Structure and microstructure of colossal magnetoresistant materials

Most manganites exhibiting colossal magnetoresistant properties are structurally very simple. They are based on a perosvskite structure with the general formula (A(1-x)A'(x))MnO3. The electric and magnetic properties strongly depend on the composition (A, A', x) and eventually on the exact oxygen content. These changing properties are strongly related to structural and microstructural changes. Indeed, the structure has many degrees of freedom. The MnO6 octahedra can not only deform, they can also rotate along their fourfold or twofold axis, giving rise to different superstructures or modulated structures. This will lower the symmetry of the structure from cubic to orthorhombic, rhombohedral or monoclinic. A lowering in symmetry will of course introduce different orientation variants (twins) and translation variants (antiphase boundaries). These microstructural changes are reviewed here through a transmission electron microscopy study of bulk as well as thin film colossal magnetoresistance materials. For thin films grown on a single crystal substrate the misfit with the substrate is another very important parameter, which determines the structure and the microstructure. We review different methods of accommodating the stress induced by the substrate: elastically, through interface dislocations, through pseudo-periodic twinning, through formation of antiphase domains or through a phase transition in the film. (C) 2004 IOP Publishing Ltd.