Half-metallic surfaces in thin-film Ti2MnAl0.5Sn0.5.

Materials exhibiting a high degree of spin polarization in electron transport are in demand for applications in spintronics-an emerging technology utilizing a spin degree of freedom in electronic devices. Room-temperature half-metals are considered ideal candidates, as they behave as an insulator for one spin channel and as a conductor for the other spin channel. In addition, for nano-size devices, one has to take into account possible modification of electronic structure in thin-film geometry, due to the potential presence of surface/interface states. It has been shown that typically these states have a detrimental impact on half-metallicity, i.e. their presence results in reduced spin-polarization. Here, we employ density functional calculations to explore an inverse Heusler compound, Ti2MnAl0.5Sn0.5, which exhibits half-metallic electronic structure in bulk geometry. In particular, this material behaves as a regular metal for majority-spin, and as a semiconductor for minority-spin states. We show that in thin-film geometry, the type of termination surface has a decisive effect on half-metallicity of this material. In particular, we analyze six possible termination configurations, and show that for four of them, energy states emerge in the minority-spin band gap, significantly reducing the spin polarization of Ti2MnAl0.5Sn0.5. At the same time, our calculations indicate that two termination surfaces preserve half-metallic properties of this material. This result is somewhat unexpected, as most of the available literature reports reduction of the spin-polarization due to the presence of surface states. Thus, our results show that a judicious choice of the termination surface may be a crucial factor in nano-device applications, where highly spin-polarized current is needed.

Effect of boron doping on nanostructure and magnetism of rapidly quenched Zr2Co11-based alloys.

The role of B on the microstructure and magnetism of Zr16Co82.5-x Mo1.5B x ribbons prepared by arc melting and melt spinning is investigated. Microstructure analysis show that the ribbons consist of a hard-magnetic rhombohedral Zr2Co11 phase and a minor amount of soft-magnetic Co. We show that the addition of B increases the amount of hard-magnetic phase, reduces the amount of soft-magnetic Co and coarsens the grain size from about 35 nm to 110 nm. There is a monotonic increase in the volume of the rhombohedral Zr2Co11 unit cell with increasing B concentration. This is consistent with a previous theoretical prediction that B may occupy a special type of large interstitial sites, called interruption sites. The optimum magnetic properties, obtained for x = 1, are a saturation magnetization of 7.8 kG, a coercivity of 5.4 kOe, and a maximum energy product of 4.1 MGOe.