On the half-metallicity of Co2FeSi Heusler alloy: Point-contact Andreev reflection spectroscopy andab initiostudy

Abstract

Co${}_{2}$FeSi, a Heusler alloy with the highest magnetic moment per unit cell and the highest Curie temperature, has largely been described theoretically as a half-metal. This conclusion, however, disagrees with point contact Andreev reflection (PCAR) spectroscopy measurements, which give much lower values of spin polarization, $P$. Here, we present the spin polarization measurements of Co${}_{2}$FeSi by the PCAR technique, along with a thorough computational exploration, within the DFT and a GGA+$U$ approach, of the Coulomb exchange $U$ parameters for Co and Fe atoms, taking into account spin-orbit coupling. We find that the orbital contribution (${m}_{o}$) to the total magnetic moment (${m}_{T}$) is significant, since it is at least 3 times greater than the experimental uncertainty of ${m}_{T}$. The account of ${m}_{o}$ radically affects the acceptable values of $U$. Specifically, we find no values of $U$ that would simultaneously satisfy the experimental values of the magnetic moment and result in the half-metallicity of Co${}_{2}$FeSi. On the other hand, the ranges of $U$ that we report as acceptable are compatible with spin polarization measurements (ours and the ones found in the literature), which all are within approximately the 40--60 $%$ range. Thus, based on reconciling experimental and computational results, we conclude that (a) spin-orbit coupling cannot be neglected in calculating Co${}_{2}$FeSi magnetic properties, and (b) Co${}_{2}$FeSi Heusler alloy is not half-metallic. We believe that our approach can be applied to other Heusler alloys such as Co${}_{2}$FeAl.