Phase equilibria and magnetic properties of Heusler-type ordered phases in the Co–Mn–Ga ternary system

• Phase diagrams of 800 °C, 900 °C and 1000 °C in the Co–Mn–Ga ternary system were first established.
• STEM–HAADF observations reveled that Mn2CoGa alloy has the L21b-type ordered structure.
• Magnetic moments of CoxMn(76−x)Ga24 alloys followed the generalized Slater–Pauling rule in a wide composition range.

Phase equilibria at 800, 900 and 1000 °C, as well as order–disorder and magnetic transitions of Heusler-type ordered phase were investigated in the Co–Mn–Ga ternary system. The β phase with bcc structure including the Heusler-type ordered phase exists in a wide composition range of the ternary system. It was confirmed by STEM–HAADF observation that the crystal structure of the Heusler-type ordered alloy Mn2CoGa is L21b-type Fm3¯m (i.e., (Mn,Co)2MnGa), but not inverse-Heusler (Hg2CuTi) type F4¯3m. The L21/B2 order–disorder transition temperature shows a maximum at the stoichiometric composition of Co2MnGa and the Curie temperature of the L21 phase decreases with increasing Ga content. Magnetic moments converted from magnetization curves at 4.2 K for CoxMn(76−x)Ga24 alloys well follow the generalized Slater–Pauling rule in the composition of 25 ⩽ x ⩽ 50, and magnetization curves indicate very small high-field magnetic susceptibility.