Antiferromagnetic resonance and high magnetic field properties of NaNiO2

Magnetisation measurements up to 23 T and submillimeter wave ESR in the frequency region 48-380 GHz have been performed on NaNiO2 powders at low temperature. Also typical spectra above the Neel temperature T-N are shown. At 4 K the magnetisation shows a spin-flop transition at 1.8 T and saturation at 10 T. M-S = 1 mu (B)/Ni confirms the low spin state of the Ni3+ ions. The susceptibility exhibits: a maximum at T-N = 20 K with mu (eff) = 1.85 mu (B) and theta = +36 K. NaNiO2 is an A-type antiferromagnet: we derive J(F) = 13 K and J(AF) = -1 K for the interactions between Ni ions within and between adjacent layers, respectively. The AFMR spectra yield an energy gap of 52.5 GHz, in agreement with the spin-flop value derived from the magnetisation. The anisotropy of the g factor observed at 100 K with g(perpendicular to) > g(\\) can be attributed to the Jahn-Teller effect for Ni3+ ions in the low spin state, which stabilises the \3z(2) - r(2)> occupation. We finally comment on the isomorphic controversial Li1-xNi1+xO2 compound.