Geometric influence on the net magnetic moment in LaCoO3 thin films

In bulk form, LaCoO3 is not magnetic. In thin film form, this compound can become magnetic. Although magnetism has traditionally been attributed to a thermally-induced intermediate state or to oxygen vacancy defects, we find that strain causes antiferromagnetic interactions at high temperatures, and a weak ferromagnetism at low temperatures. The different magnetic behaviors of LaCoO3 films grown on LaAlO3 and SrTiO3 are related to the Co-O-Co bond angles and the constraints imposed on the Co-O bond lengths by the substrate geometries. Long-range magnetic order occurs below T ~ 90 K when the Co-O-Co bond angle is greater than 163 degrees, consistent with the behavior of bulk and nanoparticles forms of LaCoO3. A LaAlO3 substrate prevents magnetic long-range order at low temperatures near the film-substrate interface and collinear antiferromagnetic sublattices away from the interface. At low temperatures, the antiferromagnetically ordered sublattices are non-collinear in films grown on SrTiO3 substrates, leading to a significant net moment. Films gown on LaAlO3 with a SrTiO3 strained buffer layer at the interface are not magnetic, showing that a purely chemical interaction with the SrTiO3 is not responsible for the observed phenomena.

This article was published in Journal of Materials Research and is available here.