  We observe interfacial ferromagnetism in superlattices of the paramagnetic
metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a
thickness dependent metal-insulator transition and we find the emergence of
ferromagnetism to be coincident with the conducting state of LaNiO3. That is,
only superlattices in which the LaNiO3 layers are metallic exhibit
ferromagnetism. Using several magnetic probes, we have determined that the
ferromagnetism arises in a single unit cell of CaMnO3 at the interface.
Together these results suggest that ferromagnetism can be attributed to a
double exchange interaction among Mn ions mediated by the adjacent itinerant
metal.
