High-Temperature Ferromagnetism in CaB2C2

We report a high Curie-temperature ferromagnet, CaB₂C₂. Although the compound has neither transition metal nor rare earth ions, the ferromagnetic transition temperature T_c is about 770 Kelvin. Despite this high T_c, the magnitude of the ordered moment at room temperatures is on the order of 10^-4 Bohr magneton per formula unit. These properties are rather similar to those of doped divalent hexaborides, such as Ca_1-xLa_xB₆. The calculated electronic states also show similarity near the Fermi level between CaB₂C₂ and divalent hexaborides. However, there is an important difference: CaB₂C₂ crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes-in contrast with CaB₆. Thus, the disputed threefold degeneracy, specific to the cubic structure, in the energy bands of divalent hexaborides turns out not to be essential for high-temperature ferromagnetism. It is the peculiar molecular orbitals near the Fermi level that appear to be crucial to the high-T_c ferromagnetism.