Minority-spin conducting states in Fe substituted pyrite CoS₂

There has been a longstanding debate whether the pyrite CoS₂or its alloys with FeS₂are half metallic. We argue using first principles calculations that there is a finite occupation of minority-spin states at the Fermi level throughout the series Co1-xFe_xS₂. Although the exchange-correlation functional influences the specifics of the electronic structure, we observe a similar trend with increasing Fe concentration in both local density approximation and generalized gradient approximation calculations. Specifically, even as band filling is decreased through Fe substitution, the lowest-lying conduction band in the minority-spin channel broadens such that these states keep getting lowered relative to the Fermi level, which is contrary to the expectations from a rigid band picture. Furthermore, the exchange splitting decreases as more Co atoms are replaced by Fe, and this again brings the minority-spin states closer to the Fermi level. These two mechanisms, in conjunction with the experimental observation that minority-spin bands cross the Fermi level in stoichiometric CoS₂, indicate that minority-spin charge carriers will always be present in Co1-xFe_xS₂.