17O hyperfine anomalies in La2CuO4

Magnetization data and NMR spectra for both O-17 and La-139 in La2CuO4 are reported for temperatures T ranging from 285 to 600 K. We find a small, positive paramagnetic state hyperfine coefficient C, corroborating earlier findings that C for the doped phase is not dominated by the Cu-O transferred coupling, contrary to simple models proposed up to now. Near and below TN the planar O-17 shift is negative, highly anisotropic, and similar to 100 times larger in magnitude. We propose a new and more complex magnetic structure in the ordered phase to account for these data.