Emergent Ferromagnetism at LaFeO₃/SrTiO₃ Interface Induced by Spin-State Transition

Creating and manipulating magnetic states through the interface is of particular interest for developing novel spintronics. Despite recent progress, challenges remain in understanding emergent interface magnetism. In magnetic insulators with complex structures, multiple mechanisms can play their roles in modifying interfacial magnetism, and the spatial extent of modifications is often confined to a few unit cells near the interface. Therefore, a comprehensive characterization approach with the highest spatial resolution and precision is required for an unequivocal understanding of the origin of interfacial magnetism. We present a novel conducting ferromagnetic state at the epitaxial LaFeO₃/SrTiO₃ interface, where the atomistic origin is traced down using a combination of atomic resolution imaging/spectroscopy and first principal calculations. Our result shows that the interfacial ferromagnetism originates from spin state transitions of Fe ions within the first few unit cells near the interface, which is mediated by two factors. First, the oxygen octahedral volume shrinks to maintain oxygen connectivity with the substrate. Further, polar mismatch leads to the accumulation of extra charge on Fe site. Both factors combined result in a checkerboard-type arrangement of low and high-spin Fe ions at the LaFeO₃ interface, leading to the observed interfacial ferromagnetism. The result here demonstrates a new paradigm of creating interfacial magnetic states by combining multiple degrees of freedom, and also presents a new routine to understand the atomistic origin of emergent magnetism.