SPIN-ECHO STUDY OF MAGNETISM IN BI2CUO4

The zero-field nuclear spin-echo spectra of Cu and Bi exhibiting a loc al magnetic field were obtained in the antiferromagnetic (AF) state of the Bi2CuO4 crystals, grown by a hydrothermal technique. The crystals have an orthorhombic unit cell in comparison with the tetragonal P4/n cc symmetry previously assumed. The analysis of the resonance frequenc ies and the intensity behaviour showed the existence of (i) two crysta llographically and magnetically inequivalent sites both for Cu and Bi atoms, and (ii) a large enhancement associated with the presence of a spontaneous magnetic moment. The new results can be understood by taki ng into account the lowering of symmetry due to an orthorhombic distor tion of our crystals. The magnetic structure with Cu moments parallel to CUO4 planes was found more preferable to explain the experimental d ata.