Possible ferromagnetism in divalent boride systems

The possibility of ferromagnetic ground states of divalent boride systems has been theoretically investigated by taking into account realistic three-dimensional structures with three pairs of valence and conduction bands which overlap at the X points. It is indicated that because of the particular relationships between on-site Coulomb and exchange interactions, metallic ferromagnetism is difficult to stabilize even within a mean-field calculation. On the other hand, the ferromagnetic metal is seen to be more easily stabilized in the model of a single pair of semimetallic bands, suggesting the preference of tetragonal symmetry for ferromagnetism. Also, a comment is made against excitonic ferromagnetism.