Strain-induced phase separation in La_0.7Sr_0.3MnO₃ thin films

La_0.7Sr_0.3MnO₃ thin films having different thicknesses were grown by pulsed laser deposition with in situ reflection high energy electron diffraction diagnostics on LaAlO₃ substrates. The mismatch between film and substrate gives rise to an in-plane compressive biaxial strain, which partially relaxes in films thicker than 30 unit cells. Accordingly, the ratio between the out-of-plane and the in-plane lattice parameter (c\a) varies between 1.06 (fully strained) and 1.03 (partially relaxed). In-plane compressive strain favors the stabilization of the 3z²-r² orbitals (chain-type antiferromagnetic phase), thus giving rise to a sizeable x-ray absorption linear dichroism signal. The shape of the linear dichroism depends weakly on the c\a ratio, while its intensity strongly increases with c\a. At the same time, the metal-insulator transition temperature shifts from about 360 K towards lower temperatures with decreasing thickness, eventually reaching an insulating state for the 30 unit cells film. Low-temperature nuclear magnetic resonance spectra show a decrease of the Mn^DE double-exchange metallic contribution with decreasing the thickness, which becomes negligible in the 30 unit cells thick film. The experimental results demonstrate a strain driven competition between two stable phases: the orbital ordered chain-type insulating antiferromagnetic and the orbital disordered metallic ferromagnetic. For intermediate values of the epitaxial strain the local minimum state of the system lies in a gap region between the two stable phases. Such a region has glassy characteristics with coexisting clusters of the two phases. The strain is used as a driving force to span the glassy region.