Magnetocaloric materials can be useful in magnetic refrigeration applications, but to be practical the magneto-refrigerant needs to have a very large magnetocaloric effect (MCE) near room temperature for modest applied fields (<2 Tesla) with small hysteresis and magnetostriction, and should have a complete magnetic transition, be inexpensive, and environmentally friendly. One system that may fulfill these requirements is MnxFe2-xP1-yGey, where a combined first-order structural and magnetic transition occurs between the high temperature paramagnetic and low temperature ferromagnetic phase. We have used neutron diffraction, differential scanning calorimetry, and magnetization measurements to study the effects of Mn and Ge location in the structure on the ordered magnetic moment, MCE, and hysteresis for a series of compositions of the system near optimal doping. The diffraction results indicate that the Mn ions located on the 3f site enhance the desirable properties, while those located on the 3g sites are detrimental. The entropy changes measured directly by calorimetry can exceed 40 J/kg-K. The phase fraction that transforms, hysteresis of the transition, and entropy change can be controlled by both the compositional homogeneity and the particle size, and an annealing procedure has been developed that substantially improves the performance of all three properties of the material. On the basis of these results we have identified a pathway to optimize the MCE properties of this system for magnetic refrigeration applications.
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