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The nonlocal coherent-potential approximation provides a systematic technique for the study of short-range ordering effects in a variety of disordered systems. In its original formulation the technique, however, shows an unwanted dependence on details in the coarse-grained effective medium construction. This is particularly evident in the study of $\stackrel{P\vec}{k}$-resolved quantities, such as the Bloch spectral function and other non-site-diagonal observables. We remove the issue and recover fully physical results in first principles studies of real materials, by means of a resampling procedure first proposed for model tight-binding Hamiltonians. The prescription is further generalized to the case of complex unit cell compounds, with more than a single sublattice, and illustrated through examples from metallic alloys and disordered local moment simulations of paramagnetism in the prototype iron-based superconductor FeSe.

We studied the structural, electronic, and magnetic properties of a cubic perovskite ${\mathrm{BaFeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.5$) within the density-functional theory using a generalized gradient approximation (GGA) and a GGA + $U$ method. According to our calculations, ${\mathrm{BaFeO}}_{3}$ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with an extremely high Curie temperature (${T}_{C}$) of 700--900 K. However, such an estimate of ${T}_{C}$ disagrees with all available experiments, which report that ${T}_{C}$ of the ${\mathrm{BaFeO}}_{3}$ and undoped ${\mathrm{BaFeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ films varies between 111 and 235 K, or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for ${\mathrm{BaFeO}}_{3}$, we concluded that good agreement can be obtained when oxygen vacancies are included in our model. Thus, the relatively low ${T}_{C}$ measured in ${\mathrm{BaFeO}}_{3}$ can be explained by oxygen vacancies intrinsically present in the material. Since iron species near the O vacancy change their oxidation state from $4+$ to $3+$, the interaction between ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Fe}}^{3+}$, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing $\ensuremath{\delta}$ in ${\mathrm{BaFeO}}_{3\ensuremath{-}\ensuremath{\delta}}$, its ${T}_{C}$ decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of ${\mathrm{BaFeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above room temperature.

Spin-dependent electronic transport through multiferroic Co/PbTiO$_{3}$/Co tunnel junctions is studied theoretically. Conductances calculated within the Landauer-B\"uttiker formalism yield both a large tunnel magnetoresistance (TMR) and a large tunnel electroresistance (TER). On top of this, we establish a four-conductance state. The conductances depend crucially on the details of the electronic structure at the interfaces. In particular, the spin polarization of the tunneling electronic states is affected by the hybridization of orbitals and the associated charge transfer at both interfaces. Digital doping of the PbTiO$_{3}$ barrier with Zr impurities at the TiO$_{2}$/Co$_{2}$ interface significantly enhances the TMR\@. In addition, it removes the metalization of the barrier., Comment: 9 pages, 5 figures

Magnetoelectric coupling at the interface between $\text{PbTi}(\text{Zr}){\text{O}}_{3}$ (001) (PZT) and a Co film as well as its tuning by Zr substitutions is studied here from first principles. Chemically disordered 25% Zr-doped compositions were modeled within the supercell approach. Our calculations suggest that ferroelectric ionic displacements in PZT can be locally enhanced, especially when the interfacial Ti cations are replaced with Zr. Regarding Co films their structure tends to become hexagonal for thicker films, while rather significant changes in Co magnetization are anticipated in response to the electric polarization reversal in PZT.

The electron energy-loss near-edge structure ~ELNES! at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia ~YSZ! materials. The electronic structure of the three polymorphs of pure ZrO2 and of the doped YSZ structure close to the 33 mol %Y2O3 composition have been calculated using a full-potential linear muffin-tin orbital method ~NFP-LMTO! as well as a pseudopotential based technique. Calculations of the ELNES dipole transition matrix elements in the framework of the NFP-LMTO scheme and inclusion of core hole screening within Slater’s transition state theory enable the ELNES to be computed. Good agreement between the experimental and calculated ELNES is obtained for pure monoclinic ZrO2. The agreement is less good with the ideal tetragonal and cubic structures. This is because the inclusion of defects is essential in the calculation of the YSZ ELNES. If the model used contains ordered defects such as vacancies and metal Y planes, agreement between the calculated and experimental O K-edges is significantly improved. The calculations show how the five different O environments of Zr 2Y2O7 are connected with the features observed in the experimental spectra and demonstrate clearly the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

Relativistic full-potential calculations within the generalized gradient approximation (GGA) for a series of Mn-based Heusler alloys are presented. Calculated equilibrium lattice parameters deviate less than 1.2% from the experimental values. The main features of a half metallic system are present in the density of states for the PtMnSb and NiMnSb. We predict that PdMnSb shows half metallic character under hydrostatic pressure. The substitution of Sb in PtMnSb by Sn or Te destroys the minority spin band gap. Spin and orbital magnetic moments for all the systems are in good agreement with previous calculations and experimental data. L{sub 2,3} x-ray absorption and x-ray magnetic circular dichroism (XMCD) spectra are calculated for all the five compounds. Pt spectra present big deviations from system to system in the PtMnY (Y=Sn,Sb,Te) compounds while Mn spectra show only small deviations. For all these spectra GGA underestimates the L{sub 3}/L{sub 2} integrated branching ratio and produces a much smaller L{sub 2} peak intensity for the Ni site in NiMnSb. The XMCD sum rules are used to compute the spin and orbital magnetic moments and the results are compared to the direct calculations. (c) 2000 The American Physical Society.

Two years after the prediction of a giant spin Hall effect for the dilute Cu(Bi) alloy [Gradhand et al., Phys. Rev. B 81, 245109 (2010)], a comparably strong effect was measured in thin films of Cu(Bi) alloys by Niimi et al. [Phys. Rev. Lett. 109, 156602 (2012)]. Both theory and experiment consider the skew-scattering mechanism to be responsible, however they obtain opposite sign for the spin Hall angle. Based on a detailed analysis of the obtained theoretical results, we propose that either the formation of extremely small clusters or the influence of interface roughness and grain boundaries decorated with Bi atoms are responsible for the observed phenomenon.

The coherent potential approximation has historically allowed the efficient study of disorder effects over a variety of solid state systems. Its original formulation is, however, limited to a single-site or uncorrelated model of local substitutions. This neglects the effects of correlation and short-range ordering, often found in real materials. Recent theoretical work has shown one possible way to systematically address such shortcomings for simple materials with only one element per unit cell. We briefly review the basic ideas of such development within the framework of multiple scattering theory and suggest its generalization to materials with complex lattices. We validate our extension through a systematic comparison with a classic Cu1−cZnc reference test case and propose, for further illustration of local environment effects, the example of the yttria-stabilized cubic phase of zirconia, re-examined through various techniques for the first-principles treatment of disorder

First-principles calculations of x-ray absorption spectra (XAS) and the related x-ray magnetic circular dichroism (XMCD) are presented for ultrathin Fe layers on a BaTiO${}_{3}$ (BTO) single-crystal surface within the framework of density functional theory. We have investigated the dependence of XAS and XMCD of Fe ${L}_{2,3}$ edges as a function of Fe layer thickness (1--3 ML) and on polarization direction of BTO. The calculations give a detailed insight concerning the relation between structural, electronic, and magnetic properties at the multiferroic interface Fe/BTO. In dependence on the Fe layer thickness we find characteristic features in the related XMCD, which depend strongly on the interface structure. The critical temperature of the ultrathin Fe layer is calculated to be between 170 to 230 K.

The tunneling electroresistance (TER) for ferroelectric tunnel junctions (FTJs) with BaTiO_{3} (BTO) and PbTiO}_{3} (PTO) barriers is calculated by combining the microscopic electronic structure of the barrier material with a macroscopic model for the electrostatic potential which is caused by the ferroelectric polarization. The TER ratio is investigated in dependence on the intrinsic polarization, the chemical potential, and the screening properties of the electrodes. A change of sign in the TER ratio is obtained for both barrier materials in dependence on the chemical potential. The inverse imaginary Fermi velocity describes the microscopic origin of this effect; it qualitatively reflects the variation and the sign reversal of the TER. The quantity of the imaginary Fermi velocity allows to obtain detailed information on the transport properties of FTJs by analyzing the complex bandstructure of the barrier material., Comment: quality of figures reduced

The geometric structure of O/Fe(001)-p(1x1) is studied by surface x-ray diffraction (SXRD). After dosing of about 4 L (1 L = 10(-6) Torr s) of oxygen at room temperature we find approximately one monolayer (ML) of oxygen in an FeO-like structure forming two layer thick islands covering about 40% of a roughened Fe(001) surface. Subsequent annealing up to 500 C results in surface flattening leading to a highly ordered structure. Very precise SXRD data reveal the presence of 1 ML of oxygen atoms located in fourfold hollow sites at d((O-Fe)) = 0.48 +/- 0.08 angstrom above the first Fe layer. The first Fe-Fe-interlayer spacing is expanded to d(12) = 1.66 +/- 0.02 angstrom corresponding to an increase of 16% relative to the bulk spacing (1.43 angstrom). Density-functional calculations confirm our findings and indicate a strong dependence of the (local) layer expansion on the oxygen coverage. Our results are important for understanding the surface magnetic properties of the O/Fe(001)-p(1x1) surface in general.

As demonstrated conceptually by first-principles calculations, the Rashba spin splitting in the $6p$ states of a Bi adlayer on ${\text{BaTiO}}_{3}(001)$ can be manipulated by the electric polarization in the ferroelectric substrate. Although this spin-electric effect is moderate, with a relative change in the splitting of about 5%, the absolute splitting of about $0.24\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ is unmatched. Further, the occupied $6p$ surface states display an anisotropic dispersion and deviate significantly from the free-electron model of the Rashba effect. Our findings may pave a route for spin-electronic devices.

We present an ab initio study of the effect of iron oxidation on magnetoelectric coupling which appears at the ferromagnet/ferroelectric interface, such as ${\text{Fe}}_{2}/A{\text{TiO}}_{3}(001)$ $(A=\text{Ba},\text{Pb})$. The oxygen coverage, $c$, ranging between one half and two adsorbed O per each Fe atom was simulated within a slab geometry. Since the change in magnetization induced by electric polarization reversal remains robust below $cl3/2$, we suggest that the surface oxidation of composite multiferroics will not significantly damage their switchable magnetoelectricity.

The spins of a pair of spin-orbit split surface states at a metal surface are usually antiparallelly aligned, in accord with the Rashba model for a two-dimensional electron gas. By first-principles calculations and twophoton photoemission experiments we provide evidence that in the surface alloy Bi/Cu111 the spins of an unoccupied pair of surface states are parallelly aligned. This unconventional spin polarization, which is not consistent with that imposed by the Rashba model, is explained by hybridization of surface states with different orbital character and is attributed to the spin-orbit interaction. Since hybridization is a fundamental effect our findings are relevant for spin electronics in general.

On the basis of first-principles electronic-structure calculations we predict that epitaxial multiferroic films---fabricated as ultrathin Fe films deposited on ${\text{TiO}}_{2}$-terminated (001) surfaces of $A{\text{TiO}}_{3}$ perovskites $(A=\text{Pb},\text{Ba})$---exhibit an unexpected change in their magnetic structure with increasing Fe-film thickness. The magnetic order changes from ferromagnetic, with a magnetization of about $3{\ensuremath{\mu}}_{B}/\text{atom}$ for the 1 ML system, to ferrimagnetic with almost vanishing magnetization upon deposition of a second Fe layer. Ferromagnetic order is restored for thicker Fe films but with significantly reduced magnetization as compared to Fe bulk. The effect is understood in terms of hybridization of electronic states and geometric structure. The magnetoelectric coupling affects the size of the magnetic moments moderately; a spin-reorientation transition is not found.

We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T ,i n metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced local moment magnetic fluctuations are described within the relativistic generalization of the disordered local moment theory from which the T dependence of the magnetization, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry, L10-ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m 2 for a broad range of values of m. This is the same trend that we have previously found in L10-ordered FePt and which agrees with experiment. We also study a magnetically soft cubic magnet, the Fe50Pt50 solid solution, and find that its small magnetic anisotropy constant K1 rapidly diminishes from 8 eV to zero. K1 evolves from being proportional to m 7 at low T to m 4 near the Curie temperature. The accounts of both the tetragonal and cubic itinerant electron magnets differ from those extracted from single ion anisotropy models and instead receive clear interpretations in terms of two ion anisotropic exchange.

Using the fully relativistic spin-polarized Korringa-Kohn-Rostoker method, we study the prototypical soft magnet, bcc iron. We investigate how its magnetic anisotropy (MAE) varies as a function of volume, band filling, and tetragonal distortions of the crystal lattice. We follow the trends of the linear magnetostriction and magnetic permeability. We find that a slight reduction in band filling and modest lattice expansion produces a significant magnetic softening of this model system. We explore whether this situation can be realized by doping bcc Fe with vanadium. Treating the compositional disorder with the coherent potential approximation, we calculate the magnetic anisotropy and magnetostriction trends of iron-rich ${\mathrm{Fe}}_{1\ensuremath{-}c}{\mathrm{V}}_{c}$ disordered alloys and find the behavior to accord with the predictions from the bcc Fe model. In particular we find that for $c\ensuremath{\approx}0.1$ the MAE is very small and the linear magnetostriction is zero. We propose ${\mathrm{Fe}}_{0.9}{\mathrm{V}}_{0.1}$ as a high permeability material. Fair agreement with experimental values for the MAE and magnetostriction of both Fe and FeV is found.

We present ab initio calculations of the static paramagnetic spin susceptibility for hcp iron at finite temperatures and for a range of pressures. The dominant magnetic fluctuations in hcp Fe are found to be incommensurate antiferromagnetic, characterized by the wave vector ${\mathbf{q}}_{\mathrm{inc}}=(0.56,0.22,0).$ We show that ${\mathbf{q}}_{\mathrm{inc}}$ is linked to a Fermi-surface nesting feature. For the lowest pressure $\ensuremath{\sim}16\mathrm{GPa}$ at which hcp Fe forms, we find that these modes become unstable below a N\'eel temperature ${(T}_{N})$ of 69 K. ${T}_{N}$ rapidly diminishes with increasing pressure. We therefore predict that hcp Fe will be found to have an incommensurate spin-density-wave-ordered state over a small pressure range starting with the onset of hcp phase. We note the coincidence with the superconductivity recently found in this material.

Atomistic configurations of yttria-stabilized zirconia between 3 and 10 mol % ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ were relaxed using the pseudopotential technique. The results showed a phase transition to the cubic (c) $({\mathrm{ZrO}}_{2}{)}_{100\ensuremath{-}x}({\mathrm{Y}}_{2}{\mathrm{O}}_{3}{)}_{x}$ at $x\ensuremath{\sim}10\mathrm{mol}%.$ The electron-energy-loss near-edge spectra, calculated using the linear muffin-tin orbital method and relaxed defect geometry, agree with experiment. In the displacive limit of the double-well potential model, the vibration modes, corresponding to a soft phonon of $c\ensuremath{-}{\mathrm{ZrO}}_{2},$ were calculated for each composition of yttria-stabilized zirconia. The effect of anharmonicity yields the fine structure in the spectral density which is associated with stabilization at $xl10\mathrm{mol}%.$ In studying the phonon dynamics, we use the displacement probability density which quantifies accurately the transition temperature above which the c phase is stabilized.

The electron energy-loss near-edge structure ~ELNES! at the O K edge has been studied in yttria-stabilized zirconia ~YSZ!. The electronic structure of YSZ for compositions between 3 and 15 mol % Y2O3 has been computed using a pseudopotential-based technique to calculate the local relaxations near the O vacancies. The results showed phase transition from the tetragonal to cubic YSZ at 10 mol % of Y2O3, reproducing experimental observations. Using the relaxed defect geometry, calculation of the ELNES was carried out using the full-potential linear muffin-tin orbital method. The results show very good agreement with the experimental O K-edge signal, demonstrating the power of using ELNES to probe the stabilization mechanism in doped metal oxides.