Ferroelastic Nature and Ferromagnetism in Epitaxial LaCoO3

Abstract

Magnetism in strained LaCoO3 (LCO) thin films has been an intriguing scientific subject for the last decade. While the bulk LCO is non-magnetic, strained LCO thin films exhibit robust ferromagnetism with a phase transition temperature (Tc) of 80 K.[1] Since this system is a representative example where epitaxial strain induces non-bulk magnetic properties, it has drawn lots of attentions in the field of magnetic oxide thin films. Despite extensive research over the past decade[2-6], the consensus on the origin of ferromagnetism has not yet to be achieved. A main challenge of this system is a complex microstructure inside the film; the existence of planar defects (seen as dark stripes in high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) images) and their association with oxygen vacancies are important issues. Through the systematic STEM-based investigations, we verified that the planar defects always exist in epitaxial LCOs which undergo a ferroelastic response and the pristine planar defects do not involve oxygen vacancies. Furthermore, we have recently found that the compressed units sandwiched by the dark stripes are responsible for ferromagnetism in epitaxial LCOs. In this presentation, we will show that the ferroelastic nature of ferromagnetic structural units can account for many unusual magnetic properties of this system.