Comments on the use of the Landau theory in some order-order transitions

The application of the Landau theory of phase transitions to a two-sublattice magnet results in a broad range of magnetic transitions. The use of a two-component order parameter enables one to describe both order-disorder and order-order transitions. In particular, the metamagnetic transitions exhibit the thermal hysteresis. It is shown that the Kittel model is a special case of the Landau expansion. Finally, an alternative one-component order parameter expansion is discussed.