Electron-spin-lattice relaxation in GdBa2Cu3O6+x

A method of directly measuring electron-spin-lattice relaxation times T1 as short as 10-9--10-10 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 Gd3+ ions was studied in high-Tc superconductor GdBa2Cu3O6+x at 0.4≤x≤0.84 in the temperature range 77--300 K. It was found that spin-spin interactions between Gd3+ ions have no influence on the T1 value, so Gd3+ can be used as a spin probe even in concentrated substances. The relaxation rate W=(TT1)-1 is shown to consist of two parts, W0 and WAF, resulting from the quasiparticle contribution and the antiferromagnetic fluctuations, respectively. Both contributions reveal a spin gap opening with Δ/kB=200 K. The temperature variation of W0 is in agreement with published 89Y NMR data in YB2Cu3O6+x, the W0 values being proportional to the 89Y Knight shifts. The ratio WAF/W0 shows Curie-Weiss behavior and increases strongly as x decreases. This implies incomplete filtering of the antiferromagnetic fluctuations over the finite-sized Gd3+ 4f orbitals and symmetry breaking due to oxygen vacancies