Spin excitations in the antiferromagnet NaNiO2
  In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S = 1=2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferro- magnetic 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 bi-axial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane. The direction of the easy plane is constrained by the collective Jahn-Teller distortion occurring in this material at 480 K. 
Sophie De Brion (LCMI), C. Darie (LC), M. Hopzapfel (LCMI), D.   Talbayev, L. Mihaly, F. Simon, A. Janossy, G. Chouteau (LCMI)