Electronic Structure and Magnetic Properties of Ferromagnetic Shape Memory Alloys CoNiAlThis Paper was Presented at the Spring Meeting of the Japan Institute of Metals, held in Tokyo, on April 1, 2004

To expand our knowledge of electronic structures and magnetic properties for ferromagnetic shape memory Co-Ni-Al alloys, which undergo a thermoelastic martensitic transformation from a B2 to an L10 structure, we have performed first-principles band calculations for the composition of Co:Ni:Al = 1:1:1, i.e., (Co1=3Ni2=3)(Co1=3Al2=3) and (Ni1=3Co2=3)(Ni1=3Al2=3). Their electronic structures have been calculated for the supercell structure, including a cubic-tetragonal distortion and a spin polarization. The obtained total energy indicates that (Co1=3Ni2=3)(Co1=3Al2=3) may be more appropriate than (Ni1=3Co2=3)(Ni1=3Al2=3). In the paramagnetic state of (Co1=3Ni2=3)(Co1=3Al2=3), it has been found that the transformation from the B2 to the L10 structure comes from the change of the environment of the Co atoms (Co[2e]) on the original Al sites and, other Co and Ni atoms. These features are reflected in their d-orbital density of states (DOS). The Co[2e] atoms also play an important role in the magnetic transition between the paramagnetic and ferromagnetic states. The Co[2e] atoms carry magnetic moments corresponding to those of fcc Co (hcp Co) and the energy gain due to the spin polarization is brought. The origin of the spin polarization can be attributed to the similarity of their environment. This is confirmed by the similarity of their d-orbital DOSs. The ‘‘band energy’’ estimated from total DOS shows that the changes of DOS near the Fermi level bring the band-Jahn-Teller-type stabilization of the distorted structure.