We report the ac susceptibility study of with . In addition to the single-ion effect at (single-ion effect peak temperature) corresponding to the spins as that in spin ice and a possible spin freezing peak at , a new peak associated with is observed in at nonzero magnetic field with a characteristic temperature . increases linearly with in a wide composition range ( at 5 kOe). Both application of a magnetic field and increasing doping with enhance . The peak is found to be thermally driven with an unusually large energy barrier as indicated from its frequency dependence. These effects are closely related to the crystal-field levels and the underlying mechanism remains to be understood.