Christian Mix

Origin of the spin Seebeck effect in compensated ferrimagnets

Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.

Magnetic domain structure of La0.7Sr0.3MnO3 thin-films probed at variable temperature with scanning electron microscopy with polarization analysis

The domain configuration of 50 nm thick La0.7Sr0.3MnO3 films has been directly investigated using scanning electron microscopy with polarization analysis (SEMPA), with magnetic contrast obtained without the requirement for prior surface preparation. The large scale domain structure reflects a primarily four-fold anisotropy, with a small uniaxial component, consistent with magneto-optic Kerr effect measurements. We also determine the domain transition profile and find it to be in agreement with previous estimates of the domain wall width in this material. The temperature dependence of the image contrast is investigated and compared to superconducting-quantum interference device magnetometry data. A faster decrease in the SEMPA contrast is revealed, which can be explained by the techniques extreme surface sensitivity, allowing us to selectively probe the surface spin polarization which due to the double exchange mechanism exhibits a distinctly different temperature dependence than the bulk magnetization.

Multiferroic and structural properties of BiFeO3 close to the strain induced phase transition on different substrates

Single phase BiFeO3 (BFO) thin films have been grown strained on different substrates by pulsed laser deposition. The crystal structure and symmetry were investigated by detailed X-ray diffraction and reciprocal space maps as a function of the layer thicknesses of BFO and La0.7Sr0.3MnO3 (LSMO) buffer layers. Corresponding changes in surface morphology and ferroelectric domain structure were observed. The influence of the BFO layer thickness and the use of a LSMO buffer layer proved to be crucial for good ferroelectric and structural properties of BFO. Limits for the stability of bulk-like or strain dominated and ferroelectric domain structures are found for growth under different levels of compressive strain.

Thermal conductivity of half-Heusler superlattices

Thin films and superlattices (SLs) of TiNiSn and ZrHfNiSn layers have been grown by dc magnetron sputtering on MgO (100) substrates to reduce the thermal conductivity, aiming for improvement of the thermoelectric figure of merit ZT. The thermal conductivity of 1 Wm−1K−1 was measured by the differential 3ω method for an SL with a periodicity of 8.8 nm. In addition to x-ray diffraction analysis of the SL crystal structure, smooth interfaces were confirmed by scanning/transmission electron microscopy.

Direct observation of temperature dependent magnetic domain structure of the multiferroic La0.66Sr0.34MnO3/BiFeO3 bilayer system by x-ray linear dichroism- and x-ray magnetic circular dichroism-photoemission electron microscopy

Low-thickness La0.66Sr0.34MnO3 (LSMO)/BiFeO3 (BFO) thin film samples deposited on SrTiO3 were imaged by high resolution x-ray microscopy at different temperatures. The ultra-thin thickness of the top layer allows to image both the ferromagnetic domain structure of LSMO and the multiferroic domain structure of the buried BFO layer, opening a path to a direct observation of coupling at the interface on a microscopic level. By comparing the domain size and structure of the BFO and LSMO, we observed that, in contrast to LSMO single layers, LSMO/BFO multilayers show a strong temperature dependence of the ferromagnetic domain structure of the LSMO. Particularly, at 40 K, a similar domain size for BFO and LSMO is observed. This indicates a persistence of exchange coupling on the microscopic scale at a temperature, where the exchange bias as determined by magnetometer measurements is vanishing.

Conductance control at the LaAlO3/SrTiO3-interface by a multiferroic BiFeO3 ad-layer

Multilayered BiFeO3 (BFO)/LaAlO3 (LAO) thin film samples were fabricated on SrTiO3 (STO) substrates by pulsed laser deposition. In this work, the ferroelectric polarization of a multiferroic BFO ad-layer on top of the quasi-two-dimensional electron gas (2DEG) at the LAO/STO interface is used to manipulate the conductivity of the quasi-2DEG. By microstructuring the conductive area of the LAO/STO-interface, a four-point geometry for the measurement of the resistivity was achieved. Piezo force microscopy allows for imaging and poling the spontaneous ferroelectric polarization of the multiferroic layer. The resistance changes showed a linear dependence on the area scanned and a hysteretic behavior with respect to the voltages applied in the scanning process. This is evidence for the ferroelectric polarization of the multiferroic causing the resistance changes. Coupling the antiferromagnetic BFO layer to another ferromagnetic layer could enable a magnetic field control of the conductance of the quasi-2DEG at t...