ESR in Hexagonal ABX₃Type Antiferromagnets. II. KNiCl₃: Easy-Plane Anisotropy Case

Abstract

We report the results of ESR and magnetic measurements on single crystals of KNiCl 3 whose chemical unit cell at room temperature is described by enlarging that of CsNiCl 3 to \(\sqrt{3}a\), \(\sqrt{3}a\), c , containing three –NiCl 3 – chains (two on the A-site and one on the B-site). The intrachain exchange interaction constant was estimated to be J 0 / 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, J 1 AB and J 1 AA , and taking six sublattices, we investigate the normal modes for the motions of the sublattice magnetizations. The frequency-field diagram calculated with J 1 AB / k B =-1.3×10 -2 K and J 1 AA / k B =-1.1×10 -2 K is in good agreement with the experimental one.