Engineering Multiferroic Garnet-Perovskite and Spinel-Perovskite Nanocomposites

Nanocomposite oxide thin films consist of two phases that grow epitaxially on a substrate. Of particular interest are multiferroic nanocomposites consisting of pillars of a ferrimagnetic spinel such as CoFe2O4 (CFO) within a matrix of a ferroelectric perovskite such as BiFeO3 (BFO) grown on a perovskite substrate. BFO/CFO nanocomposites exhibit magnetoelectric coupling mediated by strain transfer at the vertical interfaces. We first discuss how patterning of substrates enables control of the locations and geometry of the self-assembled pillars. In particular, we describe the fabrication, microstructure and ferroic properties of ‘sideways multilayers’ consisting of parallel fins of CFO and BFO grown on (111) and (110)-oriented substrates patterned using a focussed ion beam process, including the magnetic and ferroelectric domain structure. We then demonstrate nanocomposites made from perovskites and garnets. Iron garnets (IGs) are ferrimagnetic insulators of composition R3Fe5O12, where R is Y, Bi or a rare earth. They exhibit a range of useful magnetic properties including spin orbit torque switching, relativistic domain wall velocities of km/s, spin wave-driven domain wall motion, and magnetooptical activity. The garnet structure is incompatible with a perovskite substrate, and deposition of garnet compositions onto perovskite substrates leads to the formation of Fe-rich orthoferrites RFeO3. Instead, we use a garnet substrate patterned with a thin seed layer of a perovskite or nanosheets of perovskite-structured Ca2Nb3O10. Growth of garnet on these heterogeneous substrates produces IG over the bare garnet substrate and Fe-rich orthoferrite over the perovskite seed, with vertical garnet/perovskite interfaces, where both phases have the same composition. The ferroic properties of these structurally-modulated films will be described, as well as their utility in magnetooptical and magnonic devices.