A method of directly measuring electron-spin-lattice relaxation times ${\mathit{T}}_1$ as short as ${10}^{\mathrm{-}9}$–${10}^{\mathrm{-}10}$ s is developed. The technique is based on the relation between the EPR absorption magnitude and the response of longitudinal spin magnetization to a radio-frequency modulation of microwave power. By means of this method, the electron-spin-lattice relaxation of ${\mathrm{Gd}}^{3+}$ ions was studied in high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{GdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ at 0.4≤x≤0.84 in the temperature range 77–300 K. It was found that spin-spin interactions between ${\mathrm{Gd}}^{3+}$ ions have no influence on the ${\mathit{T}}_1$ value, so ${\mathrm{Gd}}^{3+}$ can be used as a spin probe even in concentrated substances. The relaxation rate W=(${\mathit{TT}}_1$${)}^{\mathrm{-}1}$ is shown to consist of two parts, ${\mathit{W}}_0$ and ${\mathit{W}}_{\mathrm{AF}}$, resulting from the quasiparticle contribution and the antiferromagnetic fluctuations, respectively. Both contributions reveal a spin gap opening with Δ/${\mathit{k}}_{\mathit{B}}$=200 K. The temperature variation of ${\mathit{W}}_0$ is in agreement with published ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR data in ${\mathrm{YB}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$, the ${\mathit{W}}_0$ values being proportional to the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ Knight shifts. The ratio ${\mathit{W}}_{\mathrm{AF}}$/${\mathit{W}}_0$ shows Curie-Weiss behavior and increases strongly as x decreases. This implies incomplete filtering of the antiferromagnetic fluctuations over the finite-sized ${\mathrm{Gd}}^{3+}$ 4f orbitals and symmetry breaking due to oxygen vacancies.

• Received 19 December 1994

DOI:https://doi.org/10.1103/PhysRevB.52.1290