High-T_c ferromagnetism in the SrB₆ family: A case of doped spin-1 Mott insulator in a valence-bond solid phase

Doped divalent hexaborides such as Sr_1-xLa_xB₆ exhibit a high T_c ferromagnetism. We isolate a degenerate pair of 2p orbitals of boron with two valence electrons, invoke electron correlation and Hund coupling, to suggest that the undoped state is better viewed as a spin-1 Mott insulator; it is predicted to be a type of 3d Haldane gap phase with a spin gap ∼0.1eV, much smaller than the charge gap of >1.0eV seen in angle-resolved photoemission spectroscopy. The experimentally seen high T_c “ferromagnetism” is argued to be a complex magnetic order in disguise—either a canted six-sublattice antiferromagnetic (≈120°) order or its quantum melted version, a chiral spin liquid state, arising from a type of double-exchange mechanism.