Nb-doping in (Mn,Fe)2(P,Ge) giant magnetocaloric materials

The influence of Nb substitutions on the structure, magnetoelastic transition, and magnetocaloric properties is investigated in MnFe1−xNbxP0.76Ge0.24 (x = 0, 0.02, 0.04, 0.6) compounds. Powder XRD show that the compounds crystallize in the hexagonal Fe2P-type crystal structure (space group P6̄2 m) and contain minor amount of MnO impurity phase. At a low applied magnetic field (<1.6 T), the isothermal entropy change of the Nb-substituted MnFe0.98Nb0.02P0.76Ge0.24 sample is higher than that of the parent or more substituted samples, reaching a maximum of 14.0 J kg−1 K−1. Nb substitutions are found to efficiently reduce the thermal hysteresis in Ge-based Fe2P magnetocaloric materials; for instance, only 6 at. % of Nb for Fe substitution reduces the thermal hysteresis from 7.6 to 2.6 K while preserving giant magnetocaloric effect. Direct ΔTad measurements confirm the cyclic character of the giant magnetocaloric effect for x = 0.02.