ESR in Hexagonal ABX₃ Type Antiferromagnets. II. KNiCl₃: Easy-Plane Anisotropy Case

Abstract

We report the results of ESR and magnetic measurements on single crystals of KNiCl₃ whose chemical unit cell at room temperature is described by enlarging that of CsNiCl₃ to \sqrt3a, \sqrt3a, c, containing three –NiCl₃– chains (two on the A-site and one on the B-site). The intrachain exchange interaction constant was estimated to be J₀⁄k_B=−15 K. The magnetic anisotropy was found to be the easy-plane type and D⁄k_B=−0.86 K. Three-dimensional ordering occurs at T_N=8.2 K. In the ordered state an ESR mode which is unexplainable in terms of the usual two-sublattice model was observed. Introducing two kinds of interchain exchange interactions, J₁^AB and J₁^AA, and taking six sublattices, we investigate the normal modes for the motions of the sublattice magnetizations. The frequency-field diagram calculated with J₁^AB⁄k_B=−1.3×10⁻² K and J₁^AA⁄k_B=−1.1×10⁻² K is in good agreement with the experimental one.