Theory of Ferromagnetism inCa1−xLaxB6

Novel ferromagnetism in ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{B}}_6$ is studied in terms of the Ginzburg-Landau theory for excitonic-order parameters, taking into account symmetry of the wave functions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.