Enhanced Interfacial Ferromagnetism in (111)-Oriented LaNiO$_3$/CaMnO$_3$ Superlattices

Interfacial ferromagnetism (i-FM) in complex oxides is a promising route for generating magnetic phenomena. An interesting example is FM in CaMnO$_3$ (CMO)-based superlattices (SLs), which is attributed to Mn-Mn double exchange (DE). In (LaNiO$_3$)$_N$/(CaMnO$_3$)$_M$ (LNO/CMO) SLs on (001)-oriented LaAlO$_3$, i-FM arises from Mn-Mn DE and Ni-Mn superexchange (SE). The relative contribution from these mechanisms can be tuned via cooperative distortions of the oxygen octahedra. For CMO, a G-type antiferromagnet, (001) planes are fully spin compensated, while (111) planes are fully uncompensated. Therefore, one may expect (111)-oriented SLs to have enhanced interfacial coupling, giving us insight into the interplay between the magnetic mechanisms in these SLs. We have fabricated LNO/CMO SLs on (111)-oriented LaAlO$_3$. We observe a significant enhancement in i-FM compared to (001)-oriented LNO/CMO SLs. Magnetometry measurements show that a (111)-oriented N=4, M=4 SL has more than a threefold enhancement in i-FM compared to the (001)-oriented N=4, M=4 SL with ~1.0 μB/int. Mn for (111) and ~0.3 μB/int. Mn for (001) SLs. These results are consistent with enhanced Ni-Mn SE arising from increased interfacial Ni-Mn bonding as a result of the (111) growth axis.