Interfacial Ferromagnetism in ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ Superlattices

We observe interfacial ferromagnetism in superlattices of the paramagnetic metal ${\mathrm{LaNiO}}_3$ and the antiferromagnetic insulator ${\mathrm{CaMnO}}_3$. ${\mathrm{LaNiO}}_3$ exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of ${\mathrm{LaNiO}}_3$. That is, only superlattices in which the ${\mathrm{LaNiO}}_3$ layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of ${\mathrm{CaMnO}}_3$ 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.

Figure 1

X-ray reflectivity and theoretical fit of a $(6,8{)}_{10}$ superlattice showing total thickness fringes as well as first, second, and third order superlattice reflections (marked by arrows). (Inset) High resolution STEM Z-contrast image of the same sample.

Figure 2

(a) X-ray absorption spectra (XAS) of the Mn $L_{3,2}$ edge of a (6,8) superlattice. This spectra is consistent with a Mn valence between $3.9+$ and $4.0+$, very near bulk CMO. (b) X-ray magnetic circular dichroism of the Mn $L_{3,2}$ edge showing asymmetry associated with a magnetic response in the Mn ions.

Figure 3

(a) Fitted polarized neutron reflectivity with standard error at 800 mT and 15 K at the first order superlattice reflection of a (5,8) superlattice. (b) We show below the model used to obtain this fit, in which we assume one unit cell of magnetized CMO.

Figure 4

(a) Magnetic hysteresis loops for a series of typical (2,8), (4,8), and (6,8) superlattices. (Inset) Resistivity vs temperature from 5 to 350 K for (2,8), (4,8), and (6,8) superlattices showing a thickness dependent metal-insulator transition. This transition coincides with the transition from FM to nonmagnetic behavior of the superlattices. (b) Magnetic hysteresis loops showing scaling of the magnetic moment with the number of interfaces rather than CMO layer thickness. (Inset) Magnetic moment vs temperature from 10 to 150 K. A FM transition is observed between 75 and 80 K only in $n\ge 4$ superlattices.