Search

In spin-transport experiments with spin currents propagating through an antiferromagnetic (AFM) material, the antiferromagnet is mainly treated as a passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the N\'eel temperature and decreasing by lowering the temperature. To study the role of antiferromagnetism in local and nonlocal spin-transport experiments, we send spin currents through NiO of various thicknesses placed on ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below $100\phantom{\rule{4pt}{0ex}}\mathrm{K}$ that is unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence are similar to those for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in a single crystal of the rare-earth antiferromagnet DyFeO3 with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the Fe3+ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy, and the Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence of the signal on the magnetic field strength as evidence for field-induced order of the Dy3+ moments up to room temperature. At and below the Morin temperature of 50 K, the SMR monitors the spin-reorientation phase transition of Fe3+ spins. Below 23 K, additional features emerge that persist below 4 K, the ordering temperature of the Dy3+ magnetic sublattice. We conclude that the combination of SMR and SSE is a simple and efficient tool to study spin reorientation phase transitions and sublattice magnetizations.

In spintronics and spin caloritronics, the understanding of spin angular momentum transfer across heavy metal/ferromagnetic insulator interfaces is of fundamental importance. There has been much controversy about the presence of magnetic proximity effects at such interfaces. Using element-selective x-ray and angle-resolved magnetotransport measurements, the authors present here a systematic study of Pt/Y${}_{3}$Fe${}_{5}$O${}_{12}$ heterostructures, with different order of the layer stacking resulting in different interface properties. They demonstrate that combined temperature-dependent x-ray and magnetotransport studies are essential to unambiguously confirm or exclude the presence of magnetic proximity effects.

We investigate the influence of the barrier thickness of ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$-based magnetic tunnel junctions (MTJs) on the laser-induced tunnel magneto-Seebeck (TMS) effect. Varying the barrier thickness from 1 to 3 nm, we find a distinct maximum in the TMS effect for a 2.6-nm barrier thickness. This maximum is measured independently for two barrier materials, namely, ${\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}$ (MAO) and MgO. Additionally, samples with a MAO barrier exhibit a high thermovoltage of more than $350\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}$ in comparison to $90\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}$ for the MTJs with a MgO barrier when heated with the maximum laser power of 150 mW. Our results allow for the fabrication of improved stacks when dealing with temperature differences across MTJs for future applications in spin caloritronics, the emerging research field that combines spintronics and thermoelectrics.

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of ${\mathrm{Ni}}^{2+}$ ions out of NiO into ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ ultrathin films, resulting in off-stoichiometric nickel ferrite--like thin layers. We synthesized epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/NiO bilayers on Nb-doped ${\mathrm{SrTiO}}_{3}$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400 ${}^{\ensuremath{\circ}}\mathrm{C}$, 600 ${}^{\ensuremath{\circ}}\mathrm{C}$, and 800 ${}^{\ensuremath{\circ}}\mathrm{C}$ under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions and the ${\mathrm{Fe}}^{3+}$ ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films.

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, $\text{Pt}/{\mathrm{Ni}}_{33}{\mathrm{Fe}}_{67}, \text{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (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 $\ensuremath{\delta}$ and absorption $\ensuremath{\beta}$ across the $\mathrm{Pt}\phantom{\rule{0.28em}{0ex}}{\mathrm{L}}_{3}$-absorption edge.

We present a comparison of the tunnel magneto-Seebeck effect for laser-induced and intrinsic heating. Therefore, Co40Fe40B20/MgAl2O4 and Co25Fe55B20/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23% of the MgO MTJ emphasizes the influence of the CoFe composition. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alternatively be explained by the Brinkman model and the given junction properties. Especially, we are able to demonstrate that the symmetric contribution is solely influenced by the barrier asymmetry. Thus, we conclude that the symmetry analysis used for the intrinsic method is not suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect.

Transverse magneto-thermoelectric effects are studied in permalloy thin films grown on MgO substrates. We find that small parasitic magnetic fields below 1 Oe can produce artifacts of the order of 1 % of the amplitude of the anisotropic magneto-thermopower which is also detected in the experiments. The measured artifacts reveal a new source of uncertainties for the detection of the transverse spin Seebeck effect. Taking these results into account we conclude that the contribution of the transverse spin Seebeck effect to the detected voltages is below the noise level of 20 nV., Comment: 9 pages

We present results on the longitudinal spin Seebeck effect (LSSE) shown by semiconducting ferrimagnetic NiFe2O4/Pt films from room temperature down to 50K base temperature. The temperature dependence of the conductivity has been studied in parallel to obtain information about the origin of the electric potentials detected at the Pt coverage of the ferrimagnet in order to distinguish the LSSE from the anomalous Nernst effect. Furthermore, the dependence of the LSSE on temperature gradients as well as the influence of an external magnetic field direction is investigated.