The application of Landau's theory of continuous phase transitions to magnetic phase transitions

The conventional group theoretical results of Landau's theory of continuous (i.e. second order) phase transitions are extended to cover the case of a transition between two magnetically ordered phases; this involves the use of the irreducible corepresentations of the magnetic space groups of the two phases between which the transition occurs. An example is given to illustrate the application of this theory. It is shown that while, for some systems, the correct conclusions can be obtained by neglecting the anti-unitary symmetry operations that contain theta , the operation of time reversal, this is not always the case.