Interaction between intra-grain currents and transport current in bulk YBaCuO*

The hysteric magnetization curves of transport and grain currents are measured for a ring shaped sample by applying an a.c. magnetic field and using pickup coils. The dependence of the magnetization curves on the amplitude of the applied a.c. magnetic field can be explained by a simple model. This model takes into account the time dependence of the transport and the intra-grain current. We will show that the mutual interaction of the grain and transport currents cannot be neglected in our samples. At first the influence of an additional d.c. background field on the transport current seems quite unexpected, but this d.c. field dependence can be interpreted too within our model.