DISCONTINUITY IN THE MAGNETIZATION OF SINGLE-CRYSTAL TL2BA2CUO6+DELTA WITH H//AB

Using a low-field dc SQUID susceptometer, we have measured the zero-field-cooled magnetization M(H) of a single crystal of T1-2201 (T-c = 82.5K) with the field applied parallel to the CuO2 planes. For field sweeps carried out below T = 50K, a large jump in magnetization is observed at H = H-d, close to the field of first flux penetration. It is thought that the discontinuity in the magnetization arises from the destruction of Josephson screening currents between the CuO2 planes at a field close to the lower critical field H-c1.