Tailoring properties of Heusler alloys by elemental substitution and electron counting: (Co${}_{2-\hat{I}\pm }$Mn${}_{\hat{I}}\pm$)FeGe, Co${}_2$(Fe${}_{1-{\hat{I}}^2}$Mn${}_{\hat{I}}^2$)Ge, and (Co${}_{2-\hat{I}\pm }$Fe${}_{\hat{I}}\pm$)MnGe

Abstract

Rational material design by elemental substitution is useful in tailoring materials to have desirable properties. Here we consider three non-equivalent substitutional series based on Co${}_2$FeGe, viz; (Co${}_{2-\hat{I}\pm }$Mn${}_{\hat{I}}\pm$)FeGe, Co${}_2$(Fe${}_{1-{\hat{I}}^2}$Mn${}_{\hat{I}}^2$)Ge, (Co${}_{2-\hat{I}\pm }$Fe${}_{\hat{I}}\pm$)MnGe ($0\le \hat{I}\pm \le 2,0\le {\hat{I}}^2\le 1$), and study how material properties evolve with the interchange of Mn, Fe, and Co in Co${}_2$FeGe. In all three schemes, single-phase compounds can be obtained over a wide range of compositions: $0.125<\hat{I}\pm <1.375$ for (Co${}_{2-\hat{I}\pm }$Mn${}_{\hat{I}}\pm$)FeGe, $0\le {\hat{I}}^2\le 1$ for Co${}_2$(Fe${}_{1-{\hat{I}}^2}$Mn${}_{\hat{I}}^2$)Ge, and $0<\hat{I}\pm <1.50$ for (Co${}_{2-\hat{I}\pm }$Fe${}_{\hat{I}}\pm$)MnGe. All the single-phase compounds crystallise in fcc structure with chemical ordering consistent with the ``4-2'' rule of Butler et al. The compounds are soft ferromagnets with low temperature saturation magnetisation agreeing with the Slater-Pauling rule. Very high Curie temperatures are measured, with values up to 1000 K for lower Mn concentrations. First principle calculations indicate, in the most stable atomic configuration, Mn prefers sharing sublattice with Ge, also consistent with the 4-2 rule. The calculations further predict half-metallic behaviour for (Co${}_{1.625}$Mn${}_{0.375}$)FeGe, while finding other compositions to be nearly half-metallic. Upon comparing the results of the three series, it is found that single-phase alloys occur for a specific range of valence electrons per unit cell ($\sim 28.5-29.75$), and that even for multi-phase samples the structural, magnetic, and electronic properties depend primarily on the number of valence electrons and not on the specific substitution scheme employed.

34 pages, 32 figures