Crystalline and magnetic structures and ferromagnetic resonance study of Ni-Co-Mn-Ge Heusler alloy system

A series of (Ni1-xCox)(2)MnGe and (Ni0.2Co0.8)(2)Mn1+yGe1-y bulk X(2)YZ-type full Heusler alloys were synthesized by arc melting. Our results show that the structure of the (Ni0.2Co0.8)2Mn(1+y)Ge(1-y) alloys evolved from hexagonal Ni2(I)n-type paramagnetic phase, and the cubic L2(1) 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)(2)MnGe alloys evolved from the L2(1)+Ni5Mn4Ge3 impurity phase to the pure L2(1) ferromagnetic one with the increase of x from 0 to 1.0. Magnetic tests indicated that ferromagnetic coupling occurred between Ni-X/Co-X and Mn-Y ions (i.e., Ni, Co, and Mn ions in X, X, and Y sites, respectively) and antiferromagnetic coupling between Ni-X/Co-X and Mn-Z ions (i.e., Mn ions in Z sites) for the L2(1) structure of the alloys. High-resolution transmission-electron-microscopy studies confirmed the Heusler L2(1) structure of (Ni0.2Co0.8)(2)MnGe alloys, and revealed that Ni ions repelled each other in the Co-rich (Ni1-xCox)(2)MnGe alloy, and it brought out the secondary phase in the Ni-rich (Ni1-xCox)(2)MnGe alloy. Ferromagnetic resonance results showed that the Lande g factor for the L2(1) (Ni0.2Co0.8)(2)MnGe alloys was 2.17. However, it was only 2.04 for the (Ni0.2Co0.8)(2)Mn0.6Ge1.4 alloy due to its hexagonal crystal field. The large damping coefficient of 0.22 for the L2(1) (Ni0.2Co0.8)(2)MnGe alloy probably originated from the few antiferromagnetic-coupling Mn atoms. (C) 2017 Elsevier B.V. All rights reserved.