Long-range magnetic ordering in the high-T_{c} superconductors RBa_{2}Cu_{3}O_{7-δ} (R=Nd, Sm, Gd, Dy, and Er)

Abstract

Low-teperature specific-heat C measurements have been made on the high-$T_c$ superconductors R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (R=Nd, Sm, Gd, Dy, and Er; δ≊0.1) for 0.5≲T≤30 K. Features in the specific heat associated with magnetic ordering of the trivalent R ions were observed for all of these compounds. The values of the magnetic ordering temperature $T_M$, defined as the temperature of the peak of the specific-heat anomaly due to magnetic ordering, are 0.52, 0.61, 2.25, 0.90, and 0.60 K for the compounds with R=Nd, Sm, Gd, Dy, and Er, respectively. Calculations of the magnetic entropy from the C vs T data suggest that the R ions in these compounds have doublet ground states, except for R=Gd, which has an eightfold degenerate ground state. The values of $T_M$ for the compounds with R=Nd, Sm, Dy, and Er are in reasonable agreement with the deGennes factor scaling of $T_M$ for the compound with R=Gd. The deGennes factor scaling of $T_M$ is consistent with expectations based on the Ruderman-Kittel-Kasuya-Yosida interaction, although the agreement may be fortuitous since recent specific-heat measurements by Dunlap et al. on metallic and semiconducting (oxygen-defiicient) ${\mathrm{GdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ suggest that dipole-dipole interactions are responsible for the magnetic order in these materials.