Polarization reduction in half-metallic Heusler alloys: the effect of point defects and interfaces with semiconductors

Half-metallic full-Heusler alloys represent a promising class of materials for spintronic applications. However, (i) intrinsic point defects in Heusler compounds can be detrimental with respect to their predicted 100% spin polarization at the Fermi level and (ii) when joined to mainstream semiconductors the presence of interface states-which destroys half-metallicity-can degrade their performance. Here, we present an overview of recent first-principles calculations performed to explore both these issues. In particular, we focus on ab initio FLAPW calculations performed for Co2MnGe and Co2MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well as non-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings show that Mn antisites, due to their low formation energies, can easily occur, in excellent consistency with experimental observations, and that they do not destroy half-metallicity. On the other hand, Co antisites, which also show a modest formation energy, give rise to defect states at the Fermi level. As for the [001]-ordered interfaces, we show that the strong hybridization in proximity to the junction gives rise to rather broad interface states that locally destroy half-metallicity. However, the bulk gaps (both in the minority spin channel for the Heusler alloy and for both spin channels in the semiconducting side) are fully recovered within a few layers away from the junction.