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 (T-C(M)) 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 T-2 followed by an AFM to FM-like transition around T-1, 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 Delta S-M 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 d(1). 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. (C) 2016 Elsevier Ltd. All rights reserved.