Reduced Hysteresis and Enhanced Giant Magnetocaloric Effect in B-Doped all- d -Metal Ni - Co - Mn -

The all-d-metal $\mathrm{Ni}\text{\ensuremath{-}}(\mathrm{Co})\text{\ensuremath{-}}\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Ti}$-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 \ensuremath{\Delta}${T}_{\mathrm{hys}}$ (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 (\ensuremath{\Delta}${S}_{m}$) 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 ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.25em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for an applied magnetic field change (\ensuremath{\Delta}${\ensuremath{\mu}}_{0}H$) of 5 T (30.2 J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.25em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for \ensuremath{\Delta}${\ensuremath{\mu}}_{0}H$ = 7 T). Additionally, due to the obtained low thermal hysteresis \ensuremath{\Delta}${T}_{\mathrm{hys}}$, the maximum reversible \ensuremath{\Delta}${S}_{m}^{\mathrm{rev}}$ amounts to 18.9 J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.25em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 283 K for \ensuremath{\Delta}${\ensuremath{\mu}}_{0}H$ = 5 T (22.0 J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.25em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 281 K for \ensuremath{\Delta}${\ensuremath{\mu}}_{0}H$ = 7 T), which is competitive to the traditional $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Mn}$-$X$-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 $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Ti}$-based magnetic Heusler compounds.