Materials research for new magnetic refrigeration using metamagnetic transition

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Because of high thermodynamic efficiency, magnetic refligeration has been an attractive technology for the low temperature generation. The search for new materials with a large magnetocaloric effects (MCE) in the temperature range of interest is strongly desired for further improvement of magnetic refrigeration. Some of the intermetallic compounds containing rare earth elements R undergo a first-order-magnetic transition from a ferrimagnetic to paramagnetic state at low temperatures. Above TィイD2cィエD2, these materials exhibit a metamagnetic transition in appropriate magnetic fields, which suggests a large MCE. In this research, we have studied the MCE of ErCoィイD22ィエD2 and DyMnィイD22ィエD2GeィイD22ィエD2. The nature of magnetic transition RcoィイD22ィエD2 and RMnィイD22ィエD2GeィイD22ィエD2 was also studied and discussed.In EiCoィイD22ィエD2, we have found the large MCE in 32-40 K. The maximum height of the isothermal entropy change, ΔS and the adiabatic temperature change, ΔTィイD2adィエD2 in a field change of 0 to 8 T are 12 J/K mol and 13 K,respectively, which are larger than those of other R compounds in the same temperature range. The substitution of 10%Ni for Co lowers Tc from 32 K to 13 K without reduction of the MCE. These results strongly suggest Er(CoィイD21-xィエD2NiィイD2xィエD2)ィイD22ィエD2 has a high potential for working materials of magnetic refrigeration. From the studies on DyMnィイD22ィエD2GeィイD22ィエD2, we have revealed the nature of the MCE of first-order magnetic transition systems. Namely, the maximum values of the MCE are not sensitive to magnetic field, while the temperature range with the large MCE is increased linearly with increasing magnetic field. In other words, the MCE of the first-order magnetic transition systems is large in relatively weak magnetic fields, although it extends over a narrow temperature range. These characteristics are advantageous to the active magnetic regenerative materials.