Half-metallic ferromagnetism in hexagonal M Al7 N8 and cubic M Al3 N4 (M=Cr and Mn) from first principles

Motivated by recent reports on high Curie temperatures in Cr- and Mn-doped aluminum nitrides and the fabrication of heavily transition-metal-doped ones with room-temperature ferromagnetism, we study systematically 12.5% transition-metal substituted aluminium nitrides, hexagonal CrAl7 N8 and MnAl7 N8, using a full-potential density-functional method. We optimize fully the crystal structures and prove the stability of their ferromagnetism against possible antiferromagnetic orders. Their volumes are expanded only by about 1% compared with that of AlN. Every d electron of +3 cations of Cr and Mn contributes one Bohr magneton to the moment. Our calculations show that both of them are half-metallic ferromagnets with half-metallic gaps larger than 1 eV. The transition-metal substitution creates 10 transition-metal d -dominated impurity bands in the semiconductor gap of AlN. The hybridization of the d states with the N p ones yields a large spin exchange splitting of the d states which drives the ferromagnetism. The half-metallicity is attributed to the wide minority-spin gap across the Fermi level and the favorable majority-spin density of states of the d -dominated bands in addition to the above factors. 25% Cr and Mn substituted cubic aluminum nitrides, cubic CrAl3 N4 and MnAl3 N4, are studied in the same way and proved to share almost all the properties of the two hexagonal ones. The mechanism for the ferromagnetism should be useful for understanding nitride-based diluted magnetic semiconductors. These half-metallic transition-metal aluminum nitrides, at least some of them, could be useful in spintronics.