Large inverse magnetocaloric effect induced by antiferromagnetically magnetostructural transition in Mn^sub 0.8^Fe^sub 0.2^Ni^sub 1-x^Cu^sub x^Ge compounds

The effects of Cu substitution for Ni atoms on the magnetic and magnetocaloric properties have been investigated in Mn0.8Fe0.2Ni1-xCuxGe (x = 0.05–0.3) compounds. The substitution of Cu leads to the larger separation of Mn atoms, which stabilizes the hexagonal phase and thus reduces the structural transition temperature Tstr. Moreover, the larger separation of nearest-neighbor Mn atoms weakens the ferromagnetic (FM) coupling in TiNiSi-type orthorhombic phase and results in the spiral antiferromagnetic (AFM) and more antiparallel AFM state with increasing Cu content. Similarly, the Ni2In-type hexagonal phase also changes from FM state to weak-magnetic spin-glass-like state with the introduction of Cu. The x = 0.05 compound shows a conventional magnetocaloric effect (MCE) due to the magnetostructural transformation from PM hexagonal phase to FM orthorhombic phase. On the contrary, the x = 0.1 compound undergoes the antiferromagnetically magnetostructural transition from FM hexagonal phase to AFM orthorhombic phase, which leads to a high inverse MCE. This large inverse MCE could be utilized in novel magnetic refrigerator to improve the refrigeration efficiency and applied for magnetic refrigeration of natural gas liquefaction.