Thermoelectric effects and anisotropy in magnetic films

It was the purpose of this thesis to contribute to a better understanding of spin caloritronic phenomena and thermoelectric effects as well as the anisotropy of magnetic thin films. Mostly this work was motivated by the recent discovery of the spin Seebeck effect (SSE) in Japan: a generation of a pure spin current across the interface between magnetic|nonmagnetic materials upon application of the temperature gradient along (transversal SSE) or across (longitudinal SSE) the interface. As the experimental configuration for the TSSE involves the heat flows, special care to spurious temperature gradients has to be taken. Semiconducting GaMnAs and insulating yttrium iron garnet (YIG) magnetic thin films were investigated. As nonmagnetic material platinum was chosen, providing also the opportunity to detect pure spin currents via the spin Hall effect (SPHE) in it. Starting with the measurement of in-plane voltages, transverse to the in-plane temperature gradient and measured along the platinum stripes, by sweeping the external magnetic field at different in-plane directions (TSSE geometry), symmetric and asymmetric contributions were observed in both, the GaAs/GaMnAs/Pt and GaAs/GaMnAs systems. The former was clearly attributed to the planar Nernst effect, arising from an in-plane temperature gradient along the sample, while the latter was shown to be result of the anomalous Nernst effect (ANE) caused by spurious out-of-plane temperature gradients. Using the ANE constant that was measured upon deliberately applying an out-of-plane temperature gradient, it was estimated that a rather small temperature drop of ΔT z ≈ 12 nK across the 200 nm magnetic film thickness can be responsible for the appearance of spurious ANE signals, leading to the asymmetry of the thermovoltages registered in the PNE/TSSE configuration. Thus, the TSSE if it exists at all in the GaMnAs sample, is very likely to be negligible. The transport measurements in the insulating YIG sample in TSSE/PNE configuration demonstrated no field dependencies, supporting the idea that the signals in the semiconducting GaMnAs film originated not from the TSSE, but rather from the conventional ANE. The voltages in ANE/LSSE configuration the characteristic LSSE signals were observed. This thesis is also devoted to the magnetic anisotropy investigation in GaMnAs thin film by applying wide-field Kerr microscopy simultaneously to galvanomagnetic measurements. Upon sweeping the magnetic field the development of magnetic domains was traced: at low temperatures (5 K) the sample exhibits a clear two-fold switching of magnetization, while at higher temperatures (above 20 K) only a one-step switching was detected, indicating a strong temperature dependence of the magnetic anisotropy. The same behavior was derived from the transport measurements in the PHE configuration. Rotating the field in the plane of the thin film and analyzing the resulting angle dependencies of the transversal voltage, we obtained the anisotropy constants K u and K c . The temperature dependence of the K u / K c -ratio showed a gradual substitution of the cubic anisotropy, prevailing at low temperatures, by a uniaxial one with rising temperature. For the purpose of the magneto-optical investigation and in-depth analysis of the magnetic anisotropy, within the frame of this thesis a new hardware and software realization for quantitative Kerr microscopy was developed. The principles of such a technique were formulated already years ago by Rave et al, but the experimental realization was limited to materials with in-plane surface magnetization and static domain imaging due to the manual adjustment of the microscope sensitivity. The realization of a separation of in-plane|out-of-plane magnetic contrast, suggested in this thesis, disposes those complications and improves the method, providing the opportunity for a simultaneous measurement of both components of magnetization in an automatic regime and, thus, allowing the quantitative analysis of the magnetization reorientation processes in magnetic media. This technique was successfully applied to the quantitative investigation of the magnetic domain structure in GaMnAs film, and was also demonstrated on a number of other materials, including a permalloy (Ni 81 Fe 19 ) patterned film element and the separation of the in-plane and out-of-plane components of magnetic contrast in sintered NdFeB polycrystals. In summary, this work contributes to a deeper understanding of the contribution of the conventional thermoelectric effects to the exploding field of spin caloritronic. It points out that thermoelectric phenomena have to be treated with care and that magnetic anisotropy and its temperature dependence have to be considered in any analysis of spin caloritronic phenomena. The thesis extends the existing methods of investigating magnetic anisotropy of magnetic films by introducing a novel technical realization of quantitative Kerr microscopy.:1 Introduction 1 2 Theoretical background 6 2.1 Thermoelectricity 6 2.2 The anomalous Hall and Nernst effects 8 2.3 Planar Hall and planar Nernst effects 9 2.4 Spin Hall and inverse spin Hall effects 11 2.5 Spin transfer torque and spin pumping 12 2.6 Spin caloritronics 15 2.7 Spin Seebeck effect 16 2.7.1 Experimental observation of the spin Seebeck effect 16 2.7.2 Theory of the spin Seebeck effect 23 2.8 Magnetic anisotropy of GaMnAs 26 2.9 Kerr microscopy 28 2.10 Quantitative Kerr microscopy 29 3 Experimental methods 33 3.1 Sample preparation 33 3.2 Measurement techniques 34 3.2.1 Vibrating Sample Magnetometry (VSM) 34 3.2.2 Transport measurement 35 3.2.3 Thermal conductivity measurements 38 3.2.4 Kerr Microscopy 39 4 Experimental results and discussion 43 4.1 Magnetization of GaMnAs (VSM data) 44 4.2 Thermal Properties of GaAs|GaMnAs heterostructure 46 4.3 Measurements in transversal spin Seebeck / planar Nernst effect configuration 48 4.4 Anomalous Nernst effect measurements 51 4.5 Planar Hall effect measurements and magnetic anisotropy 54 4.6 Domain studies by Kerr microscopy 60 4.7 Domain investigation in GaMnAs by quantitative Kerr microscopy 64 4.8 Measurements of YIG film in transversal spin Seebeck/planar Nernst effect configuration 67 5 Summary and conclusions 70 Appendices 73 A Selective sensitivity in Kerr microscopy 74 B Quantitative vector Kerr microscopy 81 Bibliography 87 Own publications 9