  Heterointerfaces have led to the discovery of novel electronic and magnetic
states because of their strongly entangled electronic degrees of freedom.
Single-phase chromium compounds always exhibit antiferromagnetism following the
prediction of Goodenough-Kanamori rules. So far, exchange coupling between
chromium ions via hetero-anions has not been explored and the associated
quantum states is unknown. Here we report the successful epitaxial synthesis
and characterizations of chromium oxide (Cr2O3)-chromium nitride (CrN)
superlattices. Room-temperature ferromagnetic spin ordering is achieved at the
interfaces between these two antiferromagnets, and the magnitude of the effect
decays with increasing layer thickness. First-principles calculations indicate
that robust ferromagnetic spin interaction between Cr3+ ions via
anion-hybridizations across the interface yields the lowest total energy. This
work opens the door to fundamental understanding of the unexpected and
exceptional properties of oxide-nitride interfaces and provides access to
hidden phases at low-dimensional quantum heterostructures.
