Magnetocaloric effect of Ni-Fe-Mn-Sn microwires prepared by melt-extraction technique

Small-sized materials with large surface to volume ratio favor heat transfer during magnetic refrigeration cycling and thus may help enhancing the refrigeration efficiency. Here, high Fe content Ni_44.9Fe_4.3Mn_38.3Sn_12.5 polycrystalline microwires were prepared by a melt-extraction technique. The as-extracted microwires were annealed at 1173 K for 60 min, leading to significant grain growth and formation of a secondary Fe-rich γ phase. The annealed microwire exhibits larger magnetization difference (ΔM) between the austenite and martensite phases and smaller thermal hysteresis compared to the as-extracted microwire. The annealed microwire possesses a magnetic transition to austenite at 299 K, followed by a martensitic transformation (MT) from a ferromagnetic austenite to a weak-magnetic martensite at 208 K upon cooling. Under a magnetic field of 50 kOe, the annealed microwires show a maximum magnetic entropy change ΔS_m of 6.9 J/kg·K and an effective refrigeration capacity RC_eff of 78.0 J/kg over a broad working temperature span ΔT_FWHM of 20 K around the MT. In addition, magnetic transition of the austenite gives rise to ΔS_m − 3.7 J/kg·K and RC_mag 232.5 J/kg with ΔT_FWHM of 85 K under 50 kOe.