Anomalous strain relaxation and its impact on the valence-driven spin-state/metal-insulator transition in epitaxial (Pr1-yYy)(1-x)CaxCoO3-delta

Pr-based cobaltites exhibit extraordinary phenomena where abrupt valence shifts trigger coupled structural/spin-state/metal-insulator transitions. Recent work achieved strain control of these phenomena in thin films, with epitaxial compression even stabilizing room-temperature transitions. Here, we study the thickness dependence of these effects in the model system compressively strained YAlO3(101)/(Pr0.85Y0.15)(0.7)Ca0.3CoO3-delta(001). Transport data reveal highly unusual behavior where thicker films exhibit two transitions: one at the fully strained temperature (similar to 245 K) and one near the bulk (similar to 135 K). High-resolution x-ray diffraction confirms that this is due to anomalous strain relaxation where, immediately above the critical thickness (similar to 30 nm), a film region with a near-bulk lattice parameter coexists with a fully strained region. Scanning transmission electron microscopy then reveals striking images where periodic arrays of dislocations occur in the film interior, rather than at the film-substrate interface, seeding accompanying lateral modulations in chemical doping. The unusual transport is thus a direct consequence of anomalous strain relaxation, which we discuss in detail. Intriguing behavior also arises in the ultrathin limit, where temperature-dependent transport and x-ray data reveal gradual suppression of the amplitude of the structural/metal-insulator transition but with no change in transition temperature, which we ascribe to effects of disorder in the presence of symmetry matching between the film and substrate. These results establish an unusual strain relaxation mechanism in perovskite oxide films (likely relevant to other systems) and further elucidate the sensitive strain response of these fascinating and potentially useful valence/structural/spin-state/metal-insulator transitions.