Tuning the magnetocaloric response in half-Heusler/Heusler MnNi1+xSb solid solutions

Abstract

Materials with a large magnetocaloric response are associated with a temperature change upon the application of a magnetic field and are of interest for applications in magnetic refrigeration and thermomagnetic power generation. The usual metric of this response is the gravimetric isothermal entropy change ΔSM. The use of a simple proxy for the ΔSM that is based on density functional theory (DFT) calculations of the magnetic electronic structure suggests that half-Heusler MnNiSb should be a better magnetocaloric than the corresponding Heusler compound MnNi2Sb. Guided by this observation, we present a study of MnNi1+xSb (x=0, 0.25, 0.5, 0.75, and 1.0) to evaluate relevant structural and magnetic properties. DFT stability calculations suggest that the addition of Ni takes place at a symmetrically distinct Ni site in the half-Heusler structure and support the observation using synchrotron x-ray diffraction of a homogeneous solid solution between the half-Heusler and Heusler end members. There is a maximum in the saturation magnetization at x=0.5 and the Curie temperature systematically decreases with increasing x. ΔSM for a maximum magnetic field change of ΔH=5T monotonically decreases in magnitude from -2.93Jkg-1K-1 in the half-Heusler to -1.35Jkg-1K-1 in the Heusler compound. The concurrent broadening of the magnetic transition results in a maximum in the refrigerant capacity at x=0.75. The Curie temperature of this system is highly tunable between 350 K and 750 K, making it ideal for low grade waste heat recovery via thermomagnetic power generation. The increase in ΔSM with decreasing x may be extendable to other MnNi2Z Heusler systems that are currently under investigation for use in magnetocaloric refrigeration applications.