The all-d-metal $\mathrm{Ni}\text{−}(\mathrm{Co})\text{−}\mathrm{Mn}\text{−}\mathrm{Ti}$-based Heusler alloys are found to show a giant magnetocaloric effect near room temperature and are thereby potential materials for solid-state refrigeration. However, the relative large thermal hysteresis and the moderate ferromagnetic magnetization provides limitations for real applications. In the present study, we demonstrate that introducing interstitial B atoms within ${\mathrm{Ni}}_{36.5}{\mathrm{Co}}_{13.5}{\mathrm{Mn}}_{35}{\mathrm{Ti}}_{15}$ alloys can effectively decrease the thermal hysteresis Δ$T_{\mathrm{hys}}$ (down to 4.4 K), and simultaneously improve the saturation magnetization (maximum 40% enhancement) for low concentrations of B doping (up to 0.4 at. %). In comparison to the undoped reference material, the maximum magnetic entropy change (Δ$S_m$) for the ${\mathrm{Ni}}_{36.5}{\mathrm{Co}}_{13.5}{\mathrm{Mn}}_{35}{\mathrm{Ti}}_{15}{\mathrm{B}}_{0.4}$ alloy shows a remarkable improvement from 9.7 to 24.3 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for an applied magnetic field change (Δ${\mu }_{0}H$) of 5 T (30.2 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for Δ${\mu }_{0}H$ = 7 T). Additionally, due to the obtained low thermal hysteresis Δ$T_{\mathrm{hys}}$, the maximum reversible Δ$S_m^{\mathrm{rev}}$ amounts to 18.9 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 283 K for Δ${\mu }_{0}H$ = 5 T (22.0 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 281 K for Δ${\mu }_{0}H$ = 7 T), which is competitive to the traditional $\mathrm{Ni}\text{−}\mathrm{Mn}$-$X$-based Heusler alloys (X = $\mathrm{Ga}$, $\mathrm{In}$, $\mathrm{Sn}$, $\mathrm{Sb}$). The enhancement of the magnetic moments by B doping is also observed in first-principles calculations. These calculations clarify the atomic occupancy of B and the changes in the electronic configuration. Our current study indicates that interstitial doping with a light element (boron) is an effective method to improve the magnetocaloric effect in these all-d-metal $\mathrm{Ni}\text{−}\mathrm{Co}\text{−}\mathrm{Mn}\text{−}\mathrm{Ti}$-based magnetic Heusler compounds.

• Magnetic order
• Magnetocaloric effect
• Heusler alloy
• Magnetic refrigeration