MAGNETIC-INTERACTIONS BETWEEN COPPER AND RE IN-RE BA2CU3O7-DELTA

The antiferromagnetic ordering temperatures of the rare earth (RE) mom ents in RE Ba-2 Cu3O7-delta, obtained from specific heat measurements, are roughly in agreement with the de Gennes factors, i.e., the orderi ng mechanism can be mainly seen as an indirect spin-spin exchange. How ever, the oxygen dependence of T-N is found to be reversed for the lig ht rare earths compared to the heavy rare earths. As origin for this s ystematic observation an indirect interaction between the 3d-moments o f copper and the 4f-moments of rare earths is discussed as a second or der effect. Such an interaction is supported by measurements of the 4f relaxation behavior on the Nd 1:2:3 cuprates by inelastic magnetic ne utron scattering. Here, the usual thermally driven increase of the mag netic relaxation rate is suppressed up to about 80 K. This correlates with the appearance of a spin gap found by Rossat-Mignod in Y Ba2Cu3O7 and therefore the 3d-4f coupling can be understood as an interaction of the 4f moments with a spin-fluctuation exchange in the CuO2 planes. Furthermore, the quasielastic magnetic response has a Gaussian contri bution at temperatures below 100 K, i.e., much above the long ranged o rdering temperatures T-N Magnon-like excitations appear already at sli ghtly larger temperatures than T-N. In addition the paramagnetic inela stic spectra show only little dependence of the crystal field scheme o n the oxygen concentration.