Low-field ac susceptibility was measured on bulk (Formula presented) prepared by a partial melting technique. Discontinuous jumps with heights of (Formula presented) at temperature intervals of 3-5 K were observed in the ac susceptibility. 28 jumps were observed in the temperature range between 4.2 K and the Curie temperature of 250 K, with an ac field amplitude of 1 Oe, a frequency of 117 Hz, and a dc bias field of 5 and 20 Oe. Upon increasing the frequency up to 5000 Hz, the jumps shifted to lower temperature. For a dc bias field of 50 Oe and an ac field with an amplitude of 0.1 Oe and a frequency between 117 and 5000 Hz, only three jumps with heights of about (Formula presented) at large temperature intervals of about 60 K were observed for (Formula presented) With increasing frequencies, the three jumps shifted to lower temperatures without changing jump height. The discontinuous jumps in (Formula presented) were interpreted in terms of thermally and ac-field-activated domain-wall jumps. © 1998 The American Physical Society.