The low temperature specific heat capacity of CeRu₂containing gadolinium, terbium or yttrium

Specific heat capacity measurements were made at temperatures between 1.8 and 20K in vacuum magnetic induction up to 4 T on Ce 1-x Gd x Ru 2 (x<or=0.23), Ce 1-x Tb x Ru 2 (x=0.16 and 0.20) and also on Ce 1-x Y x Ru 2 (x=0.16 and 0.20). A magnetic contribution to the specific heat, resulting from the ordering of the rare earth ions, is shown to exist even at temperatures below the superconducting transition temperature T c . The magnetic ordering is proved to grow steadily with decreasing temperature. It is not influenced by T c . For alloys containing gadolinium it is completed at absolute zero. For alloys containing terbium the ordering is complicated by a crystal field producing an overall splitting of more than 20K. The existence of a jump in the specific heat at T c shows that superconductivity is a volume effect. Whether the total volume is superconducting or not is uncertain because of lack of an appropriate theory for this jump in samples in which superconductivity and magnetic ordering coexist.