To explain the observed features of k-space photoelectron images taken on off-stoichiometric Heusler Ni49.7Mn29.1Ga21.2 single-crystals in the cubic austenitic and pseudotetragonal martensitic phases, the images were simulated theoretically. Despite the moderate structural difference of both phases, there is large difference in photoemission spectra. Analysis of the final states’ structure, matrix elements, and interface barrier scattering was performed to interpret discrepancies between the external photoemission of the austenite and martensite. The missing signal at the surface-normal emission of the martensitic phase is, ultimately, explained by repeated scatterings of escaping electrons on the interfaces between nanotwins.

Atomic ordering of ferromagnetic alloys, such as Heusler compounds, vastly influences their magnetic properties and overall usability. However, in some cases x-ray diffraction cannot unambiguously determine the correct atomic structure, which hampers further research. In this paper, we analyze the atomic ordering using spectroscopic ellipsometry-a simple tabletop method that researchers can use in situ during sample deposition. The Co2Fe(Ga0.5Ge0.5) Heusler compound was chosen for the analysis, as the atomic ordering greatly influences its spin-polarizing capabilities. The ellispometric results are confronted with x-ray diffraction and with the help of ab initio calculations the qualitative changes in the optical response are linked to the changes in atomic ordering.

A parameter-free electronic structure approach is applied to the study of stability and chemical order in the 15 substitutional body-centered cubic (bcc)-based alloys made of the six transition metals of groups 5 (V, Nb, Ta) and 6 (Cr, Mo, W) of the periodic table. The method is based on a Green's function description of the electronic structure of the random alloys. Configurational order is treated within the generalized perturbation method, and temperature effects are examined with a generalized mean-field approach. In contrast to the results summarized in the assessed phase diagrams, stability and ordering trends are predicted in a broad range of alloy composition for at least seven alloys, and explanation is found in their electronic structure properties. Short-range order results, thermodynamics analysis, and bcc-based phase diagrams are also presented.

Doping ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ by magnetic ions represents an interesting problem since it may break the time-reversal symmetry needed to maintain the topological insulator character. Mn dopants in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory has focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by a van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T}_{\mathrm{C}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T}_{\mathrm{C}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

Temperature-dependent resistivity and magnetoresistance are measured in bulk tetragonal phase of antiferromagnetic CuMnAs and the latter is found to be anisotropic both due to structure and magnetic order. We compare these findings to model calculations with chemical disorder and finite-temperature phenomena included. The finite-temperature ab initio calculations are based on the alloy analogy model implemented within the coherent potential approximation and the results are in fair agreement with experimental data. Regarding the anisotropic magnetoresistance (AMR) which reaches a modest magnitude of 0.12%, we phenomenologically employ the Stoner-Wohlfarth model to identify temperature-dependent magnetic anisotropy of our samples and conclude that the field-dependence of AMR is more similar to that of antiferromagnets than ferromagnets, suggesting that the origin of AMR is not related to isolated Mn magnetic moments.

The electronic and transport properties of the ordered B2-(Fe60Al40) phase which undergoes a continuous transition into the disordered A2-(Fe60Al40) phase are studied from first principles. The disordering is characterized as a gradual interchange of atoms between Fe and Al sublattices under the condition that the total amount of Fe and Al atoms is kept unchanged. This is the simplest model of gradual disordering of the ordered phase due to the ion irradiation. The physical properties are strongly influenced by varying local environment of Fe atoms on both sublattices. This leads to the transition between a high moment at large disorder and a very low moment in the ordered phase. Similar behavior is found also for anomalous Hall conductivity and anomalous Hall angle. Unusual behavior of the longitudinal conductivity as a function of degree of disorder is due to the shift of the Fermi level of majority states from the sp-like part to the d states with increasing ordering. The disordered phase has a large anomalous Hall angle as contrasted with a negligible anomalous Hall angle for well ordered samples, which is in agreement with recent experiment.

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-converging 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.

The spin Hall effect in fcc Pt-rich disordered solid solutions Pt-M (M = V, Nb, Ta) is studied by means of a relativistic ab initio Green-function theory.We find that the spin Hall angle of all three alloy systems exhibits similar sizable values, with a maximum slightly above 0.2, obtained for dopant concentrations around 12 at.%.This result is explained by a competition between opposite concentration trends of the intrinsic spin Hall conductivity and of the longitudinal residual resistivity while the strength of the impurity spin-orbit interaction plays only a minor role.This behavior agrees qualitatively with results of recent experiments on spin Hall torques in multilayered systems involving Pt-rich alloys with heavy and light impurities.

The high-entropy alloys AlxCrFeCoNi exist over a broad range of Al concentrations (0 < x < 2). With increasing Al content their structure is changed from the fcc to bcc phase. We investigate the effect of such structural changes on transport properties including the residual resistivity and the anomalous Hall resistivity. We have performed a detailed comparison of the first-principles simulations with available experimental data. We show that the calculated residual resistivities for all studied alloy compositions are in a fair agreement with available experimental data as concerns both the resistivity values and concentration trends. We emphasize that a good agreement with experiment was obtained also for the anomalous Hall resistivity. We have completed study by estimation of the anisotropic magnetoresistance, spin-disorder resistivity, and Gilbert damping. The obtained results prove that the main scattering mechanism is due to the intrinsic chemical disorder whereas the effect of spin polarization on the residual resistivity is appreciably weaker.

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).

The antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure is a promising material for the AFM spintronics. The resistivity measurements indicate the presence of defects. We confirmed vacancies on Mn or Cu sublattices and Mncu and Cum antisites as most probable defects in CuMnAs by our new ab initio total energy calculations. We have estimated resistivities of possible defect types as well as resistivities of samples for which the X-ray structural analysis is available. In the latter case we have found that samples with Cu- and Mn-vacancies with low formation energies have also resistivities which agree well with the experiment. Finally, we have also calculated exchange interactions and estimated the Neel temperatures by using the Monte Carlo approach. A good agreement with experiment was obtained.

We investigate theoretically the combined effect of phonons and magnons caused by finite temperatures on the electrical resistivity of nonstoichiometric half-Heusler NiMnSb alloy. The coherent potential approximation within the alloy analogy model is employed for an efficient treatment of chemical impurities, atomic displacements, and magnetic disorder. Spin fluctuations of local Mn moments are described by two models: (i) uncompensated disordered local moment approach and (ii) filling of the moments. The calculated resistivity agrees with experimental data, the agreement is good up to 600 K. We show that a strong magnetic disorder leads to a violation of the Matthiessen's rule for the resistivity. We also discuss the spin polarization of the electrical current which exceeds 90% at room temperature but it is dramatically reduced by the magnetic disorder for higher temperatures approaching the Curie point (T-C = 730 K).

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.

We present implementation of the alloy analogy model within fully relativistic density-functional theory with the coherent potential approximation for a treatment of nonzero temperatures. We calculate contributions of phonons and magnetic and chemical disorder to the temperature-dependent resistivity, anomalous Hall conductivity (AHC), and spin-resolved conductivity in ferromagnetic half-Heusler NiMnSb. Our electrical transport calculations with combined scattering effects agree well with experimental literature for Ni-rich NiMnSb with 1--2% Ni impurities on Mn sublattice. The calculated AHC is dominated by the Fermi surface term in the Kubo-Bastin formula. Moreover, the AHC as a function of longitudinal conductivity consists of two linear parts in the Ni-rich alloy, while it is nonmonotonic for Mn impurities. We obtain the spin polarization of the electrical current $P>90%$ at room temperature and we show that $P$ may be tuned by chemical composition. The presented results demonstrate the applicability of an efficient first-principles scheme to calculate temperature dependence of linear transport coefficients in multisublattice bulk magnetic alloys.

The spin-disorder resistivity (SDR) of a broad range of magneticmaterials, both ordered and disordered, is reviewed.We identify the SDR at the critical temperature with the residualresistivity of the corresponding system evaluated in the frameworkof the disordered local moment (DLM) model.The underlying electronic structure is determined in the frameworkof the tight-binding linear muffin-tin orbital method which employsthe coherent potential approximation to describe the DLM stateand chemical disorder.The DLM fixed-spin moment method is used in the case when the DLMmoment collapses.The Kubo-Greenwood approach is employed to estimate the resistivityof the DLM state.Formalism is applied to Fe and Ni and its alloys, Heusler alloys,and ordered ferromagnetic and antiferromagnetic alloys.Finally, the SDR of the Earth's core will be studied using thesame formalism.Calculations are compared with available experimental data.

In shape memory materials the fine twinned microstructure plays a fundamental role. Here we show that in the martensite phase of Ni-Mn-Ga Heusler alloy, the fine features of electronic structure are not caused by intrinsic electronic changes but mesoscopically different orientations of ferroelastic domains, i.e., twins. The cuts of Fermi surfaces of a (pseudo) tetragonally distorted Ni49.7Mn29.1Ga21.2 Heusler single crystal with a (100) surface orientation were measured using angle-resolved photoemission spectroscopy and compared with first-principles calculations. In this work we demonstrated that the measured photoelectron spectrum was a projection of three separate electronic structures originating from single a-a and two perpendicular a-c ferroelastic domains with a twinned relationship. The twinning results in pseudosplitting of the experimentally observed bands at Fermi level.

We revise critically existing approaches to evaluation of thermodynamic potentials within the Green's function calculations at finite electronic temperatures. We focus on the entropy and show that usual technical problems related to the multivalued nature of the complex logarithm can be overcome. This results in a simple expression for the electronic entropy, which does not require any contour integration in the complex energy plane. Properties of the developed formalism are discussed and its illustrating applications to selected model systems and to bcc iron with disordered local magnetic moments are presented as well.

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 a 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 a tot and AHC agree reasonably with the experiment. On the basis of calculated transport quantities a 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.

The spin-disorder resistivity (SDR) of a disordered fcc-(Ni1-x, Fe-x) alloy is determined from first principles. We identify the SDR at and above the critical temperature with the residual resistivity of the corresponding paramagnetic state evaluated in the framework of the disordered local moment (DLM) model. The underlying electronic structure is determined by means of the tight-binding linear muffin-tin orbital method, which employs the coherent potential approximation (CPA) to describe both the DLM state and the chemical disorder in alloys. An extension of the DLM fixed-spin moment method for two independent magnetic moments is used and combined with the paramagnetic lattice gas entropy to determine local moments by minimizing the corresponding free energy. The effect of phonon scattering is included through the mapping of static atomic displacements into a multicomponent random alloy which is then treated in the CPA. Finally, the Kubo-Greenwood-CPA approach is employed to estimate the SDR. We also address the problem of the validity of the Matthiessen rule at the Curie point. Good agreement of calculated and measured SDR is obtained over the whole studied concentration range; the results point to the importance of nonzero Ni magnetic moments in the limit of pure nickel.