Effect of Mo Addition on Structure and Magnetocaloric Effect in c-FeNi Nanocrystals

Nanocrystalline powders of (Fe70Ni30)100 xMox (x = 1 to 4) were produced by high-energy (SPEX) mechanical alloying. Increasing the Mo content was found to stabilize the face-centered cubic phase in mechanically alloyed nanopowders. To obtain a single c-phase, a powdered sample was solution annealed in the c-phase field and water quenched. The Curie temperature, TC, of the alloys was lowered with Mo addition, without decreasing the refrigeration capacity (RC), due to the additional temperature broadening of the magnetic entropy change. Based on previous study on the role of disorder, the additional temperature broadening was attributed to increased positional disorder and changes in the distribution of ferromagnetic exchange bonds introduced by Mo addition into the c-FeNi system. (Fe70Ni30)97Mo3 and (Fe70Ni30)96Mo4 alloys have RCFWHM values of 440 J/kg and 432 J/kg at 5 T, comparable to other prominent magnetic refrigerants operating near room temperature. The economic viability of these alloys, along with their competitive magnetocaloric properties and potential for scalable production, make them good candidate magnetic refrigerants without critical rare-earth materials.