Thermal and Electrical Transport in Ferromagnetic Metal Thin Films

The recent emergence of spin caloritronics has focused considerable attention on the interplay between spin, charge, and temperature gradients in magnetic materials. A reliable and energy efficient method for generating pure spin currents would signify an important step toward future spin-based nano-electronics that may offer lower power consumption and greater processing capabilities. To develop new technology using thermoelectric effects in magnetic thin films, it is essential to understand thermal and electrical transport through these films. One possible source of pure spin currents is the so-called spin Seebeck effect (SSE) in which a thermal gradient (∇T) applied to a ferromagnet is thought to produce a pure spin current detectable by measuring a transverse voltage (VT) generated by the inverse spin Hall effect. However, recent work on spin-dependent transport in thin film nanostructures supported by bulk substrates has underscored the difficulty in understanding thermal gradients in these systems due to uncertainty in the direction of the applied thermal gradient through a substrate with a thermal conductance several orders of magnitude larger than the sample conductance. These results suggest that early SSE experiments may have been strongly affected by other effects such as the anomalous Nernst effect. They may also have been affected by thermoelectric effects generated from planar thermal gradients such as the planar Nernst effect which develops a VT in a film with a planar thermal gradient and magnetization. In this dissertation, I introduce the concepts of thermal conductivity, the Wiedemann-Franz law, and thermoelectric effects including the Seebeck effect, the Peltier effect, and the planar Nernst effect (PNE). Next, I describe our experimental method for measuring thermal and electrical transport in non magnetic and ferromagnetic metallic thin films using suspended Si-N membrane structures. Our membrane method reduces the background thermal conductance contribution by 5 orders of magnitude when compared with similar experiments conducted on thin films supported by bulk substrates. This confinement to the plane of the platform and film ensures a thermal gradient in the x- or y-direction only. The experiment therefore enables exploration of thermoelectric effects in a completely 2-D configuration. Next, I present results of several experiments probing thermal conductivity and the Lorenz number in thin films. Both the thermal conductivity and electrical conductivity of metallic thin films is lower that bulk values from literature. The deviation of the Lorenz number from the theoretically predicted Sommerfeld value in all films indicates imbalances between the heat and charge currents in the films from scattering or additional thermal conductivity contributions from magnons and phonons. I also present results from experiments measuring the Seebeck effect or thermopower, and anisotropic magnetoresistance in ferromagnetic thin films. In these films, the thermopower scales with resistance as predicted by the Mott equation, and the magnetic field dependence of the thermopower results from the same spin-dependent scattering responsible for the AMR. I present the first results from experiments designed to probe the PNE and related effects such as the SSE in ferromagnetic thin films. The results share features previously attributed to the SSE such as linear delta T dependence and sign reversal on hot and cold sides of the sample, however, the voltage generated transverse to the applied thermal gradient is always even in applied field due to spin-dependent scattering. The data display a sinθcosθ angular dependence predicted by the PNE rather than the cosθ angular dependence expected from the SSE. In these experiments, we observe no evidence of a thermally generated spin current, and the upper limit on the SSE coefficient in our experiment is 15-30 times smaller than previously reported by experiments conducted using bulk substrates. Finally, I present first results from experiments designed to measure the Peltier effect in thin films and test the interdependence between the Peltier and Seebeck effects predicted by Onsager reciprocal relations. These are the first measurements of the Peltier effect and Onsager reciprocity in ferromagnetic thin films near room temperature, and are an important step to confirm the validity of the theoretically predicted Onsager reciprocity in these systems