Temperature dependence of the field induced magnetization reorientation in dzialoshinsky-moriya type weak ferromagnets

Using a Hamiltonian consisting of isotropic, anisotropic, and antisymmetric exchange terms plus a Zeeman term, the temperature dependence of the weak ferromagnetic-spin-flop phase transition is calculated in the molecuar field approximation. For all temperatures less than the tricritical one, the phase transition is of first order. The stability limits of the two phases and the thermodynamic phase boundary are calculated for this temperature region. For temperatures greater than the tricritical one, the second order transition boundary between the phases is calculated. A set of equations precisely defining the tricritical point in the field-temperature plane is derived and results are presented for the location of this point as a function of the parameters of the system. The methods developed in this work are general and can be applied directly to similar problems.