Wide magnetic refrigeration window in ferromagnetic shape memory alloys with intermartensitic transformation

Narrow refrigeration temperature window is a significant drawback for the magnetic refrigeration application of ferromagnetic shape memory alloys (FMSMAs). In this work, the effect of intermartensitic transformation (IMT) on the widening refrigeration temperature window was demonstrated in a Ni-Co-Mn-Sn-Ga alloy. The presence of the thermoelastic IMT behavior was corroborated by DSC, XRD, magnetization (M−T) and isothermal magnetization (M−H) measurements. The IMT appeared during both cooling and heating processes (a two-way IMT) and was essentially different from the one-way IMT (occurs only during cooling or heating process) in other FMSMAs. XRD revealed that the thermoelastic IMT process at heating stage was presumably related to the sequential structural transformations from 6 M → 4O → A. Consequently, the studied Ni-Co-Mn-Sn-Ga alloy exhibited significant inverse magnetocaloric effect (IMCE) due to the large magnetization difference between weak magnetic martensite and strong ferromagnetic austenite. The obtained magnetic entropy change (ΔS_m) showed two successive peaks, amounting to 22.9 and 23.6 J kg⁻¹ K⁻¹ at temperatures of 285.5 and 295.7 K respectively under a magnetic field change of 5 T. The refrigeration capacity (RC, 353.6 J kg⁻¹) and effective RC (RC_eff, 253.8 J kg⁻¹) with a wide magnetic refrigeration window of 17.5 K were achieved in the Ni-Co-Mn-Sn-Ga alloy, which may act as promising working materials for solid-state cooling.