Abstract
A series of (Ni1−xCox)2MnGe and (Ni0.2Co0.8)2Mn1+yGe1−y bulk X2YZ-type full Heusler alloys were synthesized by arc melting. Our results show that the structure of the (Ni0.2Co0.8)2Mn1+yGe1−y alloys evolved from hexagonal Ni2In-type paramagnetic phase, and the cubic L21 ferromagnetic phase evolved to the cubic B2 ferrimagnetic one gradually with the increase of y from −0.8 to 0.8. In addition, the structure of the (Ni1−xCox)2MnGe alloys evolved from the L21+Ni5Mn4Ge3 impurity phase to the pure L21 ferromagnetic one with the increase of x from 0 to 1.0. Magnetic tests indicated that ferromagnetic coupling occurred between NiX/CoX and MnY ions (i.e., Ni, Co, and Mn ions in X, X, and Y sites, respectively) and antiferromagnetic coupling between NiX/CoX and MnZ ions (i.e., Mn ions in Z sites) for the L21 structure of the alloys. High-resolution transmission-electron-microscopy studies confirmed the Heusler L21 structure of (Ni0.2Co0.8)2MnGe alloys, and revealed that Ni ions repelled each other in the Co-rich (Ni1−xCox)2MnGe alloy, and it brought out the secondary phase in the Ni-rich (Ni1−xCox)2MnGe alloy. Ferromagnetic resonance results showed that the Landé g factor for the L21 (Ni0.2Co0.8)2MnGe alloys was 2.17. However, it was only 2.04 for the (Ni0.2Co0.8)2Mn0.6Ge1.4 alloy due to its hexagonal crystal field. The large damping coefficient of 0.22 for the L21 (Ni0.2Co0.8)2MnGe alloy probably originated from the few antiferromagnetic-coupling Mn atoms.
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