Strain-induced crystal structure change in ultrathin films of Pr _0.7Sr_0.3MnO₃

Ultrathin films (<30 nm) of Pr_0.7Sr_0.3MnO ₃ on LaAlO₃ have been studied using transmission electron microscopy (TEM). It was shown that the films are highly uniform and defect-free, and that they are coherently strained to the smaller lattice parameter of the substrate, resulting in a tetragonal expansion perpendicular to the film plane and a change of crystal structure from the ordered orthorhombic of bulk materials to a simple tetragonal perovskite. The variation of the tetragonality with distance from the interface was also determined from high-resolution TEM images.