FERROMAGNETISM IN TRIANGULAR COBALT(II) LAYERS - COMPARISON OF CO(OH)2 AND CO2(NO3)(OH)3

This paper deals with correlations between structural and magnetic properties of two layered parent compounds, Co(OH)2 and Co2(NO3)(OH)3, which mainly differ by their interlayer distances. Measurements of magnetization, magnetic susceptibility, and specific heat show the occurrence of phase transitions at nearly the same temperatures, and this is explained by strong anisotropic in-plane interactions between cobalt(II) ions. Using HTS expansion for a triangular 2D lattice and numerical computations, the in-plane ferromagnetic interactions are shown to be J = 9.2 K for Co(OH)2 and 7.4 K for Co2(NO3)(OH)3. In the ordered state, both compounds exhibit metamagnetic behavior with critical fields mainly related to interplane interactions. A spin anisotropy crossover (from XY to Ising) is suggested to explain the magnetic findings above and below T(c).