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PACS 75.50. Gg, 75.60.Lr The relaxation of the initial magnetic permeability has been measured in polycrystalline tetravalent-doped magnetite with nominal composition Me x Fe 3-x O 4 (Me: Ti, Sn, Si) with 0 < x < 0.4. The samples have been sintered at 1400 ºC in a reducing CO 2 atmosphere. The time decay of the initial permeability after sample demagnetization has been represented by means of isochronal curves in the temperature range between 80 K and 500 K.The isochronal disaccommodation spectra for Ti and Sn-doped magnetite show the splitting of the well known process III of vacancy-doped magnetite (at 300 K) into three peaks at 240 K (IV peak), 270 K (IV') and 300 K (III). This behaviour is ascribed to the presence of tetravalent cations in octahedral sites within the spinel lattice. Two peaks at 400 K (II) and 440 K (I) also emerge, owing to the existence of pairs Me 4+ -Fe 2+ cations as well as long range diffusion of ferrous cations. The differences found among these substitutions are explained in terms of the different cationic radii of tin and titanium. On the other hand only the III and IV processes are detected in Si-doped magnetite samples, due to the preferential occupation by Si cations of tetrahedral sites according to their smaller size.

The domain wall stabilization processes m titanium doped magnetite is investigated by means of disaccommodation techniques. The time evolution of the initial mageetic susceptibility after sample demagnetization is measured m temperature mge 80 to 500 K for polycrystalline specimens with nominal composition, F*Ti,04 (0W.l). The presence of ~i @ ions in the spinel lattice changes drastically the disaccommodation spectra observed in vacancy-doped magnetite. The increase of the tetravalent cations in the lattice involves an almost linear decrease of the Venvey temperature and the spwmg of processes Dl