arXiv subjects

D. Ködderitzsch

Publications and source records attributed to D. Ködderitzsch.

15 recordsLinked to original sources

arXiv2017

Observation of the spin Nernst effect

The observation of the spin Hall effect triggered intense research on pure spin current transport. With the spin Hall effect, the spin Seebeck effect, and the spin Peltier effect already observed, our picture of pure spin current transport is almost complete. The only missing piece is the spin Nernst (-Ettingshausen) effect, that so far has only been discussed on theoretical grounds. Here, we report the observation of the spin Nernst effect. By applying a longitudinal temperature gradient, we generate a pure transverse spin current in a Pt thin film. For readout, we exploit the magnetization-orientation-dependent spin transfer to an adjacent Yttrium Iron Garnet layer, converting the spin Nernst current in Pt into a controlled change of the longitudinal thermopower voltage. Our experiments show that the spin Nernst and the spin Hall effect in Pt are of comparable magnitude, but differ in sign, as corroborated by first-principles calculations.

cond-mat.mes-hall↗

arXiv2015

Resonant impurity states in chemically disordered half-Heusler Dirac semimetals

We address the electron transport characteristics in bulk half-Heusler alloys with their compositions tuned to the borderline between topologically nontrivial semi-metallic and trivial semiconducting phases. The precise first-principles calculations based on the coherent potential approximation (CPA) reveal that all the studied systems exhibit sets of dispersionless impurity-like resonant levels, with one of them being located right at the Dirac point. By means of the Kubo formalism we reveal that the residual conductivity of these alloys is strongly suppressed by impurity scattering, whereas the spin Hall conductivity exhibits a large value which is comparable to that of Pt, thereby leading to divergent spin Hall angles.

cond-mat.mtrl-sci↗

arXiv2015

Finite temperature magnetism of FeRh compounds

The temperature dependent stability of the magnetic phases of FeRh were investigated by means of total energy calculations with magnetic disorder treated within the uncompensated disordered local moment (uDLM) approach. In addition, Monte Carlo simulations based on the extended Heisenberg model have been performed, using exchange coupling parameters obtained rom first principles. The crucial role and interplay of two factors in the metamagnetic transition in FeRh has been revealed, namely the dependence of the Fe-Fe exchange coupling parameters on the temperature-governed degree of magnetic disorder in the system and the stabilizing nature of the induced magnetic moment on Rh-sites. An important observation is the temperature dependence of these two competing factors.

cond-mat.mtrl-sci↗

arXiv2015

Dzyaloshinskii-Moriya interactions and magnetic texture in the Fe films deposited on transition-metal dichalcogenides

The magnetic properties of materials based on two-dimensional transition-metal dichalcogenides (TMDC) have been investigated by means of first-principles DFT calculations, namely Fe-intercalated bulk Fe$_{1/4}$TaS$_2$ compounds as well as TMDC monolayers with deposited Fe films. Changing the structure and the composition of systems consisting of Fe overlayers on top of a TMDC monolayers resulted in considerable variations of their physical properties. For the considered systems the Dzyaloshinskii-Moriya (DM) interaction has been determined and used for the subsequent investigation of their magnetic structure using Monte Carlo simulations. Rather strong DM interactions as well as large $D/J$ ratios have been obtained in some of these materials, which can lead to the formation of skyrmionic structures varying with the strength of the applied external magnetic field.

cond-mat.mtrl-sci↗

arXiv2015

Skyrmion magnetic structure of an ordered FePt monolayer deposited on Pt(111)

The effect of the Dzyaloshinsky-Moriya interaction on the magnetic structure of an ordered FePt monolayer deposited on Pt (111) surface has been investigated. In the ground state, the pronounced anisotropic geometry of the FePt layer with alternating Fe and Pt chains gives rise to a helimagnetic structure with a strong difference in the helicity period along the chains and perpendicular to them. In the presence of an external magnetic field, the region of stable Skyrmion magnetic structures in the $B-T$ phase diagram has been demonstrated via Monte Carlo simulations using the parameters obtained within first-principles electronic structure calculations. The present study demonstrates clearly that the ratio of the exchange coupling parameters J/D for a deposited magnetic film - being of central importance for the formation of Skyrmions - can be manipulated by growing an overlayer of 2-dimensional (2D) compounds with the atoms carrying spontaneous magnetic moments separated by the atoms of non-magnetic elements.

cond-mat.mes-hall↗

arXiv2015

Electronic, magnetic and transport properties of Fe intercalated 2H-TaS$_2$ studied by means of the KKR-CPA method

The electronic, magnetic and transport properties of Fe intercalated 2H-TaS$_2$ have been investigated by means of the Korringa-Kohn-Rostoker (KKR) method. The non-stoichiometry and disorder in the system has been accounted for using the Coherent Potential Approximation (CPA) alloy theory. A pronounced influence of disorder on the spin magnetic moment has been found for the ferro-magnetically ordered material. The same applies for the spin-orbit induced orbital magnetic moment and magneto-crystalline anisotropy energy. The temperature-dependence of the resistivity of disordered 2H-Fe$_{0.28}$TaS$_2$ investigated on the basis of the Kubo-Středa formalism in combination with the alloy analogy model has been found in very satisfying agreement with experimental data. This also holds for the temperature dependent anomalous Hall resistivity $ ρ_{\rm xy}(T) $. The role of thermally induced lattice vibrations and spin fluctuations for the transport properties is discussed in detail.

cond-mat.mes-hall↗

arXiv2015

Spin-orbit induced longitudinal spin-polarized currents in non-magnetic solids

For certain non-magnetic solids with low symmetry the occurrence of spin-polarized longitudinal currents is predicted. These arise due to an interplay of spin-orbit interaction and the particular crystal symmetry. This result is derived using a group-theoretical scheme that allows investigating the symmetry properties of any linear response tensor relevant to the field of spintronics. For the spin conductivity tensor it is shown that only the magnetic Laue group has to be considered in this context. Within the introduced general scheme also the spin Hall- and additional related transverse effects emerge without making reference to the two-current model. Numerical studies confirm these findings and demonstrate for (Au$_{1-x}$Pt$_{\rm x}$)$_4$Sc that the longitudinal spin conductivity may be in the same order of magnitude as the conventional transverse one. The presented formalism only relies on the magnetic space group and therefore is universally applicable to any type of magnetic order.

cond-mat.mes-hall↗

arXiv2013

A first-principles linear response description of the spin Nernst effect

A first-principles description of the spin Nernst effect, denoting the occurrence of a transverse spin current due to a temperature gradient, is presented. The approach, based on an extension to the Kubo-Streda equation for spin transport, supplies in particular the formal basis for investigations of diluted as well as concentrated alloys. Results for corresponding applications to the alloy system Au-Cu give the intrinsic and extrinsic contributions to the relevant transport coefficients. Using scaling laws allows in addition to split the extrinsic contribution into its skew scattering and side-jump parts.

cond-mat.mtrl-sci↗

arXiv2013

Magneto-electric and thermo-magneto-electric effects in ferromagnetic transition-metal alloys from first-principles

The electric and thermo-magneto-electric transport of the prototypical ferromagnetic transition-metal alloy system fcc-Co$_x$Pd$_{1-x}$ has been investigated on the basis of Kubo's linear response formalism. The results for the full electric conductivity tensor allow to discuss the spin-orbit induced anisotropic magneto-resistance (AMR) and the anomalous Hall effect (AHE). These are complemented by results for the corresponding thermo-magneto-electric transport properties anisotropy of the Seebeck effect (ASE) and anomalous Nernst effect (ANE). The relation between the respective response coefficients is discussed with the underlying electronic structure calculated relativistically within the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure method for disordered alloys.

cond-mat.mtrl-sci↗

arXiv2010

Coherent description of the intrinsic and extrinsic anomalous Hall effect in disordered alloys on an $ab$ $initio$ level

A coherent description of the anomalous Hall effect (AHE) is presented that is applicable to pure as well as disordered alloy systems by treating all sources of the AHE on equal footing. This is achieved by an implementation of the Kubo-Středa equation using the fully relativistic Korringa-Kohn-Rostoker (KKR) Green's function method in combination with the Coherent Potential Approximation (CPA) alloy theory. Applications to the pure elemental ferromagnets bcc-Fe and fcc-Ni led to results in full accordance with previous work. For the alloy systems fcc-Fe$_x$Pd$_{1-x}$ and fcc-Ni$_x$Pd$_{1-x}$ very satisfying agreement with experiment could be achieved for the anomalous Hall conductivity (AHC) over the whole range of concentration. To interpret these results an extension of the definition for the intrinsic AHC is suggested. Plotting the corresponding extrinsic AHC versus the longitudinal conductivity a linear relation is found in the dilute regimes, that allows a detailed discussion of the role of the skew and side-jump scattering processes.

cond-mat.mtrl-sci↗

arXiv2010

Spin projection and spin current density within relativistic electronic transport calculations

A spin projection scheme is presented which allows the decomposition of the electric conductivity into two different spin channels within fully relativistic $ab$ $initio$ transport calculations that account for the impact of spin-orbit coupling. This is demonstrated by calculations of the spin-resolved conductivity of Fe$_{1-x}$Cr$_x$ and Co$_{1-x}$Pt$_x$ disordered alloys on the basis of the corresponding Kubo-Greenwood equation implemented using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure method. In addition, results for the residual resistivity of diluted Ni-based alloys are presented that are compared to theoretical and experimental ones that rely on Mott's two-current model for spin-polarized systems. The application of the scheme to deal with the spin-orbit induced spin Hall effect is discussed in addition.

cond-mat.mtrl-sci↗

arXiv2008

Electronic transport in ferromagnetic alloys and the Slater-Pauling Curve

Experimental measurements of the residual resistivity $ρ(x)$ of the binary alloy system Fe$_{1-x}$Cr$_x$ have shown an anomalous concentration dependence which deviates significantly from Nordheim's rule. In the low ($x < 10%$) Cr concentration regime the resistivity has been found to increase linearly with $x$ until $\approx$ 10% Cr where the resistivity reaches a plateau persisting to $\approx$ 20% Cr. In this paper we present $ab$-$initio$ calculations of $ρ(x)$ which explain this anomalous behavior and which are based on the Korringa-Kohn-Rostoker (KKR) method in conjunction with the Kubo-Greenwood formalism. Furthermore we are able to show that the effects of short-range ordering or clustering have little effect via our use of the nonlocal coherent-potential approximation (NL-CPA). For the interpretation of the results we study the alloys' electronic structure by calculating the Bloch spectral function particularly in the vicinity of the Fermi energy. From the analysis of our results we infer that a similar behavior of the resistivity should also be obtained for iron-rich Fe$_{1-x}$V$_x$ alloys - an inference confirmed by further explicit resistivity calculations. Both of these alloy systems belong to the same branch of the famous Slater-Pauling plot and we postulate that other alloy systems from this branch should show a similar behavior. Our calculations show that the appearance of the plateau in the resistivity can be attributed to the dominant contribution of minority spin electrons to the conductivity which is nearly unaffected by increase in Cr/V concentration $x$ and we remark that this minority spin electron feature is also responsible for the simple linear variation of the average moment in the Slater-Pauling plot for these materials.

cond-mat.mtrl-sci↗

arXiv2008

Orbital magnetism in transition-metal systems: The role of local correlation effects

The influence of correlation effects on the orbital moments for transition metals and their alloys is studied by first-principle relativistic Density Functional Theory in combination with the Dynamical Mean-Field Theory. In contrast to the previous studies based on the orbital polarization corrections we obtain an improved description of the orbital moments for wide range of studied systems as bulk Fe, Co and Ni, Fe-Co disordered alloys and 3$d$ impurities in Au. The proposed scheme can give simultaneously a correct dynamical description of the spectral function as well as static magnetic properties of correlated disordered metals.

cond-mat.str-el↗

arXiv2007

Theory of electronic transport in random alloys with short-range order: Korringa-Kohn-Rostoker non-local coherent potential approximation

We present an ab-initio formalism for the calculation of transport properties in compositionally disordered systems within the framework of the Korringa-Kohn-Rostoker non-local coherent potential approximation. Our formalism is based upon the single-particle Kubo-Greenwood linear response and provides a natural means of incorporating the effects of short-range order upon the transport properties. We demonstrate the efficacy of the formalism by examining the effects of short-range order and clustering upon the transport properties of disordered $AgPd$ and $CuZn$ alloys.

cond-mat.mtrl-sci↗

arXiv2005

Self-interaction correction in multiple scattering theory

We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LSD method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of physical quantities averaged over ensembles of such potentials using the coherent potential approximation (CPA). This facilitates applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well as intermediate valences. As a demonstration of the method, we study the well-known $α$-$γ$ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

cond-mat.str-el↗