Anomalous Behavior of the Ni 1+ moment in bi-infinite-layered La 3 Ni 2 O 5 F

The discovery of superconductivity in hole-doped Ni 1+ systems with ``infinite layer" NiO 2 square-lattices analogous to the Cu 2+ CaCuO 2 cuprate has renewed conflicting pictures of the Cu 2+ - Ni 1+ similarities and distinctions. Recent synthesis of formal Ni 1+ La 3 Ni 2 O 5 F with two infinite NiO 2 layers per cell provides a novel member of this class. We find that Ni 1+ in this material is unusually flexible. First principles density functional theory studies reveal a single partially occupied electron band derived from density in three interstitial layers that provides self-doping to a Ni 1.09+ charge state. The blocking La(O/F)La layer provides isolation of the NiO 2 bilayer to strictly two-dimensional electronic and magnetic systems. Fixed spin moment calculations of magnetic tendencies reveal behavior unlike previous nickelates, including an essentially vanishing differential susceptibility up to a large magnetic field and a metastable ferromagnetic (FM) state. Antiferromagnetic (AFM) order is favored energetically as expected; surprisingly both FM and AFM states are obtained without correlation (Hubbard U) corrections. Doping away from half-filling by an interstitial density derived band and likely two-dimensional fluctuations account for the lack of observation of any magnetic transition.