Interface*

LaNiO₃ (LNO) is unique among rare-earth nickelates because it is a paramagnetic metal at all temperatures, while others undergo magnetic and metal-insulator transitions. Recently, we have found that the unique strain state at the (111) LaNiO₃/LaAlO₃ interface stabilizes a ferromagnetic, insulating phase in LNO. The constraints on NiO₆ octahedra at the (111)-oriented perovskite interface limit the tilts and rotations that usually relax strain in (001)-oriented films. X-ray diffraction and dynamical x-ray simulations show this results in an elongation of the out-of-plane lattice parameter in the first 5 unit cells of LNO(111) films. Transport measurements show the anomalous Hall effect and negative, hysteretic magnetoresistance in films up to 26 u.c.. This ferromagnetism can originate from Ni²⁺-Ni³⁺ superexchange interactions in the distorted, interfacial LNO(111). We also discuss the magnetic depth profile deduced from polarized neutron reflectometry. The emergence of ferromagnetism indicates a potential for increased functionality in other (111)-oriented perovskite films.