W0053

Magnetic Ordering of the Rare Earth Ions R³⁺ in R₂Ba₄Cu₇O_15-d (R=Er, Dy). G. Böttger (a), P. Fischer (a), P. Allenspach (a), A. Dönni (b), Y. Aoki (c), H. Sato (c), (a) Laboratory for Neutron Scattering, ETH Zurich & PSI, Paul Scherrer Institute, CH - 5232 Villigen PSI, Switzerland, (b) Dept. of Physics, Niigata University, Ikarashi-Ninomachi 8050, Niigata 950-21, Japan, (c) Dept. of Physics, Faculty of Science, Tokyo Metropolitan University, Japan

The polycrystalline samples of R₂Ba₄Cu₇O_15-d (R=Er, Dy; 0.05<=d<=0.7) prepared by a sol-gel-method have been investigated by X-ray and neutron diffraction, specific heat and magnetic susceptibility measurements. The high superconducting transition temperatures T_c=89 K for Er₂Ba₄Cu₇O_14.92 and T_c=91 K for Dy₂Ba₄Cu₇O_14.95 indicate a good sample quality. Specific heat measurements for Er₂Ba₄Cu₇O_15-d show a magnetic phase transition at T_N=0.54 K for d~0.1 and at T_N=0.50 K for d~0.7. For the first time we observed a quasi 3D antiferromagnetic order of the Er³⁺ ions in Er₂Ba₄Cu₇O_14.92 using neutron diffraction. The magnetic Er moments are oriented parallel to the b-axis. While the specific heat data of Er₂Ba₄Cu₇O_14.92 can be interpreted with an anisotropic 2D-Ising model, the specific heat data of Er₂Ba₄Cu₇O_14.3 can only be understood by assuming two different types of magnetic clusters to be present in this sample. Furthermore, the importance of dipolar interaction and superexchange for the observed magnetic ordering will be discussed.