ELECTRON-SPIN-LATTICE RELAXATION IN GDBA2CU3O6+X

A method of directly measuring electron-spin-lattice relaxation times T-1 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-T-c superconductor GdBa2Cu3O6+x at 0.4 less than or equal to x less than or equal to 0.84 in the temperature range 77-300 K. It was found that spin-spin interactions between Gd3+ ions have no influence on the T-1 value, so Gd3+ can be used as a spin probe even in concentrated substances. The relaxation rate W=(TT1)(-1) Lis shown to consist of two parts, W-0 and W-AF, resulting from the quasiparticle contribution and the antiferromagnetic fluctuations, respectively. Both contributions reveal a spin gap opening with Delta/k(B)=200 K. The temperature variation of W-0 is in agreement with published Y-89 NMR data in YB2Cu3O6+x, the W-0 values being proportional to the Y-89 Knight shifts. The ratio W-AF/W-0 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.