We study the electronic structures and magnetic properties of ${\mathrm{Mn}}_2\mathrm{Co}Z$ $(Z=\mathrm{Al},\mathrm{Ga},\mathrm{In},\mathrm{Si},\mathrm{Ge},\mathrm{Sn},\mathrm{Sb})$ compounds with ${\mathrm{Hg}}_2\mathrm{Cu}\mathrm{Ti}$-type structure using first-principles full-potential linearized-augmented plane-wave calculations. It is found that the compounds with $Z=\mathrm{Al}$, Si, Ge, Sn, and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the ${\mathrm{Mn}}_2\mathrm{Co}Z$ $(Z=\mathrm{Al},\mathrm{Ga},\mathrm{In},\mathrm{Ge},\mathrm{Sn},\mathrm{Sb})$ compounds. Using the x-ray diffraction method and Rietveld refinement, we confirm that these compounds form ${\mathrm{Hg}}_2\mathrm{Cu}\mathrm{Ti}$-type structure instead of the conventional $L{2}_1$ structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only the $d\text{−}d$ band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. It is the hybridization between the $\mathrm{Mn}\left(C\right)$ and Co atom that dominates the magnitude of magnetic moment of the Co atom and the sign of the $\mathrm{Mn}\left(B\right)\text{−}\mathrm{Co}$ exchange interaction. The ${\mathrm{Mn}}_2\mathrm{Co}Z$ alloys follow the Slater-Pauling rule $M_H=N_V-24$ with varying $Z$ atom. It was further elucidated that the molecular magnetic moment $M_H$ increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of $\mathrm{Mn}\left(C\right)$, while the magnetic moments of $\mathrm{Mn}\left(B\right)$ and Co are unaffected.

• Received 4 April 2007

DOI:https://doi.org/10.1103/PhysRevB.77.014424