Magnetoresistance and atomic structure of ultrathin films of (formula presented) on (formula presented)

Ultrathin films (Formula presented) of (Formula presented) on (Formula presented) exhibit properties quite different from the bulk. They become ferromagnetically ordered around 150 K but remain insulating, instead of showing the insulator-to-ferromagnetic metal transition which occurs in bulk material or thicker films at this Ca concentration. Electron microscopy shows that such films are perfectly crystalline but have a crystal structure, imposed by the lattice mismatch with the substrate, which is different from that of the bulk or thick films. An important feature is a Jahn-Teller-like distortion of the (Formula presented) octahedra which is not present in the bulk and appears to be the cause of the changed physical properties.