We present selected results of systematic studies of AC susceptibility in the series of single crystalline Fe_3-xZn_xO₄ (x < 0.04) and  Fe_{3(1-\delta )}O₄ samples. Two sets of anomalies were found: the first one connected with the Verwey transition and the second consisting of two different effects, one at 26 K, visible only for stoichiometric samples, and the other at 50 K. While the temperature position of the first anomaly (26 K) does not depend on f, the effect at 50 K shifts to higher temperatures with increasing frequency. Both effects gradually move to lower temperatures and finally disappear with increasing x and \delta . Qualitatively similar results were reported from Magnetic After Effect (MAE) technique, that records effects with the relaxation time 10⁴ higher than that characteristic for our technique.

PACS numbers: 75.30.Cr, 75.50.Dd, 75.30.Gw