Cobalt oxides have long been understood to display spin-state crossovers. A very different situation was recently uncovered in Pr-containing cobalt oxides, where a first-order spin-state/structural/metal-insulator transition occurs, driven by a remarkable Pr valence transition. Such valence transitions offer appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr_1-xCa_xCoO₃). In this work we combine epitaxy with synchrotron diffraction, scanning transmission electron microscopy with electron energy loss spectroscopy, electronic and magnetic measurements, polarized neutron reflectometry, and density-functional calculations to make the first full study of strained films of the perovskite (Pr_1-yY_y)_1-xCa_xCoO_3-δ. Remarkably, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature (Fig. 1). The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression (Fig. 2), exposing a potential novel spin-state quantum critical point 1.

Work supported by DOE through the Center for Quantum Materials.