arXiv subjects

Hubert Ebert

Publications and source records attributed to Hubert Ebert.

At least 55 records · Page 3Linked to original sources

arXiv2019

Surface resonance of the Heusler half metal Co2MnSi probed by SX-ARPES

Heusler compounds are promising materials for spintronics with adjustable electronic properties including 100% spin polarization at the Fermi energy. We investigate the electronic states of AlOx capped epitaxial thin films of the ferromagnetic half metal Co2MnSi ex-situ by soft X-ray angular resolved photoemission spectroscopy (SX-ARPES). Good agreement between the experimental SX- ARPES results and photoemission calculations including surface effects was obtained. In particular, we observed in line with our calculations a large photoemission intensity at the center of the Brillouin zone, which does not originate from bulk states, but from a surface resonance. This provides strong evidence for the validity of the previously proposed model based on this resonance, which was applied to explain the huge spin polarization of Co2MnSi observed by angular-integrating UV-photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2018

Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide

The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.

cond-mat.mtrl-sci↗

arXiv2018

Signature of a highly spin polarized resonance state at Co2MnSi(001)/Ag(001) interfaces

We investigated interfaces of halfmetallic Co2MnSi(100) Heusler thin films with Ag(100), Cr(100), Cu and Al layers relevant for spin valves by high energy x-ray photoemission spectroscopy (HAXPES). Experiments on Co2MnSi samples with an Ag(100) interface showed a characteristic spectral shoulder feature close to the Fermi edge, which is strongly diminished or suppressed at Co2MnSi (100) interfaces with the other metallic layers. This feature is found to be directly related to the Co2MnSi(100) layer, as reflected by control experiments with reference non-magnetic films, i.e. without Heusler layer. By comparison with HAXPES calculations, the shoulder feature is identified as originating from an interface state related to a highly spin polarized surface resonance of Co2MnSi (100).

cond-mat.mtrl-sci↗

arXiv2018

Large Spin Hall Effect in an Amorphous Binary Alloy

We investigate the spin Hall effect of W-Hf thin films, which exhibit a phase transition from a segregated phase mixture to an amorphous alloy below 70% W. The spin Hall angle was determined with a planar harmonic Hall voltage technique. Due to the accompanying jump in resistivity, the spin Hall angle shows a pronounced maximum at the composition of the phase transition. The spin Hall conductivity does, however, reduce from W to Hf with a weak discontinouity across the phase transition. The maximum spin Hall angle of $θ_\mathrm{SH} = -0.25$ is obtained for amorphous W$_{0.7}$Hf$_{0.3}$. A detailed comparison with spin Hall conductivities calculated from first principles for hcp, fcc, and bcc solid solutions provides valuable insight into the alloying physics of this binary system.

cond-mat.mtrl-sci↗

arXiv2017

Temperature-dependent charge transport in the compensated ferrimagnet Mn$_{1.5}$V$_{0.5}$FeAl from first principles

We present an ab-initio study of the temperature-dependent longitudinal and anomalous Hall resistivities in the compensated collinear ferrimagnet Mn$_{1.5}$V$_{0.5}$FeAl. Its transport properties are calculated using the general fully relativistic Kubo--Bastin formalism and their temperature dependency is accounted for magnetic and structural disorder. Both scattering sources, together with the residual chemical disorder, were treated equally provided by the CPA (Coherent Potential Approximation) SPR-KKR (Spin-Polarized Relativistic Korringa-Kohn-Rostoker) method. All calculated properties showed good agreement with a recent experimental results, providing useful specific information on the chemical and magnetic arrangement as well as on the influence of disorder. Finally, we demonstrated that the anomalous Hall effect in such compensated systems occurs regardless of the vanishing net spin moment.

cond-mat.mtrl-sci↗

arXiv2017

Native defects in the Co$_2$Ti$Z$ ($Z=$ Si, Ge, Sn) full Heusler alloys: formation and influence on the thermoelectric properties

We have performed first-principles investigations on the native defects in the full Heusler alloys Co$_2$Ti$Z$ ($Z$ one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that Co vacancies (Vc) in all compounds and the Ti$_\text{Sn}$ anti-site exhibit negative formation energies. The smallest positive values occur for Co in excess on anti-sites (Co$_Z$ or Co$_\text{Ti}$) and for Ti$_Z$. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced, changes in the electronic structure above the half-metallic gap. Two of the systems, Vc$_\text{Co}$ and Co$_Z$, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co$_2$Ti$Z$ compounds as being due to unintentionally created native defects.

cond-mat.mtrl-sci↗

arXiv2017

Fermi surface and effective masses in photoemission response of the (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ superconductor

The angle-resolved photoemission spectra of the superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ have been investigated both experimentally and theoretically. Our results explain the previously obscured origins of all salient features of the ARPES response of this paradigm pnictide compound and reveal the origin of the Lifshitz transition. Comparison of calculated ARPES spectra with the underlying DMFT band structure shows an important impact of final state effects, which results for three-dimensional states in a deviation of the ARPES spectra from the true spectral function. In particular, the apparent effective mass enhancement seen in the ARPES response is not an entirely intrinsic property of the quasiparticle valence bands but may have a significant extrinsic contribution from the photoemission process and thus differ from its true value. Because this effect is more pronounced for low photoexcitation energies, soft-X-ray ARPES delivers more accurate values of the mass enhancement due to a sharp definition of the 3D electron momentum.

cond-mat.supr-con↗

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↗

arXiv2016

Vectorial spin-polarization detection in multichannel spin-resolved photoemission spectroscopy using an Ir(001) imaging spin filter

We report on spin- and angular resolved photoemission spectroscopy using a high-resolution imaging spin filter based on a large Ir(001) crystal enhancing the effective figure of merit for spin detection by a factor of over $10^3$ compared to standard single channel detectors. Furthermore, we review the spin filter preparation, and its lifetime. The spin filter efficiency is mapped on a broad range of scattering energies and azimuthal angles. Large spin filter efficiencies are observed for the spin component perpendicular as well as parallel to the scattering plane depending on the azimuthal orientation of the spin filter crystal. A spin rotator capable of manipulating the spin direction prior to detection complements the measurement of three observables, thus allowing for a derivation of all three components of the spin polarization vector in multichannel spin polarimetry. The experimental results nicely agree with spin-polarized low energy electron diffraction calculations based on a fully relativistic multiple scattering method in the framework of spin-polarized density functional theory.

physics.ins-det↗

arXiv2016

Magnetocrystalline anisotropy of FePt: a detailed view

To get a reliable ab-initio value for the magneto-crystalline anisotropy (MCA) energy of FePt, we employ the full-potential linearized augmented plane wave (FLAPW) method and the full-potential Korringa-Kohn-Rostoker (KKR) Green function method. The MCA energies calculated by both methods are in a good agreement with each other. As the calculated MCA energy significantly differs from experiment, it is clear that many-body effects beyond the local density approximation are essential. It is not really important whether relativistic effects for FePt are accounted for by solving the full Dirac equation or whether the spin-orbit coupling (SOC) is treated as a correction to the scalar-relativistic Hamiltonian. From the analysis of the dependence of the MCA energy on the magnetization angle and on the SOC strength it follows that the main mechanism of MCA in FePt can be described within second order perturbation theory. However, a distinct contribution not accountable for by second order perturbation theory is present as well.

cond-mat.mtrl-sci↗

arXiv2016

Local environment effects in the magnetic properties and electronic structure of disordered FePt

Local aspects of magnetism of disordered FePt are investigated by ab initio fully relativistic full potential calculations, employing the supercell approach and the coherent potential approximation (CPA). The focus is on trends of the spin and orbital magnetic moments with chemical composition and with bond lengths around the Fe and Pt atoms. A small but distinct difference between average magnetic moments obtained when using the supercells and when relying on the CPA is identified and linked to the neglect of the Madelung potential in the CPA.

cond-mat.mtrl-sci↗

arXiv2016

Spin polarization and attosecond time delay in photoemission from spin degenerate states of solids

After photon absorption, electrons from a dispersive band of a solid require a finite time in the photoemission process before being photoemitted as free particles, in line with recent attosecondresolved photoemission experiments. According to the Eisenbud-Wigner-Smith model, the time delay is due to a phase shift of different transitions that occur in the process. Such a phase shift is also at the origin of the angular dependent spin polarization of the photoelectron beam, observable in spin degenerate systems without angular momentum transfer by the incident photon. We propose a semi-quantitative model which permits to relate spin and time scales in photoemission from condensed matter targets and to better understand spin- and angle-resolved photoemission spectroscopy (SARPES) experiments on spin degenerate systems. We also present the first experimental determination by SARPES of this time delay in a dispersive band, which is found to be greater than 26 as for electrons emitted from the sp-bulk band of the model system Cu(111).

cond-mat.mes-hall↗

arXiv2016

Study of the hyperfine fields in the BaFe$_2$As$_2$ family and its relation to the magnetic moment

The hyperfine field $B_\text{hf}$ and the magnetic properties of the BaFe$_2$As$_2$ family are studied using the fully relativistic Dirac formalism for different types of substitution. The study covers electron doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, hole doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ and also isovalently doped Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$ for a wide range of the concentration $x$. For the substituted compounds the hyperfine fields show a very strong dependence on the dopant type and its concentration $x$. Relativistic contributions were found to have a significantly stronger impact for the iron pnictides when compared to bulk Fe. As an important finding, we demonstrate that it is not sensible to relate the hyperfine field $B_\text{hf}$ to the average magnetic moment $μ$ of the compound, as it was done in earlier literature.

cond-mat.supr-con↗

arXiv2016

Impact of finite temperatures on the transport properties of Gd from first principles

Finite temperature effects have a pronounced impact on the transport properties of solids. In magnetic systems, besides the scattering of conduction electrons by impurities and phonons, an additional scattering source coming from the magnetic degrees of freedom must be taken into account. A first-principle scheme which treats all these scattering effects on equal footing was recently suggested within the framework of the multiple scattering formalism. Employing the alloy analogy model treated by means of the CPA, thermal lattice vibrations and spin fluctuations are effectively taken into account. In the present work the temperature dependence of the longitudinal resistivity and the anomalous Hall effect in the strongly correlated metal Gd is considered. The comparison with experiments demonstrates that the proposed numerical scheme does provide an adequate description of the electronic transport at finite temperatures.

cond-mat.mtrl-sci↗

arXiv2016

In-plane anisotropic magnetoresistance in antiferromagnetic Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$

Using the Kubo-Greenwood formalism the resistivity anisotropy for electron doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, hole doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ and isovalently doped Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ in their antiferromagnetic state has been calculated in order to clarify the origin of this important phenomenon. The results show good agreement with experiment for all cases without considering impurity states extending over several unit cells or temperature induced spin fluctuations. From this it is concluded that the resistivity anisotropy at low temperatures is primarily caused by an in-plane anisotropic magnetoresistance. Accounting for the band dispersion with respect to $k_z$ is however mandatory to explain the results, showing the importance of the three-dimensional character of the electronic structure for the iron pnictides. Furthermore, it is shown that the counterintuitive sign of the resistivity anisotropy is no fundamental property but just a peculiarity of the anisotropic band structure.

cond-mat.supr-con↗

arXiv2016

Influence of spin-orbit coupling on the magnetic dipole term $T_α$

The influence of the spin-orbit coupling (SOC) on the magnetic dipole term $T_α$ is studied across a range of systems in order to check whether the $T_α$ term can be eliminated from analysis of x-ray magnetic circular dichroism spectra done via the spin moment sum rule. Fully relativistic Korringa-Kohn-Rostoker (KKR) Green function calculations for Co monolayers and adatoms on Cu, Pd, Ag, Pt, and Au (111) surfaces were performed to verify whether the sum over magnetic dipole terms $T_{x}+T_{y}+T_{z}$ is zero and whether the angular dependence of the \ta\ term goes as $3\cos^{2}θ-1$. It follows that there are circumstances when the influence of the SOC on $T_α$ cannot be neglected even for 3$d$ atoms, where the SOC is nominally small. The crucial factor appears to be the dimensionality of the system: for 3$d$ adatoms, the influence of SOC on $T_α$ can be significant while for monolayers it is always practically negligible. Apart from the dimensionality, hybridization between adatom and substrate states is also important: small hybridization enhances the importance of the SOC and vice versa.

cond-mat.mtrl-sci↗

arXiv2016

Illustrative view on the magnetocrystalline anisotropy of adatoms and monolayers

Even though it has been known for decades that the magnetocrystalline anisotropy is linked to the spin-orbit coupling (SOC), the mechanism how it arises for specific systems is still subject of debate. We focused on finding markers of SOC in the density of states (DOS) and on employing them for understanding the source of magnetocrystalline anisotropy for the case of adatoms and monolayers. Fully relativistic ab-initio KKR-Green function calculations were performed for Fe, Co, and Ni adatoms and monolayers on Au(111) to investigate changes in the orbital-resolved DOS due to a rotation of magnetization. In this way one can see that a significant contribution to the magnetocrystalline anisotropy for adatoms comes from pushing of the SOC-split states above or below the Fermi level. As a result of this, the magnetocrystalline anisotropy energy crucially depends on the position of the energy bands of the adatom with respect to the Fermi level of the substrate. This view is supported by model crystal field Hamiltonian calculations.

cond-mat.mtrl-sci↗

arXiv2016

A fully relativistic description of spin-orbit torques by means of linear response theory

Symmetry and magnitude of spin-orbit torques (SOT), i.e., current-induced torques on the magnetization of systems lacking inversion symmetry, are investigated in a fully relativistic linear response framework based on the Kubo formalism. By applying all space-time symmetry operations contained in the magnetic point group of a solid to the relevant response coefficient, the torkance expressed as torque-current correlation function, restrictions to the shape of the direct and inverse response tensors are obtained. These are shown to apply to the corresponding thermal analogues as well, namely the direct and inverse thermal SOT in response to a temperature gradient or heat current. Using an implementation of the Kubo-Bastin formula for the torkance into a first-principles multiple-scattering Green's function framework and accounting for disorder effects via the so-called coherent potential approximation (CPA), all contributions to the SOT in pure systems, dilute as well as concentrated alloys can be treated on equal footing. This way, material specific values for all torkance tensor elements in the fcc (111) trilayer alloy system Pt | Fe$_x$Co$_{1-x}$ | Cu are obtained over a wide concentration range and discussed in comparison to results for electrical and spin conductivity, as well as to previous work - in particular concerning symmetry w.r.t. magnetization reversal and the nature of the various contributions.

cond-mat.mtrl-sci↗