Formation, magnetic properties and electronic structure of CoFeGa2: An XYZ2 type Heusler alloy analogue

A new Mn-free XYZ₂ type Heusler-like alloy CoFeGa₂ was synthesized, both X-ray diffraction and scanning electron microscope measurements suggest it is a fine single phase. CoFeGa₂ basically crystallizes in B2 structure with Ga occupying A, C sites and Co, Fe randomly occupying B, D sites. Simultaneously, a small amount of Fe/Co-Ga antisite disorder can also exist. CoFeGa₂ has a Curie temperature of 62 K and a saturation magnetization of 0.81 μ_B/f.u. at 5 K. The bifurcation between the zero-field-cooling and field-cooling thermomagnetic curve indicates the co-existence of ferromagnetic and antiferromagnetic state at low temperatures. First-principles calculations were performed to discuss the properties of CoFeGa₂. The preferable occupation of Ga for A, C sites is related to the strong p-d covalent hybridization between Ga and Co/Fe atoms, as can be determined from the electronic structure and phonon spectrum calculations. However, the highly-ordered L2₁ structure with the lowest total energy is nonmagnetic. Comparing with L2₁, the B2 disorder structure is only 0.02 eV higher in energy, and importantly, has a non-zero total moment of 0.36 μ_B/f.u. Fe/Co-Ga antisite disorder can increase the total moment of CoFeGa₂ more rapidly. The small amount of Fe/Co-Ga disorder in the sample can also contribute to the saturation magnetization. All this indicates a disorder induced ferromagnetic character in CoFeGa₂.