B-site ordering and strain-induced phase transition in double-perovskite La 2 NiMnO 6 films.

The magnetic and electrical properties of complex oxide thin films are closely related to the phase stability and cation ordering, which demands that we understand the process-structure-property relationships microscopically in functional materials research. Here we study multiferroic thin films of double-perovskite LaNiMnO epitaxially grown on SrTiO, KTaO, LaAlO and DyScO substrates by pulsed laser deposition. The effect of epitaxial strains imposed by the substrate on the microstructural properties of LaNiMnO has been systematically investigated by means of advanced electron microscopy. It is found that LaNiMnO films under tensile strain exhibit a monoclinic structure, while under compressive strain the crystal structure of LaNiMnO films is rhombohedral. In addition, by optimizing the film deposition conditions a long-range ordering of B-site cations in LaNiMnO films has been obtained in both monoclinic and rhombohedral phases. Our results not only provide a strategy for tailoring phase stability by strain engineering, but also shed light on tuning B-site ordering by controlling film growth temperature in double-perovskite LaNiMnO films.