Interfacially Coupled Ferroelectric-Ferromagnetic Multiferroics

In this thesis I investigate the coupling of ferroelectric materials with thin film in-plane ferromagnets as a means of achieving voltage control of magnetism.

The strain-coupling of a (111)-cut barium titanate substrate with a sputtered CoFeB thin film has been investigated by Kerr microscopy. A one-to-one pattern transfer from ferroelectric domains to ferromagnetic domains has been observed through the imprinting of magnetoelastic anisotropy which couples magnetic domain walls to ferroelectric domain walls, where the orientation of the anisotropy rotates. We have observed two magnetoelastic coupled states in which the rotation of these magnetoelastic anisotropy axes is either 60° or 120°.

I have further investigated the magnitude of the magnetoelastic anisotropy change with temperature through the use of optical crystostat attachments to standard Kerr microscopes. The barium titanate substrate experiences three phase transitions with temperature from cubic above 420 K, to a polar tetragonal phase below 420 K, a polar orthorhombic phase below 290 K and a polar rhombohedral phase below 185 K. We observe corresponding transition behaviour in the coupled ferromagnetic layer in the magnitude of the magnetoelastic anisotropy measured from hard-axis hysteresis loops. These changes were significant, varying between 13.6 kJ/m3 and 32.9 kJ/m3 across the temperature range of interest. This has a huge impact on the domain wall properties, with the domain wall width changing significantly as the magnitude of this anisotropy changes depending on the angle of the magnetoelastic state simulation (60° or 120°) and the head-to-head or head-to-tail nature of the domain wall.

Based on these observations I try to understand the differences in the properties of the domain walls by means of micromagnetic simulation. For an arbitrary magnetoelastic anisotropy angle, I investigate what the impact of changing the various micromagnetic properties (saturation magnetization, anisotropy strength, magnetic field) is for each magnetoelastic angle. The resulting two-parameter landscapes are complex and reveal a strong dependence on the underlying magnetoelastic angle that can lead to the significant differences in properties seen in the previous chapters.

Finally, I present efforts to grow thin film ferroelectric materials using pulsed laser deposition. I have focused on trying to grow the single-phase multiferroic bismuth ferrite epitaxially with a conducting strontium ruthenate underlayer, a significant task that involves the optimization of two materials. I present an optimized set of conditions for both materials that I believe result in a good base from which to use this material in coupled all thin-film ferroelectric/ferromagnet heterostructures.