Intrinsic magnetic centers and microdomains in oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 6.5} and TmBa{sub 2}Cu{sub 3}O{sub 6+x}

We measured the influence of magnetic centers (MC{close_quote}s) on the nuclear relaxation in 123 compounds using {sup 169}Tm NMR and {sup 63}Cu NQR in TmBa{sub 2}Cu{sub 3}O{sub 6+x} (x=0, 0.2, 0.3, 0.4, 0.5, 0.6), and {sup 89}Y NMR and {sup 63}Cu NQR in YBa{sub 2}Cu{sub 3}O{sub 6.5}. Particular attention is paid to the region 0.5{le}x{le}0.6 in order to deal with both a well-defined ortho II structure and a high enough MC concentration. The experiments reveal a two-component nuclear relaxation in the superconducting compounds at temperatures above 1 K, especially pronounced in the x=0.5 samples. The relaxation data give evidence for microphase separation due to oxygen disorder in the basal CuO{sub x} planes. In the well-annealed samples, the MC{close_quote}s predominating the relaxation have S=1/2. The concentrations of these MC{close_quote}s are of the order of 3{percent} in the disordered (nonsuperconducting) microphase and {approximately}0.3{percent} in the ordered (superconducting) one. {copyright} {ital 1997} {ital The American Physical Society}