UC Irvine

This dissertation explores the study of magnetization dynamics in a ferromagnetic thin film and nanoscale ferromagnets. In bulk ferromagnets, nonlinear interactions generally couple each spin wave eigenmode to a continuum of other available modes through multi-magnon scattering. The multi-magnon scattering can potentially limit an achievable amplitude of spin wave modes by pumping energy into other energy-degenerate modes. For example, two-magnon scattering process in the presence of impurities and defects is known to act as a channel of magnetic damping in ferromagnetic thin films. I present an observation of the two-magnon scattering in epitaxial La0.7Sr0.3MnO3 (LSMO) and LSMO/Pt thin films, investigate its impact on the evaluation of low magnetic damping in LSMO, and properties of spin transport through the LSMO/Pt interface for potential nanodevice applications.

Magnetic damping is a critical parameter that determines the speed and energy efficiency of the magnetic nanodevice such as spin-torque memory and oscillators. In a nanomagnet, the geometric confinement breaks translational invariance of the system and discretizes the spin wave spectrum, which helps to suppress the kinematically allowed multi-magnon scattering. The suppression of multi-magnon scattering enables an unusual type of nonlinear interactions and excitation processes in nanoscale ferromagnets that are qualitatively different from that in bulk ferromagnets. In this regard, I report an observation of nonlinear resonant three-magnon scattering and its effect in the damping of nanoscale magnetic tunneling junctions (MTJs). The spectral lineshape of a spin wave resonance undergoing three-magnon scattering exhibits a minimum at the resonance frequency in sharp contrast to the amplitude maximum seen in the linear resonance regime. This unusual behavior arises because the damping parameter of a spin wave ceases to be frequency-independent and itself becomes a resonant function of the excitation frequency. Also, such resonant nonlinear damping dramatically alters the response of a nanomagnet to antidamping spin-torque in a counterintuitive way - the antidamping torque can increase the damping of a spin wave mode that undergoes the nonlinear resonant scattering.

Lastly, I present an experimental demonstration of electric-field driven parametric excitation of a spin wave eigenmode in nanoscale MTJ. This work shows that the microwave electric field applied across the MTJ electrode efficiently couples to the out-of-plane component of oscillating magnetization via voltage-controlled magnetic anisotropy (VCMA) in the system. The threshold voltage of parametric excitation is found to be well below 1 Volt, which makes it attractive for magnonic nanodevices such as spin wave logic. The electric-field driven parametric excitation of magnetization is a versatile method for generating short-wavelength spin wave and thus results in this work pave the way towards energy-efficient excitation of magnetization dynamics in thin films of metallic ferromagnets and nanodevices based on magnetic multilayers.