Electronic Structure of Ni<SUB>3</SUB>AlX<I><SUB>y</SUB></I> (X=B, C, H; 0<<I>y</I><1)

Abstract

The electronic structures of anti-perovskite-type intermetallic compound Ni 3 AlX y (X ¼ B, C, H; 0 < y < 1) have been calculated using coherent-potential approximation (CPA) within the local-density approximation (LDA). Ferromagnetic moment in Ni 3 Al rapidly decreases with increasing y for every dopant X, even though the lattice is more expanded than non-doped Ni 3 Al. (Received September 16, 2005; Accepted November 10, 2005; Published March 15, 2006) Keywords: Ni 3 AlX y , Ni 3 Al, itinerant magnetism, electronic structure, Korringa-Kohn-Rostokar method, coherent-potential approximation Itinerant weak ferromagnetism is paid attention again since ferromagnetic superconductors are successively found, recently. The most fascinating example is ZrZn 2 , 1) which is considered as a typical itinerant ferromagnet in 1970's. UGe 2 2) and URhGe 3) are also found to be ferromagnetic superconductors. Superconductivity near ferromagnetic phase is also found in many compounds, such as MgCNi 3 , 4)Sr 2 RuO 4 , 5) and "-Fe, 6) etc. However, the mechanism of the superconductivity, and the conditions which bring about these exotic superconductivity, are still not clear.On the other hand, Ni 3 Al has attracted much attention because of its unique mechanical properties in the past three decades. Moreover, this compound shows a typical itinerant weak ferromagnetism, and because of its simple crystal structure, extensive experimental and theoretical studies are developed. One interesting result is that in Ni 3 Al under high pressure the ferromagnetic phase collapses, and this highpressure nonmagnetic phase shows a non-Fermi-liquid behavior.7) Recently, in order to realize strong ferromagnetism by lattice expansion, several chemical doping was tried. All of the Ni 3 AlX y (X ¼ B, C, H) show the lattice expansion, but contrary to the simple expectation, any of them does not show ferromagnetism. 8) We have shown that Ni 3 AlB cannot have Stoner-like ferromagnetism by an ab-initio band calculation, because of its low density of states at the Fermi level mainly due to the strong hybridization between Ni-d and B-p states. 9) However, 100% doped Ni 3 AlB is not synthesized yet, thus more realistic calculation for offstoichiometric compound Ni 3 AlB y is desirable.Our purpose is to calculate and predict a magnetic phase diagram of Ni 3 AlX y . For this purpose we performed systematic ab-initio calculations for Ni 3 AlX y (X ¼ B, C, H; 0 < y < 1). Ni 3 Al takes the simple cubic Cu 3 Au-type crystal structure, and the dopants occupy the 1b-site, which is a large void in Ni 3 Al. The scheme we used in our calculations is Korringa-Kohn-Rostokar (KKR) method. The crystal potential is approximated by the muffin-tin (MT) potential and the atomic-sphere approximation is used. MT radii are set as 0:429a for Al, 0:370a for Ni, and 0:195a for X(dopant), respectively, where a denotes the lattice constant. When X is not occupied, empty sphere is inserted at that site. For finite doping compounds the coherent-potential approximation (CPA) are applie...