This work studies the spin-spin interaction between the conduction electrons and localized spins in high-${\mathit{T}}_{\mathit{c}}$ superconductors. We report the ESR at 9.74 GHz of low concentration of Gd substituting the Eu in single crystals of ${\mathrm{EuBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ (x≊0.85). At low temperatures just above ${\mathit{T}}_{\mathit{c}}$, the Gd ESR spectra in the c axis direction is almost 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 transition, which remains partially resolved. This transition has a Dysonian line shape and shows a Korringa behavior. The narrowing of the spectra 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 Δ${\mathit{H}}_{\mathit{K}}$/T=0.5 G/K. From this value we calculated the exchange interaction ${\mathit{J}}_{\mathit{Ss}}$ between the Gd spins, S, and the conduction electrons, s, using reported values for conduction electron susceptibility. We obtain ${\mathit{J}}_{\mathit{Ss}}$=0.5±0.1×${10}^{\mathrm{-}3}$ eV, a value that is two or three orders of magnitude smaller than those observed in metals. It explains the small effect of magnetic ions on the ${\mathit{T}}_{\mathit{c}}$ in the R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ compounds. (R = rare earth). From the angular dependent spectra of Gd:${\mathrm{EuBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ we obtain zero field terms of ${\mathit{b}}_2^0$=-1800 MHz, ${\mathit{b}}_4^0$=-180 MHz, and ${\mathit{b}}_4^4$=+600 MHz. These values are not significantly different from those obtained for Gd in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$. © 1996 The American Physical Society.

• Received 2 March 1995

DOI:https://doi.org/10.1103/PhysRevB.53.12430