Spin excitations in the antiferromagnet NaNi O2

In NaNi O2, Ni3+ ions form a quasi-two-dimensional triangular lattice of S=1 2 spins. The magnetic order observed below 20 K has been described as an A type antiferromagnet with ferromagnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The bulk of the results are interpreted in terms of a phenomenological model involving biaxial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane.