UCLA

Controlling magnetic properties using other parameters (i.e. voltage, strain, and current) than the magnetic field has been extensively investigated to reduce the physical dimensions and energy consumption of spintronic devices. Among several options, the strain is the most energy-efficient way to modulate magnetic properties. The high-quality and functional magnetoelastic materials are a critical component to realize the strain-mediated multiferroic concept, and they are the focus of this dissertation. Various thin-film magnetoelastic materials having different key features have been investigated to satisfy different requirements of multiferroic devices.

In Chapter 2, the high-quality polycrystalline Terfenol-D films, showing the highest magnetostriction at room temperature, are fabricated by optimizing the deposition conditions. The elemental magnetic moments of the Terfenol-D films are investigated by using X-ray magnetic circular dichroism technique. The spin and orbital moments of each element are separated by sum rule analysis. The correlation between the elements’ orbital moments and crystalline anisotropy of the films is figured out as a function of temperature. The large difference of the orbital moments between Dy and Tb is partly responsible for enhanced crystalline anisotropy of Terfenol-D films at low temperatures.

Chapter 3 focuses on amorphous magnetoelastic TbFe films representing exchange spring magnetic (ESM) behavior. The ESM TbFe films are enabled by composition gradient formed through the film thickness. The gradient produces Tb and Fe-dominant regions, and their exchange coupling at the interface creates an ESM behavior. By applying a mechanical strain to the film, a two-step switching with a negative coercive field is both modulated and eliminated showing the promise of 180� switching. The modulation is caused by relatively higher magnetostriction coefficient in the Tb-dominant region compared to Fe-dominant producing larger changes in magnetic anisotropy as the strain is applied.

Chapter 4 presents the development of amorphous TbFe films having perpendicular magnetic anisotropy (PMA) and the correlations between process parameters, intermediate properties, and PMA. For this work, a wafer curvature technique is used to measure the residual stress of deposited films. From the correlations between parameters, it is turned out that both of residual film stress and atomic composition strongly impact on PMA property simultaneously.