Magnetic refrigeration near room temperature with Fe2P-based compounds

The alloy MnFeP0.45As0.55 has been identified as a promising material to be used as magnetic refrigerant for room-temperature applications. This alloy, which is derived from the well-known iron pnictide Fe2P, exhibits a first-order temperature- and field-induced magnetic phase-transition. The Curie temperature can be varied by substitutions into the pnictide sublattice. We studied the effect of Ge substitution in this material. Most strikingly, we find a strong increase of the magnetic ordering temperature up to 570 K and a concomitant increase of the ordered magnetic moment up to 4.25 mu(B)/f.u. for the alloy MnFeP0.5Ge0.5. On substitution with the large Ge ions, the expected lattice expansion is very anisotropic. We observe an increase and decrease of the a and c parameter of the hexagonal lattice, respectively. Compared with the (P,As) series, for a similar volume change we find a much stronger increase of T-C. This confirms the importance of competing exchange interactions in this interesting alloy system. (C) 2004 Elsevier B.V. All rights reserved.