Analysis of magnetic aftereffects in strontium hexagonal ferrites with W-type stoichiometry.

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of W phase (SrO·9Fe[SUB2]O[SUB3]). The samples have been prepared by means of standard ceramic techniques at different temperatures in the 1250 °C<T<1420 ° range in air as well as CO[SUB2] sintering atmospheres and then rapidly quenched to provide the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as the main phase. In the temperature range between 80 and 500 K, the magnetic disaccommodation is represented by means of isochronal curves. The isochronal spectra show the presence of different after-effect processes whose maxima lie at 365 K, 300 K, and 165 K (respectively, A, B, and D peaks). They correspond to thermally activated relaxation processes with activation energies of 1 eV, 0.8 eV, and 0.5 eV, respectively. Processes A and B are connected to ionic reorientations of ferrous cations and lattice vacancies within the octahedral sites in spinel-like S blocks whereas D peak is associated with a similar process in face-sharing octahedral sites located in the hexagonal R blocks of the lattice. Regarding the corresponding isochronal spectra of barium W-type ferrites, we can observe that the C process centered at 240 K does not appear in Sr hexaferrites. This suggests that the relaxation processes which take place in the hexagonal R blocks are not favorable in Sr hexaferrites, probably due to the smaller size of the Sr ion. [ABSTRACT FROM AUTHOR]