Magnetism and half-metallicity in bulk and (1 0 0) surface of Heusler alloy Ti2CoAl with Hg2CuTi-type structure

Using the first-principles calculations within density functional theory (DFT), we investigate the electronic and the magnetic properties of the bulk and the (1 0 0) surface of Hg2CuTi-type Heusler alloy Ti2CoAl. In the Ti2CoAl bulk, the calculated equilibrium lattice constant of 6.2 Å is detected. Due to different surroundings, the atomic magnetic moment (AMM) of Ti1 and that of Ti2 show distinct difference and the magnetic contribution of Ti1 is slightly larger than that of Ti2. In addition, our work indicate that the competition between RKKY mechanism and direct magnetic exchange among d-electron atoms plays a dominating role in determining the total magnetism. As for the Ti2CoAl (1 0 0) surface, we have studied five terminations and such studies reveal that the surface Ti1 and Co atoms prefer to move towards vacuum and the slab respectively while the surface Ti2 and Al atoms almost hold their positions unmoved in comparison with those in the bulk. Moreover, the analysis on surface magnetic behaviors show that owing to the surface effect, the AMM of surface Ti1 is enhanced obviously while that of Co atom is decreased in contrast with the bulk value and the AMM of Ti2 is very close to the bulk value. Further investigation on the PDOS indicate that because of surface states, the calculated five atom-terminations fail to preserve the half-metallicity which can be observed in the bulk. Different from the conventional Co-based Heusler alloy with Cu2MnAl-type structure, it is predicted with higher surface spin polarization in CoCo-termination than TiCo-, TiAl- and TiTi-termination, moreover the AlAl atomic surface also presents more superiority to other calculated surface systems.

► Hg2CuTi-type Ti2CoAl is different from conventional Cu2MnAl-type structure.
► The atomic magnetic moments of Ti1 are different from that of Ti2 in bulk.
► The surface properties reveal that all terminations lose the half-metallicity.
► CoCo-, TiCo-, TiAl-, AlAl-termination are predicted with high spin polarization.