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 0.89 Fe 0.11 NiGe 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 μ 0 H up to 7 T, which is ~ 34.0% higher than that of − 51.8 J kg −1 K −1 in the host alloy Mn 0.89 Fe 0.11 NiGe 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 0.89 Fe 0.11 NiGe 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. • B causes a nonlinear change of the magneto-structural transition temperature of Mn 0.89 Fe 0.11 NiGe. • Adding 0.2 at% B enhances magnetic entropy change by~ 34.0%. • The minor boron atoms will increase the cell volume and stabilize the martensitic phase. • Nano-precipitates at high B contents hinder the phase transition and decrease the transition temperature.