Effects of d-band hole doping on electronic structure, magnetism, and superconductivity in Fe- and Co-doped MgCNi₃

Electronic structures and magnetism of the nonoxide perovskite MgCNi_3-xT_x compounds (x=0, 1, 2, and 3; T=Co and Fe), based on recently found superconducting MgCNi₃, have been investigated with first-principles all-electron full-potential linearized augmented plane-wave calculations within the local-(spin) density approximation. From the results of total-energy calculations, it is expected that (i) the suppression of superconductivity occurs faster for the Fe-doped case than for the Co-doped case with increase of x, and (ii) ferromagnetic transitions occur when x≥2 for the Co-doped cases, while the Fe-doped cases become ferromagnetic before x=2. From the calculated density of states, it is found that the superconductor MgCNi₃ becomes paramagnetic and then ferromagnetic, as we increase the number of minority spin d-band holes via Co and Fe doping at Ni sites. A hypothetical system, MgC(FeCoNi), has also been calculated and found to be ferromagnetic with magnetic moments of 0.97, 0.24, and 0.03 μ_B for Fe, Co, and Ni, respectively, which are roughly proportional to the number of d-band holes of minority spin. These features are well explained by d-band spin splitting, as in conventional metallic ferromagnets.