Electrical manipulation of antiferromagnets with specific symmetries offers the prospect of creating novel, antiferromagnetic spintronic devices. Such devices aim to make use of the insensitivity to external magnetic fields and the ultrafast dynamics at the picosecond timescale intrinsic to antiferromagnets. The possibility to electrically switch antiferromagnets was first predicted for Mn2Au and then experimentally observed in tetragonal CuMnAs. Here, we report on the electrical switching and detection of the Néel order in epitaxial films of Mn2Au. The exponential dependences of the switching amplitude on the current density and the temperature are explained by a macroscopic thermal activation model taking into account the effect of the Joule heating in Hall cross devices and we observe that the thermal activation plays a key role in the reorientation process of the Néel order. Our model analysis shows that the electrically set Néel-state is long-term stable at room temperature, paving the way for practical applications in memory devices. arXiv:1706.06983v2 [cond-mat.mtrl-sci]

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity was measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6 ± 0.1) µB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02 µB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect. In spintronics1 and spin caloritronics 2 pure spin currents can be generated in ferromagnetic insulators (FMIs) by spin pumping 3 , the spin Hall effect 4 and the spin Seebeck effect 5 . Since these spin currents play an important role in spintronic applications, an understanding of the generation, manipulation and detection of spin currents is an important topic of research. A common spin current detection technique uses a nonferromagnetic metal (NM) thin film grown on a ferromagnet (FM). The inverse spin Hall effect 6 converts the spin current into a transverse voltage in the NM. Pt is commonly used as NM due to its large spin Hall angle 7 , but has generated some controversy in the interpretation because of its closeness to the Stoner criterion, which can induce, e.g., Hall or Nernst effects due to the proximity to the FM 8 .For a quantitative evaluation of the spin Seebeck effect (thermal generation of spin currents) one has to exclude or separate various parasitic effects. It is reported 5 that in transverse spin Seebeck experiments a spin current is generated perpendicular to the applied temperature gradient which is typically aligned in-plane. For ferromagnetic metals (FMMs) with magnetic anisotropy, the planar Nernst effect 9 can contribute 10 due to the anisotropic magnetothermopower. Furthermore, out-of-plane temperature gradients due to heat flow into the surrounding area 11 or through the electrical contacts 12 can induce an anomalous Nernst effect (ANE) [13][14][15] or even an unintended longitudinal spin Seebeck effect as recently reported 16 .The longitudinal spin Seebeck effect (LSSE) 17 describes a spin current that is generated parallel to the temperature gradient, which is typically aligned outof-plane to drive the parallel spin current directly into the NM material. For FMMs or semiconducting ferromagnets an ANE can also contribute to the measured voltage 18 . Furthermore, for NM materials close to the Stoner criterion a static magnetic proximity effect in the NM at the NM/FMI interface can lead to a proximity induced ANE 8 . If an in-plane temperature gradient is applied, a proximity induced planar Nernst effect ...

We fabricated NiFe 2 O 4 thin films on MgAl 2 O 4 (001) substrates by reactive dc magnetron co-sputtering in a pure oxygen atmosphere at different substrate temperatures. The film properties were investigated by various techniques with a focus on their structure, surface topography, magnetic characteristics, and transport properties. Structural analysis revealed a good crystallization with epitaxial growth and low roughness and a similar quality as in films grown by pulsed laser deposition. Electrical conductivity measurements showed high room temperature resistivity (12 Ωm), but low activation energy, indicating an extrinsic transport mechanism. A band gap of about 1.55 eV was found by optical spectroscopy. Detailed x-ray spectroscopy studies confirmed the samples to be ferrimagnetic with fully compensated Fe moments. By comparison with multiplet calculations of the spectra we found that the cation valencies are to a large extent Ni 2+ and Fe 3+ .

YPtBi, a topological semimetal with very low carrier density, was recently found to be superconducting below Tc = 0.77 K. In the conventional theory, the nearly vanishing density of states around the Fermi level would imply a vanishing electron-phonon coupling and would therefore not allow for superconductivity. Based on relativistic density functional theory calculations of the electron-phonon coupling in YPtBi it is found that carrier concentrations of more than 10 21 cm −3are required to explain the observed critical temperature with the conventional pairing mechanism, which is several orders of magnitude larger than experimentally observed. It is very likely that an unconventional pairing mechanism is responsible for the superconductivity in YPtBi and related topological semimetals with the Half-Heusler structure.A series of Half-Heusler compounds with heavy elements were predicted to have topologically non-trivial band order [1][2][3]. These compound have high cubic symmetry, however without inversion symmetry. The normal band order with the s-like, twofold degenerate Γ 6 state sitting above the p-like, fourfold degenerate Γ 8 -state is inverted in some of these compounds due to spin-orbit coupling. In their natural state, the cubic symmetry leads to a band degeneracy at Γ around the Fermi level, rendering them semimetals with topologically non-trivial band order (topological semimetals) and very low density of states (DOS) at the Fermi level D(E F ). By breaking the cubic symmetry with some amount of uniaxial strain, the compounds can be made insulating, so they could become 3D topological insulators [4]. They would exhibit metallic surface states with Dirac-like dispersion, i.e., the electrons behave as massless particles with ultrahigh mobility, while at the same time being insulating in the bulk. These surface states are topologically protected as long as time-reversal symmetry is preserved, i.e., they are protected against scattering from non-magnetic impurities. Indeed, for some of these materials there is experimental evidence for topologically nontrivial bandstructures and the presence of Dirac surface states [5][6][7], although none were found to be insulating in the bulk. Some compounds from this class were found to be superconductors with critical temperatures up to 1.8 K, e.g. LaPtBi, LuPtBi, LuPdBi, YPtBi,. Compounds of the type RPdBi (R is a lanthanide with an open 4f shell) that show coexisiting local moment antiferromagnetism as well as superconductivity were found, pointing to the presence of spin triplet Cooper pairs [12], which is allowed due to the missing structural inversion symmetry [13]. Due to the topologically nontrivial band structure, novel collective excitations are possible, in particular surface Majorana fermions [14]. These could provide the basis for low-decoherence quantum processing [15].Superconducting semiconductors such as GeTe and SnTe are long known [16,17] and their superconductivity can be explained [17] with the Eliashberg theory of electron-phonon mediated superc...

We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/FM heterostructures. Different XRMR experiments are carried out and the results are evaluated for their dependence on the magneto-optical depth profile, the photon energy, the optical parameters, and the ferromagnetic material. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight into the spatial distribution of the spin polarization of a nonmagnetic metal across the interface to a ferromagnetic layer. The evaluation of the experimental results with simulations based on optical data from ab initio calculations provides the induced magnetic moment per Pt atom in the spin-polarized volume adjacent to the ferromagnet. For a series with different ferromagnetic materials consisting of Pt/Fe, Pt/Ni 33 Fe 67 , Pt/Ni 81 Fe 19 (permalloy), and Pt/Ni bilayers we find the largest spin polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni. Additional XRMR experiments with varying photon energy are in good agreement with the theoretical predictions for the energy dependence of the magneto-optical parameters and allow identifying the optical dispersion δ and absorption β across the Pt L 3 -absorption edge.

We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800 Oe at room temperature with a coercive field around 600 Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be J eff = 0.41 mJ/m 2 and K eff = 37 kJ / m 3 . The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds 325• C, however the median blocking temperature in the limit of thick MnN is 160• C. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof-of-principle we demonstrate a simple GMR stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.

In a theoretical study we investigate the electronic structure and the band gap of the inverse spinel ferrite NiFe2O4. The experimental optical absorption spectrum is accurately reproduced by fitting the Tran-Blaha parameter in the modified Becke-Johnson potential. The accuracy of the commonly applied Tauc plot to find the optical gap is assessed based on the computed spectra and we find that this approach can lead to a misinterpretation of the experimental data. The minimum gap of NiFe2O4 is found to be a 1.53 eV wide indirect gap, which is located in the minority spin channel.Today, DFT is the main tool to obtain the electronic structure of solids.1,2 A long-standing problem of electronic structure theory is the description of transition metal oxides. These exhibit strong electron-electron correlation, which is not properly accounted for by the density functional theory (DFT) with approximate local functionals. Here we focus on NiFe 2 O 4 , a ferrimagnetic inverse spinel ferrite, 3,4 which poses an example of such difficult to describe systems. Experimental investigations mostly based on optical absorption on this material found band gaps between 1.5 eV and 5 eV.5-13 Theoretical investigations on the electronic characteristics of bulk NFO using a self-interaction-corrected local spin-density approximation (SIC-LSDA) approach, 14 or by including a Hubbard correction in terms of the DFT+U method 15,16 have predicted a bandgap of around 1 eV. Sun et al. performed band structure calculations using DFT+U and a hybrid functional (HSE06).13 They obtained a bandgap of 2.7 eV with HSE06 and 1.6 eV for the DFT+U computations. Thus, there is still a lot of controversy on the band structure and the gap of NiFe 2 O 4 .The appropriate framework to discuss electron correlations and band structures is the many-body perturbation theory, e.g., within the GW approximation.17,18 Unfortunately, this approach is computationally very expensive. Tran and Blaha recently proposed an alternative, similarly accurate and computationally cheaper method to obtain the band gap directly as differences of Kohn-Sham eigenvalues: they modified the Becke-Johnson exchange potential 19 with a parameter c, so that it readswhere n σ (r) is the spin-dependent electron density and t σ (r) is the spin-dependent kinetic-energy density. v BR x,σ (r) is the Becke-Roussel potential, which models the Coulomb potential created by the exchange hole. 21 Due to the kinetic-energy dependent term in the mBJ potential, it reproduces the step-structure and derivative discontinuity of the effective exact exchange potential of free atoms. 22 The parameter c was proposed to be determined self-consistently from the density and is related to the dielectric response of the system. 23,24 c increases with the gap size and has a typical range of 1.1-1.7. 20The mBJ potential has been proposed to be combined with LDA correlation (mBJLDA). Its particular merits and limits have been reviewed by Koller et al. 25In recent publications, the performance of mBJLDA for complete band s...

Quasiparticle spectra of potentially half-metallic Co 2 MnSi and Co 2 FeSi Heusler compounds have been calculated within the one-shot GW approximation in an all-electron framework without adjustable parameters. For Co 2 FeSi the many-body corrections are crucial: a pseudogap opens and good agreement of the magnetic moment with experiment is obtained. Otherwise, however, the changes with respect to the density-functional-theory starting point are moderate. For both cases we find that photoemission and x-ray absorption spectra are well described by the calculations. By comparison with the GW density of states, we conclude that the Kohn-Sham eigenvalue spectrum provides a reasonable approximation for the quasiparticle spectrum of the Heusler compounds considered in this work.