Magnetoresistance and atomic structure of ultrathin films of La0.73Ca0.27MnO3 on SrTiO3

Ultrathin films (approximate to 6 nm) Of La0.73Ca0.27MnO3 on SrTiO3 exhibit properties quite different from the bulk. They become ferromagnetically or dered 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 a re perfectly crystalline but have a crystal structure, imposed by the latti ce 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 MnO6 octahedra which is not present in the bulk and appears to be the cause of the changed physical properties.