Metal-insulator transition, interfacial ferromagnetism, and orbital dimensional crossover in in-situ grown LaNiO₃/CaMnO₃ superlattices

Oxide heterostructures exhibiting emergent magnetic interfacial phenomena are of great interest in the design of future nanoscale spintronic devices. Here, we employ in-situ synthesis in conjunction with polarization-dependent angle-resolved photoelectron spectroscopy and x-ray magnetic circular dichroism to investigate LaNiO₃/CaMnO₃ superlattices, wherein a thickness-dependent metal-insulator transition in LaNiO₃ could be used to control the emergent interfacial ferromagnetic state in CaMnO₃ via the modulation of charge transfer across the interface. We observe the onset of the metal-insulator transition and bandgap opening in LaNiO₃ at a critical thickness of four unit cells, and a complete destruction of the Fermi surface in the ultrathin limit, as confirmed by first-principles dynamical mean field theory calculations. This is accompanied by a dimensional crossover, wherein the orbital polarization of the Ni 3d states switches to being primarily in-plane. A concomitant suppression of the interfacial Mn magnetic moment is observed below the critical thickness, but it is maximized at the onset of the metal-insulator transition.