First-order phase transition versus spin-state quantum-critical scenarios in strain-tuned epitaxial cobaltite thin films

Pr-containing perovskite cobaltites exhibit unusual valence transitions, coupled to coincident structural, spin-state, and metal-insulator transitions. Heteroepitaxial strain was recently used to control these phenomena in the model (Pr_1-yYy)_1-xCaₓCoO_3-δ system, stabilizing a nonmagnetic insulating phase under compression (with a room-temperature valence/spin-state/metal-insulator transition) and a ferromagnetic (FM) metallic phase under tension, thus exposing a potential spin-state quantum-critical point. The latter has been proposed in cobaltites and can be probed in this system as a function of a disorder-free variable (strain). We study this here via thickness-dependent strain relaxation in compressive SrLaAlO₄(001)/(Pr0.85Y0.15)0.70Ca0.30CoO_3-δ epitaxial thin films to quasicontinuously probe structural, electronic, and magnetic behaviors across the nonmagnetic-insulator/FM-metal boundary. High-resolution x-ray diffraction, electronic transport, magnetometry, polarized neutron reflectometry, and temperature-dependent magnetic force microscopy provide a detailed picture, including abundant evidence of temperature- and strain-dependent phase coexistence. This indicates a first-order phase transition as opposed to spin-state quantum-critical behavior, which we discuss theoretically via a phenomenological Landau model for coupled spin-state and magnetic phase transitions.