Trends in the Magnetocaloric Response in the Half-Heusler/Heusler System MnNi_1+xSb

The magnetocaloric effect (MCE), a change in temperature upon application of a magnetic field, has been widely studied for applications in magnetic refrigeration and thermomagnetic power generation. A density functional theory (DFT) based proxy for the gravimetric isothermal entropy change ΔS_M, the usual metric of the MCE, suggests a larger magnetocaloric response in the half-Heusler MnNiSb than in the Heusler MnNi₂Sb. Compositions between these endmembers were prepared via assisted microwave reaction. We observe a solid solution via synchrotron X-ray diffraction, and refinements agree with DFT, suggesting excess Ni is added to a symmetrically distinct site. Saturation magnetization is maximized at an intermediate composition, while the Curie temperature (T_C) decreases from 750K to 350K with increasing Ni content. The highly tunable T_C in this range is ideal for low grade waste heat recovery. ΔS_M monotonically decreases with Ni content, while concurrent broadening of the magnetic transition due to longer range magnetic interactions yields a maximum in refrigerant capacity close to the Heusler composition. The increase in |ΔS_M| with decreasing Ni may be extended to other MnNi₂Z Heusler systems that are currently under investigation for use in magnetocaloric refrigeration.