INTERACTION BETWEEN LOCALIZED AND CONDUCTION-ELECTRON SPINS IN THE HIGH-T-C SUPERCONDUCTOR GD-EUBA2CU3O6+X

This work studies the spin-spin interaction between the conduction ele ctrons and localized spins in high-T-c superconductors. We report the ESR at 9.74 GHz of low concentration of Gd substituting the Eu in sing le crystals of EuBa2Cu3O6+x (x approximate to 0.85). At low temperatur es just above T-c, the Gd ESR spectra in the c axis direction is almos t resolved. With increasing temperature, exchange effects narrow most of the spectrum into a line close to g = 2.00 except the 7/2 <-> 5/2 t ransition, which remains partially resolved. This transition has a Dys onian line shape and shows a Korringa behavior. The narrowing of the s pectra and the Korringa behavior indicate an exchange coupling between the conduction electrons and the Gd spin systems, and can be analyzed by the Barnes-Plefka theory. From the linewidth broadening of the 7/2 <-> 5/2 transition and the Barnes-Plefka theory we obtain a Korringa constant Delta H-K/T = 0.5 G/K. From this value we calculated the exch ange interaction J(Ss) between the Gd spins, S, and the conduction ele ctrons, s, using reported values for conduction electron susceptibilit y. We obtain J(Ss) = 0.5 +/- 0.1 x 10(-3) eV, a value that is two or t hree orders of magnitude smaller than those observed in metals. It exp lains the small effect of magnetic ions on the T-c in the RBa(2)Cu(3)O (6+x) compounds. (R = rare earth). From the angular dependent spectra of Gd:EuBa2Cu3O6+x we obtain zero field terms of b(2)(0)= -1800 MHz, b (4)(0)= -180 MHz, and b(4)(4)= +600 MHz. These values are not signific antly different from those obtained for Gd in YBa2Cu3O6+x.