UC Berkeley

In recent decades, one of the most active and promising areas of condensed matter research has been that of complex oxides. With the advent of new growth techniques such as pulsed laser deposition and molecular beam epitaxy, a wealth of new magnetic and electronic ground states have emerged in complex oxide heterostructures. The wide variety of ground states in complex oxides is well known and generally attributed to the unprecedented variety of valence, structure, and bonding available in these systems. The tunability of this already diverse playground of states and interactions is greatly multiplied in thin films and heterostructures by the addition of parameters such as substrate induced strain and interfacial electronic reconstruction. Thus, recent studies have shown emergent properties such as the stabilization of ferromagnetism in a paramagnetic system, conductivity at the interface of two insulators, and even exchange bias at the interface between a paramagnet and a ferromagnet. Despite these steps forward, there remains remarkable disagreement on the mechanisms by which these emergent phenomena are stabilized. The contributions of strain, stoichiometry, defects, intermixing, and electronic reconstruction are often very difficult to isolate in thin films and superlattices.

This thesis will present model systems for isolating the effects of strain and interfacial electronic interactions on the magnetic state of complex oxides from alternative contributions. We will focus first on SrRuO₃, an ideal system in which to isolate substrate induced strain effects. We explore the effects of structural distortions in the simplest case of growth on (100) oriented substrates. We find that parameters including saturated magnetic moment and Curie temperature are all highly tunable through substrate induced lattice distortions. We also report the stabilization of a nonmagnetic spin-zero configuration of Ru⁴⁺ in tetragonally distorted films under tensile strain. Through growth on (110) and (111) oriented substrates we explore the effects of different distortion symmetries on SrRuO₃ and demonstrate the first reported strain induced transition to a high-spin state of Ru⁴⁺. Finally, we examine the effects of strain on SrRuO₃ thin films and demonstrate a completely reversible universal out-of-plane magnetic easy axis on films grown on different substrate orientations. Having demonstrated the ability to tune nearly every magnetic parameter of SrRuO₃ through strain, we turn to magnetic properties at interfaces.

We study the emergent interfacial ferromagnetism in superlattices of the paramagnetic metal CaRuO₃ and the antiferromagnetic insulator CaMnO₃ and demonstrate that the interfacial ferromagnetic layer in this system is confined to a single unit cell of CaMnO₃ at the interface. We discuss the remarkable oscillatory dependence of the saturated magnetic moment on the thickness of the CaMnO₃ layers and explore mechanisms by which this oscillation may be stabilized. We find long range coherence of the antiferromagnetism of the CaMnO₃ layers across intervening layers of paramagnetic CaRuO₃. Finally, we utilize the system of LaNiO₃/CaMnO₃ to separate the effects of intermixing and interfacial electronic reconstruction and conclusively demonstrate intrinsic interfacial ferromagnetism at the interface between a paramagnetic metal and an antiferromagnetic insulator. We find that the emergent ferromagnetism is stabilized through interfacial double exchange and that the leakage of conduction electrons from the paramagnetic metal to the antiferromagnetic insulator is critical to establishing the ferromagnetic ground state.