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.

• Received 27 November 2012

DOI:https://doi.org/10.1103/PhysRevB.87.220402