Giant reversibility of magnetoresponsive properties in all- d -metal Ni-Co-Mn(Cu)-Ti Heusler alloys: Role of phase-fraction-assisted magnetostructural transition

Recently emerged all-$d$-metal Ni(Co)-Mn-Ti Heusler systems often exhibit multifunctional properties which are accompanied by magnetic-field-induced inverse martensitic transformation (IMT). Hence, the study of the transitional dynamics of IMT on the corresponding intriguing physical properties is of great importance. In this paper, we report the critical significance of field-induced austenite phase fraction ($\mathrm{\ensuremath{\Delta}}{f}_{\text{IA}}$) in achieving giant reversible magnetoresponsive properties of the optimizing Cu doped in ${\mathrm{Ni}}_{35}{\mathrm{Co}}_{15}{\mathrm{Mn}}_{34.5\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Ti}}_{15.5}$ ($x$ = 1, 2, and 3) all-$d$-metal Heusler systems. We show that the Cu substitution in the Mn site pulls the magnetic transition towards structural transitions and therefore the distance between them decreases. The evolution of $\mathrm{\ensuremath{\Delta}}{f}_{\text{IA}}$ has been investigated using the Clausius Clapeyron equation and the Landauer equation under field cycling by variation of magnetization and resistivity as a function of temperature and field for $x$ = 2 sample. A giant reversible magnetic entropy change ($\mathrm{\ensuremath{\Delta}}{\mathrm{S}}_{M}$) of $\ensuremath{\sim}$ 22.8 J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 257 K as well as a giant effective refrigerant capacity (${\mathrm{RC}}_{\text{eff}}$) of $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}610\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{4pt}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}$ in a field change of 7 T ($\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}9\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{4pt}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 269 K and ${\mathrm{RC}}_{\text{eff}}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}144\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{4pt}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}$ under 2 T) is obtained, correlating with the good geometrical compatibility between two phases (${\ensuremath{\lambda}}_{2}$ closer to 1) and the high sensitivity of phase transition temperature to the magnetic field ($\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}4.3\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{T}$). Moreover, a remarkably giant reversible magnetoresistance (MR) of $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}40%$ over 42% of the total MR in a field change of 7 T is observed, when $\mathrm{\ensuremath{\Delta}}{f}_{\text{IA}}$ is fully induced. The observed magnitude of reversible magnetocaloric effect and MR is the highest reported value so far in the all-$d$-metal Heusler family. Our findings corroborate the generality of using the $\mathrm{\ensuremath{\Delta}}{f}_{\text{IA}}$ to enhance reversible magnetoresponsive properties in conventional all-$d$-metal Heusler system. These giant magnetoresponsive characteristics over a wide temperature window may therefore lead to the all-$d$-metal Heusler system as a suitable state-of-the-art caloric material for solid-state-based technological applications.