First-principles Study Of The Electronic Structure, Magnetic Properties And Spin Polarization Of Heusler Alloy Co₂MnAl

Half-metallic alloy attracts great attentions, because it prove electron spin polarization when the alloy injects into the semiconductor. Heusler alloys Co2MnAl, due to relatively higher spin polarization and magnetism, Curie temperature over than room temperature, is considered an ideal source of spin injection.In this paler, using the projector augmented wave (PAW) method in the generalized gradient approximation (GGA) and plus on-site coulomb exchange interaction U, we systematically investigate the atomic relaxation,electronic structure, magnetism and spin-polarization in L21 and B2 structure heusler alloy Co2MnAl and its (100) surface.The results are followings:(1) The spin polarizations as well as the magnetizations are stable against small tetragonal distortions. The calculated Curie temperature Tc of the L21 phase is 789 K which is about 60 K higher than that (Tc=723 K.) of the B2 phase. The total magnetic moments of L21 and B2 phases under GGA scheme are about 4 ?B in agreement with the experimental value while the moments under GGA+U scheme have large values and are very sensitive to the U parameter. Electronic structure calculations show that there is a energy gap in the minority spin states in the L21 phase and the gap disappears in the B2 phase. Not only the L2i phase but also the B2 phase exhibits relatively high values of the spin polarization ratio P as very low density of states at the Fermi level in the minority spin band.(2) Due to the difference of Co-Mn and Co-Al bonding, the L21 and B2 structure surface atoms prefer to move toward and away the vacuum. By compared with the bulk, the spin magnetic moments of surface Co and Mn are obviously increased due to the surface effects. Our electronic structure calculations show that the gap in L21 structure bulk Co2MnAl has been destroyed by the surface states and the spin-polarization of CoCo atomic terminated surface in both structures decreases. However, the spin-polarization of MnAl atomic terminated surface is not significantly affected by the surface effects and has large value in both structures, which is potential in application for magnetic tunneling junctions.