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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Abstract-We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF ⁄ HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF ⁄ HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for 36 Ar, 58 ± 12% for 84 Kr, and 48 ± 10% for 132 Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF ⁄ HCl residue indicate that there is no Xe-HL in Saratov, but Ne isotopic ratios in the HF ⁄ HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne-Q and Ne-HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne-HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne-Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne-Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the 128 Xe ⁄ 132 Xe ratio. The 3 He and 21 Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.

We investigated the stability of the ferromagnetism of CrAs and CrSb in the zinc-blende structure against the lattice distortion, systematically. A calculation within the generalized gradient approximation using a full potential linearized augmented plane wave method was performed. We compared the ferromagnetic state and the antiferromagnetic state assuming tetragonal distortion with the lattice constants a and c changing independently and determined the spin polarization ratio in the ferromagnetic phase. The result shows that complete spin polarization (half-metallic ferromagnetism) remains stable even in the presence of large tetragonal distortion. On the other hand, our calculation shows that two monolayers of CrAs is enough to produce a half-metallic state in the CrAs/GaAs multilayer. Thus, the present result suggests that the halfmetallic nature persists in various atomic-scale superlattices made of distorted CrAs or CrSb.In recent years, there has been a great deal of research activity within a new discipline called 'spintronics' which aims at using the spins of electrons efficiently. To enhance the performance of future spintronics devices such as tunnelling magnetoresistance devices and spin fieldeffect transistors, it is desired to search for new materials having high spin polarization with ferromagnetism at room temperature. As prospective candidates, zinc-blende (zb-) CrAs and zb-CrSb have attracted the interest of many groups. Both zb-CrAs and zb-CrSb are newly predicted materials having completely spin-polarized band structure according to first-principles electronic structure calculations [1, 2] and they have now been synthesized successfully in experiments utilizing low temperature molecular-beam epitaxy [3,4]. Experimentally, the ferromagnetic transition temperature is estimated to be above 400 K.

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