Phase equilibria and stability of B2 and L21 ordered phases in the Co–Fe–Ga Heusler alloy system

The phase equilibria, A2/B2 and B2/L21 order–disorder transitions and ferromagnetic/paramagnetic transition on the Co–Fe side of the Co–Fe–Ga system were examined by electron probe microanalysis (EPMA), differential scanning calorimetric (DSC) measurement and vibrating sample magnetometer (VSM). The equilibrium compositions of interrelations mainly among γ (A1: disordered fcc-Co), α (A2: disordered bcc-Fe), β (B2: ordered bcc-CoGa or FeGa) and β′ (L21 or D03: ordered bcc-Co2FeGa or Fe3Ga) phases were determined using diffusion couples and two-phase bulk specimens. It was confirmed that a bcc single-phase region composed of α, β and β′ at 700–1000° C exists in a wide composition range and that the ferromagnetic region appearing on the Co–Fe side extends to the center of the Gibbs triangle with decreasing temperature. On the other hand, the β′ phase appears along the Fe3Ga–Co3Ga section and the maximal critical temperature of the B2/L21 order–disorder transformation was determined to be 825 °C at a stoichiometric composition expressed as Co2FeGa. The obtained phase diagram was examined in comparison with that of the Co–Fe–Al system.