Multiple magnetic transitions in MnCo1−xCuxGe driven by changes in atom separation and exchange interaction

The phase relationships, magnetic transitions, and magnetocaloric effect (MCE) of MnCo1−xCuxGe (x=0–0.5) alloys have been investigated. The substitution of Cu for Co reduces the structural transition temperature between Ni2In-type hexagonal phase and TiNiSi-type orthorhombic phase. The Curie temperature of orthorhombic martensite (TCM) also decreases with increasing x from 0 to 0.2, which is likely due to the weakening of Co-Mn and/or Co-Co interactions by non-magnetic element Cu substitution for Co atoms. In addition, two successive first-order magnetic transitions, a ferromagnetic (FM) to antiferromagnetic (AFM)-like transition around T2 followed by an AFM to FM-like transition around T1, are observed for 0.17≤x≤0.27, which have not been reported in MnCoGe-based alloys so far. An inverse MCE is observed under low field changes and then a universal curve of ΔSM is successfully constructed, proving the applicability of universal curve for AFM materials with inverse MCE. Finally, the variation of magnetic states is explained in terms of the expansion of nearest-neighbor Mn-Mn distance d1. These new findings and related discussions are very helpful for future tailoring of magnetostructural coupling and metamagnetic transition in MM'X (M, M′=transition metals, X=carbon or boron group elements) alloys. [Display omitted] •For the first time, multiple first-order magnetic transitions are observed below TCMin MnCoGe-based alloys.•The variation of magnetic states is explained in terms of the expansion of nearest-neighbor Mn-Mn distance d1.•The universal curve of ΔSM peak is constructed, suggesting its applicability in AFM materials with inverse MCE.