Route to achieving perfect B-site ordering in double perovskite thin films

Double perovskites (DP, A2BB’O6) exhibit a breadth of multifunctional properties with a huge potential range of applications, including magneto-optic and spintronic devices. However, spontaneous cation ordering is limited by the similar size and charge of B and B’ cations. We introduce a route to stimulate B-site rock-salt ordering. By growing thin films on (111)-oriented substrates, ‘in-plane’ strain acts on the intrinsically tilted oxygen octahedra of the DP and produces two different B-site cages (in size and shape), stimulating spontaneous cation ordering. For the ferromagnetic insulator La2CoMnO6, clear Co/Mn ordering was achieved by growing on (111)-oriented substrates. The difference in B-site cages was further enhanced when grown under minor (111) in-plane compressive strain, resulting in long-range ordering with a saturation magnetization of 5.8 μB/formula unit (f.u.), close to the theoretical 6 μB/f.u., without antiferromagnetic behavior. Our approach enables the study of many new ordered DPs which have never been made before. The choice of substrate is critical for forming high-quality thin films of double perovskites, a class of materials useful for spintronics. Double perovskites have a wide range of properties and a corresponding broad range of potential applications, including magneto-optic and spintronic devices. Their magnetic properties can be altered by controlling their composition and growth, but getting all the atoms to spontaneously settle at the right place is challenging. Now, Judith MacManus-Driscoll from the University of Cambridge in the UK and co-workers have shown that the key to stimulating the required ordering is by growing double perovskites along the [111] crystallographic direction with the right amount of strain. They demonstrated this by forming La2CoMnO6 on a (La,Sr)(Al,Ta)O3 (LSAT) substrate producing perfect ordering and magnetisation. This approach promises to enable the synthesis of many new ordered double perovskites. To profit of the breadth of multifunctional properties that double perovskites (A2BB’O6) exhibit, it is required to have high control on cation ordering of the B-site. We show that by growing thin films under minor in-plane strain along the (111) direction, cation ordering can be stimulated as a result of the formation of two differently sized and shaped B-sites. Such an approach enables the study of many new ordered double perovskites which have never been made before.