Abstract
The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO3 (LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO6 octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition at t = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni2+ ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni2+–Ni3+ ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.
Highlights
Complex oxide materials exhibit a wide range of correlated electron behavior representing the delicate balance of charge, spin, orbital, and lattice degrees of freedom
In a few complex oxide materials, the ferromagnetic order has been stabilized in thin film form, the bulk phase has no long-range magnetic order
LaCoO3 thin films exhibit ferromagnetism (Tc = 70 K)[1,7,8,9], while bulk phases are diamagnetic at low temperatures and paramagnetic at higher temperatures
Summary
Complex oxide materials exhibit a wide range of correlated electron behavior representing the delicate balance of charge, spin, orbital, and lattice degrees of freedom. External parameters such as electric field, magnetic field, or strain can tune the material to exhibit correlated phenomena, including ferromagnetism[1,2,3,4] and superconductivity[5,6]. In a few complex oxide materials, the ferromagnetic order has been stabilized in thin film form, the bulk phase has no long-range magnetic order. Understanding the origin of the emergent longrange magnetic order in LNO(111) films would enable us to design and tune such correlated behavior and explore interactions in possible topological phases
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