This dissertation is focused on controlling the spin state and long-range magnetic order in cobaltites by heteroepitaxial thin-film growth. I explore the growth of two different cobaltite materials, LaCoO₃ and PrCoO₃, on lattice-mismatched substrates to determine the role of epitaxial strain in giving rise to long-range magnetic order. This magnetic order is not found in the bulk cobaltite material and warrants the detailed investigations carried out in this work. I investigate changes in structure and stoichiometry that influence the electronic structure and the long-range magnetic order in these materials.

In the LaCoO₃ system, I explore the changes in structure in the films under tensile strain and compressive strain by growth on SrTiO₃, LaSrAlTaO₃, and LaAlO₃ substrates and film growth between 8 nm -133 nm thick. Substrate-dependent oxygen vacancy ordering in the films is found using microstructural characterization, presumably related to the amount of stress in each of the films. By carrying out a study of the effects on the film structure from the oxygen growth pressure, I find an overall increase in the out-of-plane lattice parameter with lower oxygen growth pressures.

These structural and stoichiometry changes in the LaCoO₃ films to trends appear to be related to the stabilization of long-range magnetic order. Highest moment is found in the films in tension (which also have the most defects) on SrTiO₃ and LaSrAlTaO₃ substrates and the lowest moment is found in films in compression on LaAlO₃. Element-specific X-ray absorption techniques reveal contributions from Co in different spin and valence states. I show how strain affects the electronic structure and distribution of these different states and relate these observations to trends observed in the magnetism. Strained films in tension have the highest amount of high spin Co³⁺ and high spin Co²⁺, while relaxed films appear to have mostly low spin Co³⁺ at 25 K. I present some scenarios to explain how these different Co ions combine to give rise to long-range ferromagnetic order in LaCoO₃ films.

In the PrCoO₃ system, I explore whether long-range magnetic order can be observed using heteroepitaxial synthesis similar to the efforts in the LaCoO₃ thin film system despite PrCoO₃ having a more stable low spin state configuration in the bulk. The PrCoO₃ films in tension are ferromagnetic, similar to the LaCoO₃ system. Thus, epitaxial strain dominates the effects of chemical pressure which stabilize a low spin state. The strained films have more high spin Co³⁺. The implication of Co sublattice ordering on the ordering of the Pr sublattice is explored using X-ray magnetic circular dichroism. A rare ordering of the Pr ions anti-parallel to the orientation of the moments on the Co sublattice appears to occur in this system. These studies demonstrate the power of heteroepitaxial synthesis to give rise to new magnetic functionality in perovskite oxide systems.