Anomalous Behavior of the Ni$^{1+}$ moment in bi-infinite-layered La$_3$Ni$_2$O$_5$F

Abstract

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 {\it 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.