CL-35 NMR-STUDY OF MAGNETIC-ORDERING IN SR2CUO2CL2 SINGLE-CRYSTAL

Cl-35 nuclear-magnetic-resonance (NMR) and relaxation measurements have been performed in a Sr2CuO2Cl2 single crystal as a function of orientation between the external magnetic field H-0 and the tetragonal c axis. The onset is found of an anisotropic inhomogeneous broadening of the NMR line without paramagnetic shift starting below about 320 K. The broadening is strongly temperature dependent and it increases with increasing H-0. The relaxation rate T1(-1), on the other hand, diverges at T(N) congruent-to 260 K in agreement with previous results in powder sample. It is suggested that the observed broadening is due to the onset at T>T(N) of a spontaneous nonzero thermal average of the Cu2+ moment lying in the a-b plane.