The time decay of the relative initial magnetic susceptibility after sample demagnetization was measured in the temperature range 80 K < T < 420 K in series of samples with initial composition BaO·6Fe2O3, that is, the stoichiometric M-type barium ferrites. These samples were sintered under various conditions of temperature and partial oxygen pressure. The results were represented by means of isochronal curves. We observed that the behavior varies drastically according to the sintering atmosphere. The underlying mechanisms responsible for the relaxation phenomena observed are pointed out in terms of the formation of different hexagonal phases and the existence of ferrous cations within the hexagonal structure.