Square to trigonal symmetry and itinerant to localized magnetism in ultra-thin SrRuO₃ films

Transition metal oxides perovskites (ABO₃) along [111] direction consists of alternating AO₃ and B planes. The B sites form a triangular lattice and two such pseudo-cubic unit cells (ABO₃) form a buckled honeycomb lattice. This provides an opportunity of creating a graphene-like structure with d-electron system and is predicted to give rise to novel quantum phases. In this work, thin films of SrRuO₃ (SRO) are grown in [001] and [111] direction and their electronic and magnetic properties are investigated as a function of film thickness. Metal-insulator transition is observed at a thickness of ~3 and ~7 unit cells in SRO (001) and SRO (111), respectively. While ferromagnetism (FM) and metallicity occur concurrently in SRO (001), FM in ultrathin SRO(111) precedes metallicity which is surprising since SRO has traditionally been described as a Stoner FM. This can be related to the magnetic sublattice symmetry and more pronounced role of localized moments in SRO(111). Our study highlights the possible breakdown of itinerant magnetism picture in trigonal symmetry and the role of structure on an atomic scale will be discussed.