High-Temperature Ferromagnetism in CaB 2 C 2

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Abstract
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 Ca1−xLaxB6. 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.