Effect of Co-Doping on the Microstructure, Martensitic Transformation Behavior, and Magnetocaloric Effect of Ni-Mn-Sb-Si Ferromagnetic Shape Memory Alloys

We have studied the effect of Co-doping on the microstructure, martensitic transformation, and magnetocaloric effect (MCE) of a Ni₅₀Mn₃₈Sb₉Si₃ alloy. Here, we further study the effect of Co-doping on the microstructure, martensitic transformation (MT), and MCE of Ni₅₀Mn₃₈Sb₉Si₃ alloy. The results show that phases α, β, and γ coexist in Ni_50−xCo_xMn₃₈Sb₉Si₃ (x = 6, 12) alloys. The MT temperatures remain almost unchanged with the addition of Co element. Above the room temperature (RT), phase α exhibits MT under external magnetic fields up to 50 kOe, and possesses a great enhancement of ΔM from 0.008 emu/g in Ni₅₀Mn₃₈Sb₉Si₃ alloy to 21.4 emu/g in Ni₃₈Co₁₂Mn₃₈Sb₉Si₃ alloy as well as an improvement of ∂M/∂T (obtained from M-T curves) that benefit ΔS_M. The MT temperature intervals of M-T curves are enlarged from 3 K in the un-doped alloy to 11 K in the water-quenched Ni₃₈Co₁₂Mn₃₈Sb₉Si₃ alloy attributed to the reduced coupling between magnetic and structural transition. This contributes to a magnetic entropy change (ΔS_M) of 4.62 J/kg K and net refrigeration capacity (RC_net) of 18.5 J/kg under a magnetic field of 50 kOe, which is superior to that of the ferromagnetic compounds containing secondary precipitates.