  Atomically engineered oxide heterostructures provide a fertile ground for
creating novel states. For example, a two-dimensional electron gas at the
interface between two oxide insulators, giant thermoelectric Seebeck
coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and
colossal ionic conductivity. Extensive research efforts reveal that oxygen
deficiency or lattice strain play an important role in determining these
unexpected properties. Herein, by studying the abrupt presence of robust
ferromagnetism (up to 1.5 uB/Mn) in LaMnO3-based heterostructures, we find the
multivalence states of Mn that play a decisive role in the emergence of
ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films. Combining
spatially resolved electron energy-loss spectroscopy, X-ray absorption
spectroscopy and X-ray magnetic circular dichroism techniques, we determine
unambiguously that the ferromagnetism results from a conventional Mn3+-O-Mn4+
double-exchange mechanism rather than an interfacial effect. In contrast, the
magnetic dead layer of 5 unit cell in proximity to the interface is found to be
accompanied with the accumulation of Mn2+ induced by electronic reconstruction.
These findings provide a hitherto-unexplored multivalence state of Mn on the
emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3
and BiMnO3, and shed new light on all-oxide spintronic devices.
