Phase transitions in an antiferromagnet with a hidden canting

Abstract

The susceptibility of a single crystal of LiCuCl 3·2H 2O, which is an antiferromagnet with Néel temperature T N = 4.4 K, has been measured at 225 Hz for different crystallographic directions, both as a function of the magnetic field (up to 110 kOe) and of the temperature (1.2–12 K). Phase diagrams were derived from these measurements, which showed more transitions than those which are usually found in simple two-sublattice antiferromagnets. The case in which the external field is parallel to the direction of antiferromagnetic alignment as well as the case in which the field is parallel to the b axis were investigated more extensively. The presence of a large Dzyaloshinsky-Moriya interaction seems quite well established, and this interaction is thought to be responsible for the unusual phase transitions found in this antiferromagnet. A new mechanism is proposed for the occurrence of phase transitions in antiferromagnets showing a hidden canting. An essential property of this process is a sudden sign reversal of the vector product of two of the four sublattice magnetization vectors, due to the existence of the Dzyaloshinsky-Moriya interaction, followed by a second-order phase transition, at which the four sublattices combine two by two, if the field is parallel to the easy spin direction. Preliminary calculations have been performed on the zero-temperature energy expression, which is expected to give a description of the most important features of the complicated behaviour of this antiferromagnet.