Understanding the Origin of Ferromagnetism in LaNiO$_{3}$/CaMnO$_{3}$ Superlattices

Interfacial ferromagnetism (FM) in transition metal oxide heterostructures is a promising route for engineering new low-dimensional devices. In 2001, FM was discovered in CaRuO$_{3}$/CaMnO$_{3}$ superlattices (SLs), which is attributed to an itinerant electron-mediated Mn-Mn double-exchange (DE). Since then we have discovered interfacial FM in (LaNiO$_{3}$)$_{N}$/(CaMnO$_{3}$)$_{8}$ SLs that is consistent with this DE interaction\footnote{A.J. Grutter et al., \textit{Phys. Rev. Lett.} \textbf{111}, 087202 (2013)}. Now we have explored even further reduced dimensionality by fabricating [(LNO)$_{n=2-7}$/(CMO)$_{4}$]$_{10}$ SLs. Transport measurements confirmed a thickness dependent metal-insulator transition, with insulating films for N$<$4. Bulk magnetometry measurements reveal interfacial FM in insulating and conducting SLs. Since there are no itinerant electrons in the insulating SLs, this FM must arise from a different source. Using x-ray absorption spectroscopy and magnetic circular dichroism, we have identified the coexistence of Ni$^{2+}$ and Ni$^{3+}$ and Ni magnetism. We therefore speculate that the FM in insulating SLs originates from a Mn-Ni superexchange interaction. We discuss the role of these interactions in interfacial FM and methods for controlling them.