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 (Pr1-yYy)1-xCaxCoO3-delta 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)0.70Ca0.30CoO3-delta epitaxial thin films to quasicontinuously probe structural, electronic, and magnetic behaviors across the nonmagneticinsulator/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.