Magnetic properties of the high-temperature superconductor R2Ba4Cu7O15-δ (R=Er, Dy)

Specific-heat measurements for Er2Ba4Cu7O15-δ show a magnetic phase transition at TN=0.54 K for δ∼0.1 and at TN=0.50 K for δ∼0.7. While the specific heat data of Er2Ba4Cu7O14.92 can be interpreted with an anisotropic 2D-Ising model, the specific heat data of Er2Ba4Cu7O14.30 can only be understood by assuming two different types of magnetic clusters to be present in this sample. Such an interpretation is supported by our inelastic neutron data demonstrating the presence of at least two distinctly different surroundings for the rare-earth ions. Furthermore, the importance of dipolar interaction and superexchange for the observed magnetic ordering will be discussed. (orig.)