This paper deals with the anisotropy induced by thermomagnetic treatment in mono crystals of Ni0.620 Co0.100 Fe 0.2602+ Fe 2.0163+ □0.004 O 42−. The induced magnetic anisotropy has been studied and an explanation of this effect is proposed. For (100) and (110) planes the experimental data agree with the calculations. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim