Abstract
A method of directly measuring electron-spin-lattice relaxation times as short as – 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 ions was studied in high- superconductor at 0.4≤x≤0.84 in the temperature range 77–300 K. It was found that spin-spin interactions between ions have no influence on the value, so can be used as a spin probe even in concentrated substances. The relaxation rate W=( is shown to consist of two parts, and , resulting from the quasiparticle contribution and the antiferromagnetic fluctuations, respectively. Both contributions reveal a spin gap opening with Δ/=200 K. The temperature variation of is in agreement with published NMR data in , the values being proportional to the Knight shifts. The ratio / shows Curie-Weiss behavior and increases strongly as x decreases. This implies incomplete filtering of the antiferromagnetic fluctuations over the finite-sized 4f orbitals and symmetry breaking due to oxygen vacancies.
- Received 19 December 1994
DOI:https://doi.org/10.1103/PhysRevB.52.1290
©1995 American Physical Society