Intensity of NMR signal in La1.90S0.10CuO4: Charge vs. spin freezing

We report on La-139 NMR and Cu-63 NQR measurements in La1.90Sr0.10CuO4 abov e its superconducting transition temperature. We find that the intensity of the signal integrated on frequencies decreases significantly on cooling fr om 60 K to 28 K for Cu-63 nuclei (wipeout effect), while it remains constan t for La-139 nuclei in this T-range. The distribution of La-139 spin-lattic e relaxation time (T-1) values in the "wipeout regime" reveals a spread of electronic fluctuation frequencies in CuO2 planes. A simple calculation, fo r spin fluctuation induced relaxation; shows that Cu-63 nuclei at sites whe re electronic fluctuations are slowest should not be observable because of too short relaxation times. This means that the Cu-63 NQR wipeout effect ma y be explained primarily by slow magnetic fluctuations.