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 MnFe0.95P0.575Si0.36B0.065 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 (ΔS). Differential scanning calorimetric measurements reveal that the decrease in ΔS 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.