Emergent Ferromagnetism at LaFeO₃/SrTiO₃ Interface Arising from a Strain‐Induced Spin‐State Transition

Abstract Creating new interfacial magnetic states with desired functionalities is attractive for fundamental studies and spintronics applications. The emergence of interfacial magnetic phases demands the fabrication of pristine interfaces and the characterization and understanding of atomic structure as well as electronic, magnetic, and orbital degrees of freedom at the interface. Here, a novel interfacial insulating ferromagnetic order in epitaxial films of LaFeO 3 grown on SrTiO 3 characterized by a combination of electron microscopy and spectroscopy, magnetometry, and density functional theory, is reported. The epitaxial strain drives a spin‐state disproportionation in the interfacial layer of LaFeO 3 , which leads to a checkerboard arrangement of low‐ and high‐spin Fe 3+ ions inside smaller and larger FeO 6 octahedra, respectively. Ferromagnetism at the interface arises from superexchange interactions between the low‐ and high‐spin Fe 3+ . Moving away from the interface the structure relaxes and the antiferromagnetic order seen in bulk LaFeO 3 is restored. It demonstrates how strain‐induced spin‐state disproportionation at the interface creates a novel ferromagnetic superexchange interaction in magnetic insulators, offering a new pathway to engineer interfacial magnetism for spintronic applications.