Specific heat near $T_N$ in (Pr, Nd, Sm, Gd, and Dy) ${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: Evidence for spin-exchange-driven ordering

Magnetic ordering at 2.2 K has been reported for Gd${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. In order to characterize the relevant magnetic interaction we have synthesized and measured the specific heat of polycrystalline $R{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$, where $R=\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\mathrm{S}\mathrm{m},\mathrm{G}\mathrm{d},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{D}\mathrm{y}$. The ordering temperatures are found to vary systematically with the de Gennes factor, suggesting the dominance of spin-spin exchange over dipolar energies for these compounds. The field dependence of the specific-heat peak for Dy${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ implies the order is of antiferromagnetic nature.