Broadening the working temperature interval in a magneto-structural coupled Ni–Mn–Ga microparticle via introducing a free surface

Abstract

In the field of magnetic refrigeration, magnetic materials that can exhibit both a giant magnetic entropy change (ΔS_m) and a widened working temperature interval (δT_FWHM) are being pursued. To obtain an enhanced magnetic entropy change, this study designed a magneto-structural coupled (i.e., overlapping the martensitic and magnetic transitions) Ni–Mn–Ga bulk ingot; however, it possessed a relatively narrow δT_FWHM of 10 K. To broaden its δT_FWHM, a microparticle with size varying from ∼38.5 to 45 μm was produced by grinding the bulk ingot and by subsequently subjecting it to stress relief annealing (SRA). Consequently, an expanded martensitic transformation width of 14 K and a broadened δT_FWHM of 15 K were achieved in the SRA state. Meanwhile, the average magnetic hysteresis was reduced from 21.8 J kg⁻¹ to 16.8 J kg⁻¹ by preparing the alloy in the microparticle configuration. Therefore, an enhanced net refrigeration capacity value of 104.0 J kg⁻¹ under 5 T was achieved in the SRA microparticle compared with that of 57.7 J kg⁻¹ in the bulk ingot. The strategy of increasing the specific surface area and regulating the internal stress can be used to expand a refrigerant's δT_FWHM, thereby helping achieve a higher magnetic refrigeration capacity in Ni–Mn–X alloys.