ORIGIN OF THE ESR LINEWIDTH IN THE NORMAL PHASE OF GDBA2CU3O7

The ESR linewidth of the superconducting oxide GdBa2Cu3O7 in the normal state is studied experimentally at 300 K. Several factors that strongly affect its shape and width-skin effect, contribution of the omega- resonance, and chemical preparation-are examined carefully, and in this context some experimental results on Gd2O3 are presented. It is shown that Q-band spectra from both GdBa2Cu3O7 powders (Lorentzian line) and single crystals (isotropic spectra) are of particular interest for theoretical analysis. The g factor of Gd3+ in GdBa2Cu3O7 is found to be 2.01+/-0.01. Experimental results are given that indicate the existence of an exchange-narrowed dipolar line, via the Van Vleck-Anderson mechanism and not the Plefka-Barnes mechanism. Both the second moment of the ESR dipolar line, and the contribution of exchange to the fourth moment for a powder and for selected field orientations in the case of a single crystal are calculated numerically. Comparison of the predictions of the Van Vleck-Anderson-Weiss model with experimental results suggests that the nonadiabatic situation is present even at the Q band, and the coupling constant between nearest neighbors is found to be 2J/k = -0.3 K. Finally, the effect of the coupling with the conduction electrons is discussed, and an experimental check is made in a dilute sample.