A study of the Meissner and shielding effects in annealed La2CuO4+δ

We have measured the field-cooled (fc) and zero-field-cooled (zfc) magnetizations of 110 degrees C annealed La2CuO4+delta for 0 < delta < 0.12. We found that the Meissner signals (fc magnetizations) and shielding signals (zfc magnetizations) are almost right on top of each other except at very low temperatures close to 5 K. In particular, at delta = 0.03, an anomalously large irreversible magnetization st low temperatures and unusual paramagnetic signals are observed at T-c. We suggest that the observed anomalies are intrinsic and are electronic in origin which is closely related to the existence of a critical hole concentration at P-c congruent to 0.06.