Magnetostructural phase transitions and magnetocaloric effect in Mn-Fe-Ni-Ge-B alloys

The search for room-temperature high-performance magnetocaloric materials is highly stimulated towards magnetic refrigeration techniques. Here we report the boron doping effect on the magnetostructural transition and magnetocaloric effect of the Mn_0.89Fe_0.11NiGe alloy. Our studies reveal that the addition of only 0.2 at% boron induces a large magnetic entropy change (∆S_m) of − 69.4 J kg⁻¹ K⁻¹ at 298.5 K under μ₀H up to 7 T, which is ~ 34.0% higher than that of − 51.8 J kg⁻¹K⁻¹ in the host alloy Mn_0.89Fe_0.11NiGe obtained at a much lower temperature of 267.5 K. With the increase of boron content, the phase transition temperature M_s showed a nonlinear change that first increased and then decreased. The investigation of the microstructure by transmission electron microscopy suggests that the minor B atoms will get into the interstitial site of Mn_0.89Fe_0.11NiGe alloy, increase the cell volume, and stabilize the orthorhombic structure of the alloy. This could be the main reason for the increase in magneto-structural transition temperatures. Otherwise, the excessive B atoms will introduce precipitates, and cause the reduction of the magnetostructural transition temperature. It suggests that proper doping point defects are effective to tailor magneto-structural transition toward achieving the magnetocaloric effect.