Direct measurement of the magnetocaloric effect (ΔTad) of Mn5−x (Fe,Co)xGe3

Magnetic field-induced adiabatic temperature change, ΔTad, measures the temperature gradient that enables a magnetic refrigerant to transfer heat from reservoirs. The ΔTad of Mn5Ge3 was evaluated by directly recording the temperature of a Mn5Ge3 sample while subjecting it to an external field. The peak ΔTad of 0.89 ± 0.006 at 0.8 T for Mn5Ge3 at its Curie temperature was consistent with the value previously obtained from specific heat capacity measurements. In addition, the peak values of ΔTad for Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 were measured to be 0.77 ± 0.009 and 0.93 ± 0.011, respectively, at 0.8 T near their respective Curie temperatures. ΔTad measurements under cyclic application of a 0.8 T field showed that the alloys, whose magnetocaloric effect stems from the second-order magnetic phase transition, exhibited no hysteresis in ΔTad and maintained their peak ΔTad within 1.3% under cyclic field. The ΔTad of sandwiched Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 plates was also measured to demonstrate that a combination of the alloys can be used to form a graded regenerator for the active magnetic regenerator cycle.