  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.
