  Polar compensation can play an important role in the determination of
interfacial electronic and magnetic properties in oxide heterostructures. Using
x-ray absorption spectroscopy, x-ray magnetic circular dichroism, bulk
magnetometry, and transport measurements, we find that interfacial charge
redistribution via polar compensation is essential for explaining the evolution
of interfacial ferromagnetism in LaNiO$_3$/CaMnO$_3$ superlattices as a
function of LaNiO$_3$ layer thickness. In insulating superlattices (4 unit
cells or less of LaNiO$_3$), magnetism is dominated by Ni-Mn superexchange,
while itinerant electron-based Mn-Mn double-exchange plays a role in thicker
metallic superlattices. X-ray magnetic circular dichroism and resonant x-ray
scattering show that Ni-Mn superexchange contributes to the magnetization even
in metallic superlattices. This Ni-Mn superexchange interaction can be
explained in terms of polar compensation at the LaNiO$_3$-CaMnO$_3$ interface.
These results highlight the different mechanisms responsible for interfacial
ferromagnetism and the importance of understanding compensation due to polar
mismatch at oxide-based interfaces when engineering magnetic properties.
