Cation Site Occupancy and Defect Engineering in Perovskite Heterostructures

Abstract

Complex oxide thin films and their heterostructures are quintessential materials for next-generation multiferroic and magnetoelectric devices. Cation and anion point defects collectively affect the structural, electrical, and magnetic properties of complex oxides. At the same time, such defects allow one to manipulate the behavior and tailor it to desired characteristics. Furthermore, the formation of atomic defects is highly influenced by the growth condition of these thin films. This thesis assesses how cation and oxygen stoichiometry and growth dynamics can control the material properties of single- and multi-phase perovskite oxide thin films.

We start with the generation of ferroelectricity in an antiferromagnet LuFeO₃ through cation antisite defects and concomitant inversion symmetry breaking. We elucidate how the crystal structure and ferroelectricity depend on cation stoichiometry. We then integrate this antiferromagnet into a self-assembled nanocomposite with a ferrimagnet [formula] in order to demonstrate interfacial magnetic exchange coupling. We discuss how the growth condition affects cation site occupancy and crystal structure. Next, we describe a nontrivial self-assembly mechanism of [formula] and CoOₓ arising from the change in oxygen pressure during deposition. Electrolyte gating of the nanocomposite can modulate the strain state and magnetization by removing oxygen vacancies, expanding the pathway of magnetoelectric coupling. We then discuss a newly discovered layering of oxygen vacancies in perovskite [formula] and identify the crystal structure using both experimental and theoretical approaches. We end by investigating the electronic and magnetic properties of [formula] related to oxygen ligand holes, as well as analyzing the effects of cation substitution and off-stoichiometry in several iron garnets.

All in all, this thesis advances the understanding of the correlation between crystal structure, atomic defects, and multiferroic and magnetoelectric properties. The observations encourage further studies of introducing and improving multiferroicity via defect engineering in underutilized material systems and exploring interfacial phenomena in various types of complex oxide heterostructures.