Electrical, Microwave, And Thermoelectric Studies Of Epitaxial Heusler Compound-Based Thin Films

Abstract

This thesis presents studies of epitaxial thin film structures based on Heusler compound ferromagnets, with particular focus on the static and dynamic properties of these structures relevant for spintronic device applications. In Chapter 1, a brief motivation is followed by an introduction to the spin physics of itinerant electronic materials, providing the framework for understanding the results presented in the following chapters. In Chapter 2, demonstrations of spin-orbit torques in epitaxial Heusler/Pt bilayers are presented. After characterizing the behavior of the spin-orbit torques through second-harmonic magnetoresistance techniques, the behaviors of the torques as a function of temperature are used to study the influence of the magnetic proximity effect on the dampinglike and fieldlike torque contributions. It is found that the dampinglike torque is due to the platinum spin-Hall effect, and is not influenced by the magnetic proximity effect. Conversely, the fieldlike torque is likely due to the interface Rashba effect, and is suppressed by the presence of the magnetic proximity effect. In Chapter 3, measurements of ferromagnetic resonance linewidths are presented for Heusler compound thin films, which are used to study the damping mechanisms of magnetization dynamics. Both intrinsic and extrinsic damping mechanisms are found, the former described by Gilbert damping and the latter due to the presence of magnon-magnon scattering processes. The Gilbert damping in these epitaxial Heusler thin films is shown to be very low relative to typical metallic ferromagnets, on the order of 10^(-4)-10^(-3) when expressed as a dimensionless Gilbert damping constant. In addition, evidence of an anisotropic Gilbert damping constant is presented for epitaxial Co(2)FeSi thin films. A methodology considering extrinsic magnon-magnon scattering contributions to the resonance linewidth is presented, revealing the characteristic lengthscale of magnetic inhomogeneity in these films. Finally, in Chapter 4 a method to measure (magneto)thermoelectric coefficients in thin films is outlined, which uses all-lithographic patterning and thermometry. Initial results for the Seebeck and anomalous Nernst coefficients in Heusler compound thin films are presented, along with interpretation.