Impact of mixed anion ordered state on the magnetic ground states of S = 1/2 square-lattice quantum spin antiferromagnets, SrNiOCl and SrNiOF

Tsujimoto, Yoshihiro*; Sugiyama, Jun; 13 of others*

The magnetic properties of the S = 1/2 two-dimensional square-lattice antiferromagnets SrNiOX (X = Cl, F) with the trivalent nickel ions in a low-spin state were studied by magnetic susceptibility, heat capacity, neutron powder diffraction, high-field electron spin resonance (ESR), muon spin rotation and relaxation (SR)measurements, and density functional theory (DFT) calculations. Both oxyhalides are isostructural to an ideal quantum square-lattice antiferromagnet SrCuOCl, but the chlorine/fluorine anion exclusively occupies an apical site in an ordered/disordered manner with an oxygen anion, resulting in the formation of highly distortedNiOX octahedra with an off-center nickel ion. The mechanism of ferromagnetic superexchange interactions and the reason for the difference between the magnetic ground states in these nickel oxyhalides are discussed.