Observation of Secondary Magnetic Transition in Tetragonal YBa₂Cu₃O_x (6.1≤x≤6.4)

Abstract

Temperature dependence of nuclear resonance spectra for the chain Cu(1) and the plane Cu(2) sites was investigated in the oxygen-deficient tetragonal YBa₂Cu₃O_x (6.1≤x≤6.4). The divergence of the spin-echo decay rate for the NQR at the Cu(1) site was observed at about 20 K, irrespective of the oxygen concentration, x, in x≥6.2. Below this critical temperature, only the NQR line broadening becomes significant without any resonance shifts. The spin-echo decay rate for the NMR at the Cu(2) site in the sample with x=6.2 also increases divergently when the temperature is raised to the critical temperature. These facts directly indicate the secondary transition associated with the moments on the Cu(2) sites at about 20 K, which is well below the Néel temperature. Some discussions are included for the magnetic structure below the transition temperature.