Origin and tuning of the magnetocaloric effect in the magnetic refrigerant Mn_1.1Fe_0.9(P_0.8Ge_0.2)

Neutron-diffraction and magnetization measurements have been carried out on a series of samples of the magnetorefrigerant Mn_1+yFe _1-yP_1-xGe_x. The data reveal that the ferromagnetic and paramagnetic phases correspond to two very distinct crystal structures, with the magnetic-entropy change as a function of magnetic field or temperature being directly controlled by the phase fraction of this first-order transition. By tuning the physical properties of this system we have achieved a magnetic-entropy change [magnetocaloric effect (MCE)] for the composition Mn_1.1Fe_0.9(P_0.8Ge_0.20) that has a similar shape for both increasing and decreasing field, with the maximum MCE exceeding 74 J/kgK -substantially higher than the previous record. The diffraction results also reveal that there is a substantial variation in the Ge content in the samples which causes a distribution of transition temperatures that reduces the MCE. It therefore should be possible to improve the MCE to exceed 100 J/kgK under optimal conditions.