Strain-induced phase separation inLa0.7Sr0.3MnO3thin films

Abstract

${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films having different thicknesses were grown by pulsed laser deposition with in situ reflection high energy electron diffraction diagnostics on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ substrates. The mismatch between film and substrate gives rise to an in-plane compressive biaxial strain, which partially relaxes in films thicker than $30\phantom{\rule{0.3em}{0ex}}\text{unit}$ 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 $3{z}^{2}\ensuremath{-}{r}^{2}$ 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 $360\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ towards lower temperatures with decreasing thickness, eventually reaching an insulating state for the $30\phantom{\rule{0.3em}{0ex}}\text{unit}$ cells film. Low-temperature nuclear magnetic resonance spectra show a decrease of the ${\mathrm{Mn}}^{\mathrm{DE}}$ double-exchange metallic contribution with decreasing the thickness, which becomes negligible in the $30\phantom{\rule{0.3em}{0ex}}\text{unit}$ 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.