Competing structural ordering tendencies in Heusler-type alloys with high Curie temperatures: Fe2CoGa1-xZnx studied by first-principles calculations
The influence of Zn substitution on the structural, magnetic, and electronic properties and lattice vibrations of ferromagnetic Fe2CoGa1−xZnx alloys in the conventional X2YZ and inverse (XY)XZ Heusler structures is investigated, by means of ab initio and Monte Carlo calculations, which predict strong ferromagnetic coupling and high Curie temperatures between 770 and 925 K. In the Ga-rich systems the inverse Heusler structure is energetically favored but no indication for a structural instability is found in contrast to Fe-Co-Ga-Zn alloys in the conventional Heusler structure. The origin of the remarkably strong preference of the cubic (c/a=1) inverse phase is believed to originate from the bcc-like environment of the two inequivalent Fe atoms and their stronger hybridization with the Co states compared to the conventional structure. In the quaternary compounds, Fe2CoGa1-xZnx, substitution of Ga by Zn reduces the energetic preference of the inverse structure caused by weakening of the Co-Fe hybridization. Simultaneously, Zn leads to higher magnetic moments and Curie temperatures because of localization effects. In addition, since Zn weakens the miscibility of the alloys, we propose that the composition of Fe2CoGa1-xZnx alloys has to be carefully chosen in order to yield an interesting future ferromagnetic shape-memory alloy system.
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