Influence of the particle size on a MnFe(P,Si,B) compound with giant magnetocaloric effect

How the microstructure affects first-order magnetic transitions (FOMT) in materials with giant magnetocaloric effect remains poorly understood. Here, we study the FOMT and giant magnetocaloric effect occurring near room temperature in MnFe 0.95 P 0.575 Si 0.36 B 0.065 particles with sizes ranging from 300 μm down to less than 15 μm. While this materials system shows a volume preserving FOMT, large anisotropic lattice discontinuities make it particularly sensitive to particle size. Grinding and sieving may lead up to 80% difference on the isothermal entropy change (Δ S). Differential scanning calorimetric measurements reveal that the decrease in Δ S does not only originate from the broadening of the transition but also involves a sudden drop in latent heat when particles are reduced from 54 μm to 31 μm, a range corresponding to about the average grain size of the bulk (26 μm). Thermal hysteresis is the largest in large particles and decreases when reducing the particle size.