Magnetic Property And Magnetocaloric Effect Of (Mn, Fe)₂(P, Si, Ge) Compounds

Transition-metal based Fe)2P-type MnFeP_1-xAs_x compounds exhibit giant magnetocaloric effect (GMCE). In order to explore whether the compounds fit for room-temperature magnetic refrigeration, we have studied the crystal structure,phase formation,Curie temperature,magnetic phase transition and magnetocaloric effect of the MnFeP_0.75-xGe_xSi_0.25,MnFeP0.5Ge0.5-xSix compound system, which replaced As by Si and Ge. In addition, we have calculated the MCE of MnFeP_0.45As_0.55 by using the Bean-Rodbell model. The main results and conclusions of this thesis are following:The relationship between the grain sizes and milling time, the size of grain, and the lattice distortion of MnFeP_0.6Ge_0.15Si_0.25 (an annealed sample) are studied by XRD method. It indicated that the grain size decreases indicated by the increase of the full width at half maximum with increasing milling time. The results show that the all the compounds crystallize in the Fe2P-type hexagonal structure. The magnetic studies show that the Curie temperature increases with increasing Ge content and the compounds exhibit the GMCE, especially MnFeP_0.75-xGexSi_0.25 compounds present colossal MCE, which originates from a first-order transition occurring near the Curie temperature. There is a large thermal hysteresis, which is the characteristics of a first-order transition, in all of these compounds. In order to further study the thermal hysteresis, we have studied the non-stoichiometric (Mn,Fe)1.1P0.5Si0.4Ge0.1 compounds. The results show that all of them keep the Fe2P-type hexagonal structure and the superfluous Mn and Fe can reduce the Curie temperature, and the superfluous Mn can decrease the thermal hysteresis, but the superfluous Fe leads to the increase of the thermal hysteresis. However, the entropy change decreased slightly. In addition, the isothermal magnetization curves and the entropy change were calculated based on Bean-Rodbell model. The theoretical calculated results show a good agreement with the experimental data. We concluded that the different field change and temperature change have less effect on the fitting results. Therefore, this work offered a simple way on the theoretical calculations of the magnetocaloric effect based on the Bean-Rodbell model.