High-Temperature Ferromagnetism in CaB 2 C 2

Abstract

We report a high Curie–temperature ferromagnet, CaB 2 C 2 . 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− x La x B 6 . The calculated electronic states also show similarity near the Fermi level between CaB 2 C 2 and divalent hexaborides. However, there is an important difference: CaB 2 C 2 crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes—in contrast with CaB 6 . 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.