  We propose a model of carrier-mediated ferromagnetism in semiconductors that
accounts for the temperature dependence of the carriers. The model permits
analysis of the thermodynamic stability of competing magnetic states, opening
the door to the construction of magnetic phase diagrams. As an example we
analyze the stability of a possible reentrant ferromagnetic semiconductor, in
which increasing temperature leads to an increased carrier density, such that
the enhanced exchange coupling between magnetic impurities results in the onset
of ferromagnetism as temperature is raised.
