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

Cl-35 nuclear-magnetic-resonance (NMR) and relaxation measurements hav e been performed in a Sr2CuO2Cl2 single crystal as a function of orien tation between the external magnetic field H-0 and the tetragonal c ax is. The onset is found of an anisotropic inhomogeneous broadening of t he NMR line without paramagnetic shift starting below about 320 K. The broadening is strongly temperature dependent and it increases with in creasing 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 powde r sample. It is suggested that the observed broadening is due to the o nset at T>T(N) of a spontaneous nonzero thermal average of the Cu2+ mo ment lying in the a-b plane.