Development and application of rare earth intermetallic compounds to working substances for extremely low-temperature refrigerator of small size

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1 Magnetic susceptibilities are measured for the rare earth intermetallic compounds of ErAg, HoAu_2, ErAu_2 and Er_2Au. As for the compounds of HoAu_2 and Er_2Au, it is found that temperature dependence of the susceptibilities make anomalies other than those found previously. These anomalies suggest the existence of new magnetic order-order transitions in these compounds.2 Specific heats of these compounds except for Er_2Au are measured between 2 and 30 K.ErAg : Mother anomaly is found at a temperature just below the Noel temperature (T_N), where the susceptibility anomaly is not observed. The peak value of specific heat is obtained to be about 20 J・K^<-1>・mol^<-3> (0.7 J・K^<-1>・cm^<-3>) at T_N. HoAu_2 : A peak in the specific heat is observed at the temperature where the new anomaly is found in susceptibility mentioned above. The specific heat at T_N has a peak value of 10 J・K^<-1>・mol^<-3> (0.27 J・K^<-1>・cm^<-3>). ErAu_2 : The maximum value of specific heat at T_N is 13 J・K^<-1>・mol^ … More <-3> (0.36 J・K^<-1>・cm^<-3>). Er_2Au : A jump of specific heat per unit volume at T_N is roughly estimated to be about 1.5 times of that of ErAg on the basis of molecular field approximation.3 The specific heats of ErAg, HoAu_2 and ErAu_2 per unit volume below 10 K are found to be lower than that of He gas which is used as a working substance of low-temperature refrigerator, It is found that the specific heats per unit volume of the compounds at temperatures below 10 K are higher than that of lead used generally for a regenerator (below 17 K in the case of ErAg). Therefore, it may be probable to make a regenerator from composite materials of these compounds and lead, so as to discharge heat of He gas below 10 K mainly to these compounds and above 10 K mainly to lead. As for the compound Er_2Au, a jump of specific heat per unit volume, which is estimated to be more than a half of the peak value of the specific heat of He gas at about 8 K, is expected around the same temperature range (T_N=6.2 K). Less