  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, we
report a novel interfacial insulating ferromagnetic order in antiferromagnetic
LaFeO3 grown on SrTiO3, characterized by a combination of electron microscopy
and spectroscopy, magnetometry, and density functional theory. The epitaxial
strain drives a spin-state disproportionation in the interfacial layer of
LaFeO3, which leads to a checkerboard arrangement of low- and high-spin Fe3+
ions inside smaller and larger FeO6 octahedra, respectively. Ferromagnetism at
the interface arises from superexchange interactions between the low- and
high-spin Fe3+. The detailed understanding of creation of emergent magnetism
illustrates the potential of designing and controlling orbital degrees of
freedom at the interface to realize novel phases and functionalities for future
spin-electronic applications.
