactions between Mn atoms in III-V and group-IV diluted magnetic semiconductors are determined from a two-step first-principles procedure. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature with the substitutional disorder treated within the framework of the coherent-potential approximation. The effective exchange pair interactions are then obtained in a second step by mapping the total energies associated with rotations of magnetic moments onto an effective classical Heisenberg Hamiltonian using the magnetic force theorem and one-electron Green functions. The formalism is applied to ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ alloys with and without As antisites, and to ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$ alloys recently studied experimentally. A detailed study of the behavior of pair exchange interactions as a function of the distance between magnetic atoms as well as a function of the concentrations of the magnetic atoms and compensating defects is presented. We have found that due to disorder and the half-metallic character of the system the pair exchange interactions are exponentially damped with increasing distance between the Mn atoms. The exchange interactions between Mn atoms are ferromagnetic for distances larger than the ones corresponding to the averaged nearest-neighbor Mn-Mn distance. The pair exchange interactions are also reduced with increasing concentrations of the Mn atoms and As antisites. As a simple application of the calculated exchange interactions we present mean-field estimates of Curie temperatures.

Magnetic properties of the europium metal in a bcc structure are studied from first principles using a two-step approach. First, the electronic structure of a ferromagnetic state is calculated in the local spin-density approximation (LSDA) to the density-functional theory whereby the highly localized $4f$ orbitals are treated as part of the atomic core. This description leads to an equilibrium lattice constant that compares well with experiment, in contrast to the standard LSDA which yields a significantly smaller atomic volume. In the second step, parameters of an effective Heisenberg Hamiltonian are derived from the self-consistent electronic structure and they are used to determine the magnetic ground state and to estimate the magnetic transition temperature. The calculated pairwise exchange interactions tend to couple the local magnetic moments of the nearest neighbors ferromagnetically. However, the interaction parameters exhibit a slow oscillatory decay as a function of the interatomic distance which makes them fully compatible with an observed spin-spiral ground state. The resulting wave vector of the spiral as well as the N\'eel temperature are in fair agreement with measured values.

The spin-dependence of the interface resistance between ferromagnetic Fe and InAs is calculated from first-principles for specular and disordered (001) interfaces. Because of the symmetry mismatch in the minority-spin channel, the specular interface acts as an efficient spin filter with a transmitted current polarisation between 98 an 89%. The resistance of a specular interface in the diffusive regime is comparable to the resistance of a few microns of bulk InAs. Symmetry-breaking arising from interface disorder reduces the spin asymmetry substantially and we conclude that efficient spin injection from Fe into InAs can only be realized using high quality epitaxial interfaces., Comment: 4 pages, 4 figures

Recent theories of spin-current-induced magnetization reversal are formulated in terms of a spin-mixing conductance $G^{mix}$. We evaluate $G^{mix}$ from first-principles for a number of (dis)ordered interfaces between magnetic and non-magnetic materials. In multi-terminal devices, the magnetization direction of a one side of a tunnel junction or a ferromagnetic insulator can ideally be switched with negligible charge current dissipation., Comment: 4 pages, 1 figure

This chapter deals with the most important quantities of the local spin density formalism. Expressions for charge densities, effective one-electron potentials, and total energies are derived within the atomic sphere approximation. Special attention is devoted to systems with only two-dimensional translational symmetry due to surfaces and interfaces as well as to systems with substitutional randomness.

This chapter describes one specific method for the configurational averaging of physical observables related to substitutionally random systems, the so-called coherent potential approximation (CPA). The calculation of such averages is greatly simplified by expressing them in terms of the Green function G(z). One of the main tasks of alloy theory is thus to find computational methods and approximations for the configurational average of G(z), 〈G(z)〉 ≡ Ḡ(z). Once 〈G(z)〉 is known the configurationally averaged one-electron observables can be calculated.

In this chapter, we derive a general expression for the Green function corresponding to a one-electron problem solved within the atomic sphere approximation. On the basis of this Green function formalism, methods for an efficient treatment of systems with two-dimensional translational symmetry are described: the tight-binding linear muffin-tin orbital method, a technique of principal layers and surface Green functions, and concepts in order to calculate important physical observables.

The influence of alloying in the spacer on the periods and the amplitudes of the oscillations of exchange coupling in magnetic multilayers is studied from first principles. The effect of substitutional randomness in the spacer is treated within the coherent potential approximation and the validity of the simplified virtual-crystal approximation is examined. As a case study, results obtained for Co(001) slabs in random fcc ${\mathrm{Cu}}_{1\ensuremath{-}x}{M}_{x}$ spacers ($M=\mathrm{N}\mathrm{i},\phantom{\rule{0ex}{0ex}}\mathrm{Z}\mathrm{n},\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{n}\mathrm{d}\phantom{\rule{0ex}{0ex}}\mathrm{A}\mathrm{u}$) are presented and discussed.

The spin-disorder resistivity (SDR) of transition metal ferromagnets (Fe, Co, Ni), rare-earth ferro- magnet Gd, and Ni2MnSn Heusler alloy is determined from first principles. We identify the SDR at the Curie temperature with the residual resistivity of the corresponding system evaluated in the framework of the disor- dered local moment (DLM) model which has the zero spin-spin correlation function. The underlying electronic structure is determined in the framework of the tight-binding linear mu! n-tin orbital method which employs the coherent potential approximation (CPA) to describe the DLM state. The DLM fixed-spin moment approach is used in the case when the DLM moment collapses. The electronic structure of hcp-Gd is determined using both the open core and LDA+U approaches. The Kubo-Greenwood approach is used to estimate the resistivity. For bcc-Fe and Ni2MnSn alloy we shall also estimate the temperature-dependent of resistivity below the Curie temperature using a semiempirical approach. Calculations are compared with the supercell Landauer-Buttiker approach developed recently as well as with available experimental data and overall good agreement is obtained.

We construct effective magnetic Hamiltonians by using the first-principles electronic-structure cal- culations. These Hamiltonians are used to determine the ground state of solids and nanostructures, and, at T > 0, using the methods of statistical mechanics, also their magnetic properties as a function of temperature. The present approach is highly flexible and it makes possible to find magnetic structure of complex systems with the accuracy of ab initio methods. As illustrations we show (i) how the magnetic structure at T = 0 and at T > 0 can be determined for systems with anisotropic exchange and for systems with several sublattices, (ii) why the magnetic moment in fcc-nickel is unstable upon reversal, and (iii) how to calculate the size of magnetic moment in fcc-Ni at the Curie temperature, which is important for explanation of the spin-disorder resistivity in Ni.

A first-principles theory of alloy ordering in two and three dimensions in the presence of ferromagnetism is described. Using the generalized perturbation method, the order-disorder transition from the bcc ${\mathrm{Fe}}_{50}$${\mathrm{Co}}_{50}$ random alloy into the ordered CsCl-structure FeCo alloy and its ordering temperature are found to agree reasonably well with available experimental data. A similar ordering tendency for the formation of the ordered c(2\ifmmode\times\else\texttimes\fi{}2) phase of the ${\mathrm{Fe}}_{50}$${\mathrm{Co}}_{50}$ surface alloy on fcc Cu(001) is investigated.

The influence of spacer randomness on the periods and the amplitudes of the oscillations of exchange coupling in magnetic multilayers is studied from first principles. The effect of disorder is treated within the coherent potential approximation. As a case study, results obtained for trilayers Co/Cu 1- x M x /Co(001), where M  Ni, Pd, and Zn, are presented and discussed.