New candidates for half-metallic materials were theoretically designed recently by Geshi et al. The materials are calcium pnictides, i.e. CaP, CaAs and CaSb. When the zinc-blende structure was assumed, these compounds showed half-metallic electronic band-structure, in which a curious flat band was found. To explain this magnetism, we investigated characters of orbitals on this flat band of CaAs. The hybridization of p states of As with d states of Ca is shown to be essential for formation of a flat band made of localized orbitals. The appearance of complete spin polarization in the flat band suggests that the flat-band mechanism is relevant for the ferromagnetism. A connection from the first-principles result to a solvable Hubbard model with a flat band is discussed.

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We have designed new materials, CaN and SrN in a rock-salt structure (RS). They are ferromagnetic nitrides. Their ferromagnetic states are quite stable. Our calculations for the formation energy and first-principles MD simulations suggested that RS-CaN is at least metastable and stable in normal condition. Based on our results we have proposed the synthesis process of RS-CaN. Our proposed process is (1) heat up α-Ca 3 N 2 until it is transformed into β-Ca 3 N 2 , (2) compress it above 50 GPa until 2Ca 3 N 2 →2CaN+Ca reaction occurs, (3) cool them down to room temperature and (4) decompress them into an ambient pressure. We consider that this high pressure and high temperature synthesis is one of the hopeful method to synthesize new materials.