A model of magneto-structural phase transition in MnAs

Abstract

Two magneto-structural phase transitions of MnAs between three phases; ferromagnetic and undistorted phase, paramagnetic and distorted phase, paramagnetic and undistorted phase; are consistently explained by a simple model Hamiltonian without an assumption of high- and low-spin states for Mn atoms. An operator is introduced to describe the distortion of the crystal, and an exchange interaction of each Mn atom pair is expressed in terms of the operator. Thus a coupled Hamiltonian for the crystal distortion and the magnetic ordering is obtained in a molecular field approximation. It is shown from the calculated result that the three phases are realised and an anomalous temperature dependence of a paramagnetic susceptibility is explained. In the presence of an external magnetic field, this model yields a field-induced transition, which is qualitatively in agreement with experimental results. Significant features of temperature dependences of the specific heat and the lattice constant are also explained by the present calculation.