The magnetic response of the Tb spins in Pb2Sr2TbCu3O7 to the applied magnetic field have been studied by ac susceptibility measurements. Both the in-phase component chi' and the out-of-phase component chi'' of ac susceptibility are found to be sensitive to the frequency but not to the strength of the driving field. At high temperatures chi' exhibits Curie-Weiss behavior, it departs from this behavior as the magnetic correlations between the To spirts develop: and a cusp which is typical of antiferromagnetic ordering is clearly revealed at lower temperatures. The correlated Tb spins respond ''better'' to a driving magnetic field which has a lower frequency. The relative maximum of the d chi'/dT versus T plot matches to the T-N determined using neutron diffraction measurements. This relative maximum continues to occur at the same temperature as the frequency of the driving field is varied. It, however, shifts to a lower temperature as a de external field is applied, indicating the antiferromagnetic coupling between the To spins are weaken by the de field. Moreover, the thermal behavior of chi'' exhibits a well-defined peak around the transition temperature, and a de field of strength 5 kOe smears the peak. (C) 1996 American Institute of Physics.
