Study of Short-Range Spin Ordering in RbNi_1-xCoxCl₃

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Short-range spin ordering of RbNi_1-xCo_xCl₃ with the substitutional concentration x smaller than 0.05 was studied by the birefringence technique. This mixed system belongs to an Ising-like Heisenberg antiferromagnet on a triangular lattice and has successive phase transitions. The upper Néel temperature, T_N1, goes up and the lower one, T_N2, goes down as an effective Ising anisotropy is amplified with gradual substituting of Ni²⁺ by Co²⁺. The temperature dependence of the birefringence Δn^ac for light propagating perpendicular to the c-axis is sensitive to the Co²⁺ concentration x at temperatures near the phase transition points. How the Ising anisotropy affects the short-range ordering of an Ising-like Heisenberg antiferromagnet on a triangular lattice is discussed by analyzing the temperature dependence of the observed birefringence.