Thin Films*

The presence of ferromagnetic (FM) order in epitaxial LaCoO₃ (LCO) thin films, in stark contrast to bulk, has gathered substantial recent attention. Due to the insulating nature of this strain-stabilized FM, however, little attention has been paid to electronic transport. Here, we present a systematic study of epitaxial LCO films (8-22 nm thick) grown under both tensile (on SrTiO₃ and LSAT) and compressive strain (on LaAlO₃ and SrLaAlO₄), emphasizing not only magnetism but also transport. Tensile films exhibit ferromagnetism with saturation magnetization 1-1.5 μ_B/Co and Curie temperature ~80 K, consistent with prior work. Polarized neutron reflectometry has also been performed, revealing a uniform magnetization depth profile with thickness. Most notably, transport, which is semiconducting in all cases, reveals an unanticipated correlation between strain-state and majority carrier type, supported by density functional theory calculations suggesting significant changes in conduction band structure under strain. This provides important clues to the origin of FM in this material.