Strain- and Electrolyte-Gating-Based Control of Magnetism in Cobaltite Thin Films

Abstract

Transition metal based perovskite oxides with the common chemical formula ABO3 exhibit diverse physical properties of fundamental and technological importance. Among these, perovskite cobaltites (where B = Co) have long been understood to display intriguing phenomena known as spin-state crossovers or transitions, where the spin of the cobalt ion changes versus temperature, pressure, etc. Recent advances in synthesizing thin films of these cobaltites have attracted significant attention and have led to realization of novel magnetic ground states not observable in bulk samples. Employing heteroepitaxial strain and electric-field effects, this thesis is aimed at understanding and controlling the emergent magnetic ground states that arise in two prototypical cobaltite thin film systems: La1-xSrxCoO3-δ and (Pr1-yYy)1-xCaxCoO3-δ, addressing several pressing open questions related to these material systems, providing avenues for several potential applications including resistive switching devices, neuromorphic computing, switchable magnetic, and photonic devices etc. Starting with LaCoO3-δ films, we first address open questions concerning electronic transport properties and correlations with the strain-stabilized ferromagnetic state under tensile strain. Detailed magnetotransport measurements reveal a striking inversion of the majority carrier type under tensile strain, from the p-type seen in bulk and under compression, to n-type under tension. We interpret these results in terms of a tensile-strain-induced redistribution of orbital occupancies towards eg states, in concert with substantial lowering of the electron effective mass. While thus far overlooked, we thus report that ferromagnetism in epitaxial LaCoO3-δ films is thus directly correlated with n-type behavior, providing important insight into the perplexing ferromagnetic state in this system. Moving on to (Pr0.85Y0.15)0.3Ca0.7CoO3-δ films, using large compressive heteroepitaxial strain, we report realization of a first-order valence-driven spin-state/metal-insulator transitions to at least 245 K (from 135 K in bulk), potentially within reach of room temperature. The obvious technological implications of this result are accompanied by new fundamental prospects also, as we additionally establish complete strain control of the electronic ground state, from a ferromagnetic metallic state under tension to nonmagnetic and insulating under compression, thereby exposing a novel potential quantum critical point. Finally, using electrolyte gating techniques, we present the first study of the voltage-induced topotactic perovskite to brownmillerite transformation across almost the entire phase diagram of ion-gel-gated La1-xSrxCoO3- (0  x  0.70), employing epitaxial films on three different substrates to understand the impact of strain. Electronic transport and operando synchrotron X-ray diffraction confirm that the perovskite to brownmillerite transformation can be driven at essentially all x, including, critically, x  0.50, where the perovskite phase is highly stable. Importantly, the threshold voltage for the transformation is tunable (between 3 V and 0 V) via Sr doping and strain, of interest for device applications. The decreasing threshold voltage with doping and strain are interpreted in terms of trends in oxygen vacancy formation enthalpy (not diffusivity), highlighting the essential role of thermodynamics (over kinetics), driven by the instability of formal Co4+ in these compounds. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications. Overall, this thesis demonstrates the power of heteroepitaxial strain and electric-field effects in developing fundamental understanding as well as control of magnetism in cobaltite thin films, highlighting abundant technological potential in these materials.