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Strong spin-orbit coupling in noncentrosymmetric materials and interfaces results in remarkable physical phenomena, such as nontrivial spin textures, which may exhibit Rashba, Dresselhaus, and other intricated configurations. This provides a promising basis for nonvolatile spintronic devices and further implications. Here, we simulate from first principles a two-dimensional electron gas in ultrathin platinum and palladium layers grown on ferroelectric PbTiO$_3$(001). The latter allows, in principle, to switch and control the spin-to-charge conversion by the polarization reversal. We show how the band structure and its Rashba splitting differ in the Pt and Pd overlayers and how these electronic features change with increasing the overlayer thickness and upon reversal of polarization. Besides, for both overlayers, we simulated their current-voltage ($I-V$) characteristics, the resistance of which upon the polarization reversal changes between 20~\% and several hundred percent. The reported findings can be used to model directly the Rashba-Edelstein effect.

Oxide interfaces between insulating perovskites can provide a two-dimensional electron gas (2DEG), which was observed initially beneath polar $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (LAO) grown on $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$(001). Here, we suggest to use a solid electrolyte $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ (LiNO) instead of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ to create in LAO/LiNO a robustly switchable and magnetic quasi 2DEG (q2DEG). This prospective phenomenon is achieved by charging and discharging lithium niobate while its $n\ensuremath{\rightarrow}p$ doping induces the alteration of magnetic order in LAO/LiNO and, thus, the spin degrees of freedom in q2DEG. On the basis of ab initio calculations and starting from the defectless and insulating LAO/LiNO superlattice, we demonstrate that either a ferromagnetic q2DEG or weakly magnetic quasi-2D hole gas emerge there due to the presence of excessive electrons or holes, respectively. These defects were simulated by one extra Li and a lithium vacancy in LiNO. By varying the $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ thicknesses up to 18 formula units, we found that (i) the spatial extent of q2DEG is 1.5 nm along the stacking direction and that (ii) the dopant position at the Al-terminated interface is energetically preferable by more than 0.5 eV compared to the deep layers. In the context of the advanced design of q2DEG, the out-of-plane electric polarization, which intrinsically appears in multiferroic LAO/LiNO, allows to switch externally the electric-field dependence of the Rashba effect. Moreover, the in-plane charge current via q2DEG may generate there a transverse spin density due to the Rashba-Edelstein effect. We discuss this scenario using the relativistic electronic structures and locally projected density of states which differ remarkably in $\mathrm{L}\mathrm{A}\mathrm{O}/{\mathrm{Li}}_{1+x}\mathrm{N}\mathrm{O}$ and $\mathrm{L}\mathrm{A}\mathrm{O}/{\mathrm{Li}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{O}$.

A two-dimensional electron gas (2DEG) can be formed at an interface between two insulators such as ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ without any additional doping. Nevertheless, structural imperfections or defects at the interface, which may arise during the growth process or be of another origin, can definitely affect 2DEG properties either increasing or reducing the 2DEG density. In this work, we study 2DEG formation at both perfect and imperfect interfaces of ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures in the framework of the density functional theory. Using a first-principles Green function method within a coherent potential approximation, we investigate consistently the development of the 2DEG density starting from a defectless interface and introducing sequentially various kinds of long-range disorder such as cation intermixing and oxygen vacancies. Finally, we evaluate the 2DEG carrier density from the Fermi surface cross sections and effective masses of the carriers, which may be directly related to the multiple transport phenomena in the system.

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.

Magnetoelectric coupling has attracted interest due to its potential to write magnetic information with electric fields. In the model system of Fe islands on Cu(111), electric fields can induce martensitic phase transitions between ferromagnetic body-centered cubic and antiferromagnetic face-centered cubic phases. Here, we present a detailed study of the dynamics and energetics of the phase transition in the electric field of the junction of a scanning tunneling microscope. Statistical measurements allow us to reveal the influence of both the electric field and the crystallographic strain on the energy landscape of the two competing phases.

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.

Using surface x-ray diffraction in combination with ab initio calculations, we demonstrate that the atomic structure of ultrathin ${\mathrm{BaTiO}}_{3}$ (BTO) films grown on Me(001) surfaces ($\mathrm{Me}=\mathrm{Fe}$, Pd, Pt) depends on subtle modifications of the interface chemical composition. A complete reversal of the surface termination from a BaO- [BTO on Fe(001)] to a ${\mathrm{TiO}}_{2}$-terminated film [BTO on Pt(001)] is observed which goes in parallel with the adsorption of submonolayer amounts of oxygen at metal hollow sites of the interface. Our results may suggest a new route to an overall control of both the surface and the interface geometry in ${\mathrm{BaTiO}}_{3}/\mathrm{\text{metal}}$ contacts.

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

Using surface x-ray diffraction and ab initio calculations we present a model of the BaTiO3(001)-(2 x 1) surface structure, which has not been considered so far. While the crystal is terminated by two TiO2 layers similarly to SrTiO3(001)-(2 x 1), we find that one out of two surface layer Ti- atoms resides in a tetragonal pyramidal oxygen environment. This peculiar geometry leads to a metallic and magnetic surface involving local magnetic moments up to 2 mu(B) in magnitude located at surface Ti and O atoms. Our results are important for the understanding of the intrinsic surface metallicity of insulating oxides in general.

Magnetoelectric coupling allows for manipulating the magnetization by an external electric field or the electrical polarization by an external magnetic field. Here, we propose a mechanism to electrically induce 180\ifmmode^\circ\else\textdegree\fi{} magnetization switching combining two effects: the magnetoelectric coupling at a multiferroic interface and magnetic interlayer exchange coupling. By means of first-principles methods, we investigate a ferroelectric layer in contact with a Fe/Au/Fe trilayer. The calculations show that the interface magnetism is strongly coupled to the ferroelectric layer. Furthermore, under certain conditions a reversal of polarization causes a sign reversal of the interlayer exchange coupling which is results in a 180\ifmmode^\circ\else\textdegree\fi{} switching of the free layer magnetization. We argue that this magnetoelectric coupling mechanism is very robust and can find applications in magnetic data storage.

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.

Using scanning tunneling microscopy experiments in combination with first-principles calculations we have studied the geometric structure of the compressed $c(7\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})$ antiphase domain structure of CO on Cu(001). We find direct evidence for structural relaxations involving an inhomogeneous CO environment characterized by molecular tilting, bending, and nonterminal sites. Our analysis solves the long-standing problem of the adsorption structure of the compressed phase and is important for understanding the physical properties of this fundamental adsorption system.

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.

From the basis of ab initio electronic structure calculations which include the effects of thermally excited magnetic fluctuations, we predict Mn-stabilized cubic zirconia to be ferromagnetic above 500 K. We find this material, which is well known both as an imitation diamond and as a catalyst, to be half-metallic with the majority and minority spin Mn impurity states lying in zirconia's wide gap. The Mn concentration can exceed 40%. The high-T-C ferromagnetism is robust to oxygen vacancy defects and to how the Mn impurities are distributed on the Zr fcc sublattice. We propose this ceramic as a promising future spintronics material.

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.