Design of Reversible Low-Field Magnetocaloric Effect at Room Temperature in Hexagonal MnMX Ferromagnets

The giant magnetocaloric effect is widely achieved in hexagonal $\mathrm{Mn}MX$-based (M = $\mathrm{Co}$ or $\mathrm{Ni}$, X = $\mathrm{Si}$ or $\mathrm{Ge}$) ferromagnets at their first-order magnetostructural transition. However, the thermal hysteresis and low sensitivity of the magnetostructural transition to the magnetic field inevitably lead to a sizeable irreversibility of the low-field magnetocaloric effect. Here, we show an alternative way to realize a reversible low-field magnetocaloric effect in $\mathrm{Mn}MX$-based alloys by taking advantage of the second-order phase transition. With introducing $\mathrm{Cu}$ into $\mathrm{Co}$ in stoichiometric $\mathrm{Mn}\mathrm{Co}\mathrm{Ge}$ alloy, the martensitic transition is stabilized at high temperature, while the Curie temperature of the orthorhombic phase is reduced to room temperature. As a result, a second-order magnetic transition with a negligible thermal hysteresis and a large magnetization change can be observed, enabling a reversible magnetocaloric effect. By both calorimetric and direct measurements, a reversible adiabatic temperature change of about 1 K is obtained under a field change of 0--1 T at 304 K, which is larger than that obtained in a first-order magnetostructural transition. To gain a better insight into the origin of these experimental results, first-principles calculations are carried out to characterize the chemical bonds and the magnetic exchange interaction. Our work provides an understanding of the $\mathrm{Mn}\mathrm{Co}\mathrm{Ge}$ alloy and indicates a feasible route to improve the reversibility of the low-field magnetocaloric effect in the $\mathrm{Mn}MX$ system.