  Interfacial charge transfer in oxide heterostructures gives rise to a rich
variety of electronic and magnetic phenomena. Designing heterostructures where
one of the thin-film components exhibits a metal-insulator transition opens a
promising avenue for controlling such phenomena both statically and
dynamically. In this letter, we utilize a combination of depth-resolved soft
X-ray standing-wave and hard X-ray photoelectron spectroscopies in conjunction
with polarization-dependent X-ray absorption spectroscopy to investigate the
effects of the metal-insulator transition in $LaNiO_{3}$ on the electronic and
magnetic states at the $LaNiO_{3}/CaMnO_{3}$ interface. We report on a direct
observation of the reduced effective valence state of the interfacial Mn
cations in the metallic superlattice with an above-critical $LaNiO_{3}$
thickness (6 u.c.) due to the leakage of itinerant Ni 3d $e_{g}$ electrons into
the interfacial $CaMnO_{3}$ layer. Conversely, in an insulating superlattice
with a below-critical $LaNiO_{3}$ thickness of 2 u.c., a homogeneous effective
valence state of Mn is observed throughout the $CaMnO_{3}$ layers due to the
blockage of charge transfer across the interface. The ability to switch and
tune interfacial charge transfer enables precise control of the emergent
ferromagnetic state at the $LaNiO_{3}/CaMnO_{3}$ interface and, thus, has
far-reaching consequences on the future strategies for the design of
next-generation spintronic devices.
