Specific heat near TN in (Pr, Nd, Sm, Gd, and Dy) Ba2Cu3O7: Evidence for spin-exchange-driven ordering

Magnetic ordering at 2.2 K has been reported for GdBa2Cu3O7. In order to characterize the relevant magnetic interaction we have synthesized and measured the specific heat of polycrystalline RBa2Cu3O7, where R=Pr,Nd,Sm,Gd,andDy. 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 DyBa2Cu3O7 implies the order is of antiferromagnetic nature.