Local Moment Formation at Cu(2) Sites around Co Atom in YBa2(Cu1-xCox)3O7+yStudied by Cu-NQR

Cu-NQR spectra in YBa 2 (Cu 1- x Co x ) 3 O 7+ y were investigated systematically from x =0 to 0.18. In the normal state, T =100 K, the integrated intensities of the spectra for both Cu(1) and Cu(2) sites decrease rapidly with increasing x , which indicates that the large region around Co atom extending to 4∼5th n.n. in Cu(1)-chain plane and 3rd∼4th n.n. in Cu(2)O 2 plane are wiped out from observation. From temperature dependence of the spectra, the magnetic ordered state associated with Co moments is confirmed to coexist with the superconducting state. The magnetic transition temperature, T M , is estimated from temperature dependence of the resonance intensities. At 1.5 K, the abnormally broad spectra ranging from 6 to 130 MHz appear, indicating the local moment formation at Cu(2) sites in the region wiped out at 100 K. Some discussions are made on the origins of the destruction of the superconductivity from the experimental results.