Structural transformations of an epitaxial La
MnO 3! thin 5lm deposited on MgO (100) substrate by metal-organic chemical vapor deposition were conjointly studied by X-ray di4raction using a conventional 4-circles-di4ractometer and by transmission electron microscopy. As reported in a previous work [Pignard et al., J. Appl. Phys., 82(9), 4445 (1997)] changes of physical properties of this material have been observed through annealing treatments: Films deposited at 6503C exhibit temperatures of semiconductor-metal and para-ferromagnetic transitions at 130 and 200 K while after successive annealings between 600 and 8003C in air, these transition temperatures increase up to 310 and 330 K, respectively. Two di4erent 5lm structures related to the evolution of these properties were determined at room temperature in their paramagnetic state. As a result, the 5lm structures were found to be monoclinic and arranged in twinned microdomains. Several structural transformations resulting of annealing treatments were deduced: (i) The as-deposited 5lm is almost single-phased with a I2/b space group structure and transforms into a mixing of two phases of I2/b and P2 1 /a space groups in a 82/18% ratio, respectively; (ii) Mn octahedra become more regular, with signi5cant increases of Mn+O+Mn bond angles, close to 1803. These structural changes are interpreted in a discussion related to an increase of the Mn 4 >/Mn 3 > ratio.
Besides, as a surprising feature, observations by transmission electron microscopy on La 0.8 MnO 3! /MgO interfaces showed that MgO exhibits domains of Fd3 cubic lattice and a cell parameter twice this of normal MgO. This seems to be the result of a strain occurring between the 5lm and the MgO substrate rather than a chemical di4usion.
2002 Elsevier Science (USA)