MAGNETIC-PROPERTIES OF THE HIGH-TEMPERATURE SUPERCONDUCTOR R2BA4CU7O15-DELTA (R = ER, DY)

Specific-heat measurements for Er2Ba4Cu7O15-delta show a magnetic phas e transition at T-N=0.54 K for delta approximate to 0.1 and at T-N=0.5 0 K for delta approximate to 0.7. While the specific heat data of Er2B a4Cu7O14.92 can be interpreted with an anisotropic 2D-Ising model, the specific heat data of Er2Ba4Cu7O14.30 can only be understood by assum ing two different types of magnetic clusters to be present in this sam ple. Such an interpretation is supported by our inelastic neutron data demonstrating the presence of at least two distinctly different surro undings for the rare-earth ions. Furthermore, the importance of dipola r interaction and superexchange for the observed magnetic ordering wil l be discussed. (C) 1998 Elsevier Science S.A.