  Giant magnetocaloric effect is widely achieved in hexagonal MnMX-based (M =
Co or Ni, X = Si or Ge) ferromagnets at their first-order magnetostructural
transition. However, the thermal hysteresis and the low sensitivity of the
magnetostructural transition to the magnetic field inevitably lead to a
sizeable irreversibility of the low-field magnetocaloric effect. In this work,
we show an alternative way to realize a reversible low-field magnetocaloric
effect in MnMX-based alloys by taking advantage of the second-order phase
transition. With introducing Cu into Co in MnCoGe 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 negligible thermal hysteresis and a large
magnetization change can be observed, enabling a large 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 get 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 a new understanding of the MnCoGe alloy and indicates a feasible route
to improve the reversibility of the low-field magnetocaloric effect in the MnMX
system.
