Search for Regenerative Materials with Large Specific Heat Based on Rare Earth-Manganese Compounds

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In recent years, much attention has been paid to rare earth (R) compounds with large specific heat at low temperatures as magnetic regenerative materials, because of the spread of small cryocooler and superconducting magnets. In the RMn_2 compounds, both R and Mn have large magnetic moments in spite of their low magnetic ordering temperatures. We have examined low temperature specific heat and related properties of this system in order to develop new regenerative materials. The following results were obtained :(1) In the case of Al substitution for Mn, the system shows spin glass at low temperatures. The specific heat and entropy become very large above 50K,but they are not enhanced at low temperatures.(2) On the other hand, in the compounds with smaller atomic distance such as HoMn_2 and ErMn_2, most of Mn atoms are nonmagnetic. Large spin fluctuations were observed in these compounds, which enhance electronic specific heat at low temperatures.(3) The crystal field is also an important factor for large specific heat. The analyzes of crystal field were developed and were successfully applied to RCo_2 and RMn_2Ge_2 compounds.Base on these results, we conclude that Cu and Ni substitution for RMn_2 is a candidate for new regenerative materials, which is under investigation. We also found that a suitable material is expected for(Dy-Er)Bi compounds.