UCLA

This thesis explores magnetization dynamics in materials to help design future low-power electromagnetic devices. In this thesis, we explore materials for multiferroic composites that can couple electricity and magnetism through voltage, rather than current, allowing for the possibility of low power control of magnetism. We study both thin film systems and explore the effect of nanostructure on strain-mediated composites, which utilize a ferroelectric material that exhibits a strain response to an applied voltage, coupled to a magnetostrictive material, which changes magnetization in response to the strain produced by the ferroelectric. In the first part of the thesis, yttrium iron garnet (YIG) is studied as a model system for low loss magnetic materials – a necessary requirement for high-frequency multiferroic devices. YIG is an ideal magnetic material for high-frequency devices, as it exhibits narrow magnetic resonances, but pure YIG has low magnetostriction. Using sol-gel chemistry, we were able to survey a range of cerium- and ruthenium-doped YIG compositions, which have both been shown to increase YIG’s magnetostriction to useful levels in bulk crystals. Homogeneously doped materials were synthesized and characterized, but the polycrystalline nature of the films led to significant magnetic losses at high frequency. In the second part of the thesis, we explore three-dimensionally coupled porous multiferroic composites. These composites were synthesized by first using block-copolymer templating to create a nanoporous magnetostrictive framework. Atomic layer deposition (ALD) was then used to partially coat the inner surface of the pores with a thin layer of ferroelectric material, the thickness of which could be varied to change the extent of residual porosity. We found that composites with larger residual porosities exhibited a larger magnetoelectric coupling, due to the mechanical flexibility of the pores, which enabled larger strains. We first studied ferroelectric lead zirconate titanate (PZT) in magnetostrictive cobalt ferrite (CFO), and observed modest increases in magnetoelectric coupling with increasing porosity. We hypothesized that this was due to the weaker ferroelectricity observed in extremely thin PZT films. Upon switching the ferroelectric to bismuth ferrite (BFO), we find that large (<50%) changes in magnetization were possible in samples with the most residual porosity.