Strain-Tuned Pr-Based Cobaltite Thin Films: Electronic Ground State Control And Strain Relaxation Effects

Abstract

Perovskite cobalt oxides, e.g., LaCoO3, have long been understood to display intriguing phenomena known as spin-state crossovers—changes in the cobalt ion spin-state in response to temperature, pressure, or other stimuli. More recently, a very different situation was uncovered in perovskite cobaltites containing praseodymium (e.g., Pr0.5Ca0.5CoO3). In these materials, a remarkable praseodymium valence transition drives a coupled, first-order spin-state/structural/metal-insulator transition. Such a valence transition, especially when coupled to spin-state and metal-insulator transitions, offers highly appealing material functionality, but has thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). This thesis demonstrates that thin-film heteroepitaxial strain enables wide control over the electronic and magnetic ground states of such Pr-based cobaltites, and provides a facile path to boosting their valence transition temperatures to a practical range.It is first shown that in thin films of (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of the valence-driven spin-state/structural/metal-insulator transition to at least 291 K, i.e., room temperature, with broad technological implications. Wide control of the electronic ground state is demonstrated, from ferromagnetic metal under epitaxial tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point in this material’s epitaxial strain “phase diagram.” These achievements then motivate a study of strain relaxation in thin film (Pr0.85Y0.15)0.7Ca0.3CoO3-δ. It is shown that, contrary to conventional strain relaxation theory, films grown under large compression beyond a critical thickness develop an anomalous, two-layered structure, exhibiting “fully-strained” and “strain-relaxed” properties simultaneously. The underlying mechanism of strain relaxation has not been previously observed, and this work therefore adds an entry to the growing list of “alternative” mechanisms of strain relaxation in oxide thin films. Intriguing ultrathin-film behavior is also described, specifically the suppression of the high-temperature metallic state without a change in metal-insulator transition temperature, generating new knowledge related to interfacial “dead layers” in epitaxial perovskite oxide films. With the basic shape of the (Pr1-yYy)1-xCaxCoO3-δ epitaxial strain phase diagram established, finally it is shown that the phase space between the discrete strains imposed by commercially-available substrates can be explored using partially-strain-relaxed films. In the compressive-to-tensile transition region of the strain phase diagram, it is demonstrated that the uniform nonmagnetic insulator ground state gives way to phase coexistence with an emergent ferromagnetic metallic phase as compressive strain is relaxed. Such electronic phase coexistence evidences a first-order transition between this material’s two disparate, strain-stabilized ground states as a function of epitaxial strain, in contrast to a quantum critical point scenario, of broad scientific interest.