Low frequency spin dynamics in undoped and Sr-doped La2CuO4

We report Cu63 NQR and NMR studies of low frequency spin dynamics in the paramagnetic state of La2-xSrxCuO4 for 0≤x≤0.15 in the temperature range up to 900 K. For x=0, the results were in good agreement with the theoretical predictions for an S=1/2 two-dimensional quantum Heisenberg antiferromagnet. For Sr-doped samples, we found that d spins exhibit, above 300 K, the characteristic behavior expected for a local moment system even for the highest-Tc phase La1.85Sr0.15CuO4, and cross over to an itinerant regime before superconductivity sets in.