Magnetization Dynamics in Thin Film and Multilayer Structures

Abstract

Spintronics is a field of research that seeks to exploit the spin rather than the charge ofthe electron for information-technology applications, with the promise of computational devices that use less energy while being faster and more powerful. A major challenge in this field has been the understanding and control of how the energy contained in a system of electron spins is transferred, and ultimately lost, to the rest of the material. This thesis presents experimental measurements of magnetization damping using ferromagnetic resonance in a variety of different thin films and multilayer structures, along with unique ways of understanding the physical mechanisms that cause damping. First, the effect of an extrinsic two-magnon scattering mechanism on the magnetization damping is demonstrated in a series of Heusler alloy thin films. A model of two-magnon scattering is developed to fit the data, and particular emphasis is placed on the mechanisms which cause the effect to be stronger in the Heusler films. It is then shown how two-magnon scattering can shift the resonance frequency, an effect that is almost always neglected, which is important due to the ubiquity of using ferromagnetic resonance measurements to extract magnetic anisotropy energies. The following portion of the thesis deals primarily with magnon-phonon coupling and its effect on damping. A mechanism of magnetization damping due to magnon-phonon coupling is shown to dominate the overall damping in a series of Fe0.7Ga0.3 alloy thin films. The mechanism causes a giant anisotropy of the damping, with the damping coefficient varying by as much as a factor of 10 depending on the orientation of the magnetization. This mechanism is extrinsic, and so it is important to account for when measuring the intrinsic damping of a material. Finally, a phonon pumping mechanism is demonstrated in a series of Co/Pd multilayers. Phonon pumping causes a resonant damping of the magnetization dynamics, at a frequency that is determined by the total thickness of the multilayer. The temperature dependence is much stronger than expected, which underscores the importance of magnetic boundary conditions in the problem. There is also a resonance frequency shift that accompanies the resonant damping, which can be predicted accurately using linear response theory.