High-Temperature Ferromagnetism in CaB ₂ C ₂

<jats:p> We report a high Curie–temperature ferromagnet, CaB <jats:sub>2</jats:sub> C <jats:sub>2</jats:sub> . Although the compound has neither transition metal nor rare earth ions, the ferromagnetic transition temperature <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> is about 770 Kelvin. Despite this high <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> , the magnitude of the ordered moment at room temperatures is on the order of 10 <jats:sup>−4</jats:sup> Bohr magneton per formula unit. These properties are rather similar to those of doped divalent hexaborides, such as Ca <jats:sub> 1− <jats:italic>x</jats:italic> </jats:sub> La <jats:sub> <jats:italic>x</jats:italic> </jats:sub> B <jats:sub>6</jats:sub> . The calculated electronic states also show similarity near the Fermi level between CaB <jats:sub>2</jats:sub> C <jats:sub>2</jats:sub> and divalent hexaborides. However, there is an important difference: CaB <jats:sub>2</jats:sub> C <jats:sub>2</jats:sub> crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes—in contrast with CaB <jats:sub>6</jats:sub> . 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- <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> ferromagnetism. </jats:p>