Magnetic entropy changes with zero hysteresis loss in the vicinity of the first-order phase transition in Ni$_{\mathrm{2}}$Mn$_{\mathrm{0.55}}$Co$_{x}$Cr$_{\mathrm{0.45-}}_{x}$Ga Heusler alloys

The observation of mitigated drawbacks in a material exhibiting a first-order magnetostructural phase transition near room temperature as well as moderate magnetocaloric effects are pertinent towards the realization of energy-efficient and environmentally-friendly solid state refrigeration technologies. The main drawbacks of a first-order material are hysteresis losses, which dramatically reduce cooling efficiency. Here, we present an experimental study on a set of NiMn$_{\mathrm{0.55}}$Co$_{x}$Cr$_{\mathrm{0.45-}}_{x}$Ga ($x=$ 0, 0.1) Heusler alloys. X-ray diffraction, dc magnetization, and differential scanning calorimetry measurements have been performed on these materials. At room temperature, the alloys were found to crystallize in the tetragonal martensite structure (\textit{P6}$_{3}$\textit{/mcm}). Magnetization measurements showed that both samples exhibited a single first-order magnetostructural phase transition, and that the replacement of Cr with Co shifted the transition temperature from 250 K ($x=$ 0) to 290 K ($x=$ 0.1), while narrowing the thermal hysteresis from 4 K to 1.7 K. Magnetization measurements also revealed that the Co-substituted alloy exhibited near-zero magnetic hysteresis, including near $T_{\mathrm{C}}$. Calorimetric data showed that the first-order magnetostructural phase transition of the Co-substituted alloy had near-perfect reproducibility over many thermal cycles. For a magnetic field change of 0 -- 2 T, the Co-substituted sample exhibited a magnetic entropy change and refrigerant capacity of 3.14 J kg$^{\mathrm{-1}}$ K$^{\mathrm{-1}}$ and 54.54 J kg$^{\mathrm{-1}}$, respectively.