On the Critical Behaviour and Phase Diagram of Ising-like Metamagnet in a Transverse Field

Abstract

A. behaviour of an Ising system in a transverse magnetic field T (applied to the easy direction) has been the subject of several theoretical [1-4] and experimental studies [5, 6]. An important feature of such system is a crossover near the zero-temperature from the quantum to the classical behaviour. It has been studied theoretically [2-4] and observed in a simple two-sublattice Ising-type antiferromagnet. MnCl2 •4H20 (MnCl2 •4D20) at temperatures in a range 100 < T < 350 mK [5, 6]. It is interesting, however, to consider the Ising-type two-sublattice system, when except of the transverse field T a field H parallel to the easy direction is applied. In such a case we have metamagnet, in which the phase transition can be driven by three parameters, namely, F, H, and T. This problem has been recently reported in Ref. [7], where the system in the quantum regime has been investigated. The aim of the present paper is to show that in the three-dimensional space of parameters F, H, and T not only critical lines, in F, H [7] and T, H [8]-planes but also a surface of critical points exist, which is bounded by the line of tricritical points. Consider an Ising-like metamagnet described by the following Hamiltonian [7] (cf. also Ref. [8]):