2015 Theses Doctoral

Novel torques on magnetization measured through ferromagnetic resonance

Li, Yi

New torques acting on magnetization in metallic ferromagnets, accompanied by new terms to the Landau-Lifshitz-Gilbert (LLG) equation which governs GHz magnetization dynamics, are important for both the fundamental understanding of magnetism and applications in magnetoelectronic devices. In this thesis, we have carried out experimental investigations of several proposed novel torques acting on magnetization dynamics using broadband ferromagnetic resonance (FMR) between 2-26 GHz. The FMR technique is well-suited for materials studies, as it investigates unpatterned (sheet-level) films with relatively high throughput, enabling comparison of the response of several room-temperature, device-relevant ferromagnetic alloys (e.g. Ni₇₉Fe₂₁, or ‘Py’, Co, and CoFeB.)
The common aspect of the torques which we have investigated by FMR is their origin in nonequilibrium spin populations, related to spin transfer torque. In Chapter 3 we have identified intrinsic “inertial” torques on magnetization, significant only at very high frequencies (up to 300 GHz), where the electron population cannot quite keep pace with the precession of magnetization. In Chapters 4 and 5 we have studied torques from “pumped” pure spin current due to the texture of precessing magnetization (intralayer spin pumping) and the precession of noncollinear magnetizations in trilayer structures (spin pumping). These three studies extend understanding of magnetism and magnetization dynamics at room temperature, and in limits of high speed and small dimension relevant for emerging applications.