B-site ordering and strain-induced phase transition in double-perovskite La₂NiMnO₆ 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 La₂NiMnO₆ epitaxially grown on SrTiO₃, KTaO₃, LaAlO₃ and DyScO₃ substrates by pulsed laser deposition. The effect of epitaxial strains imposed by the substrate on the microstructural properties of La₂NiMnO₆ has been systematically investigated by means of advanced electron microscopy. It is found that La₂NiMnO₆ films under tensile strain exhibit a monoclinic structure, while under compressive strain the crystal structure of La₂NiMnO₆ films is rhombohedral. In addition, by optimizing the film deposition conditions a long-range ordering of B-site cations in La₂NiMnO₆ 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 La₂NiMnO₆ films.