We have studied the hysteresis loops of RBa₂Cu₃O₇ (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar and Chaddah based on an extension of Bean’s model. The anomalies indicate lowH_c1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these highT_c superconductors.

We present DC and low frequency AC magnetization measurements on various RBa₂Cu₃O₇ superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s model. Low field anomalies predicted within this model are observed, and yieldH_c1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison is made with the other existing data and it is demonstrated that earlier quoted values ofH_c1 are gross overestimates.

We have measured the transport critical current densityJ_cof sintered YBa₂Cu₃O₇, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJ_cwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.

We present a calculation of the isothermal magnetization hysteresis curves appropriate to highT_csuperconductors. We discuss the nature of the low field anomaly as one goes from this strong pinning case to the weak pinning case. We show that the shape of the equilibrium (thermodynamic) magnetization curve is recovered in the limit ofJ_capproaching zero.