Moessbauer studies of the H2 reduction effects on magnetic properties of Ba-Sr hexagonal ferrite

The concept of composite magnets with hard materials and soft materials have been applied for increasing specific saturation magnetization (σs) of M-type hexagonal ferrites. We have synthesized Ba0.5Sr0.5Fe10O19 and Ba0.3Sr0.7Fe10CoTiO19 by the sol-gel method. These Ba-Sr ferrite particles were reduced in H2 gas with varying temperature from 250 deg. C to 500 deg. C. As the reduction temperature increases, the patterns of X-ray diffraction show that the peaks of α-Fe appear above 250 deg. C and the pattern corresponding to M-type hexagonal ferrite of Ba0.5Sr0.5Fe10O19 disappears above 450 deg. C. However, that for M-type hexagonal ferrite can be found at 450 deg. C in Ba0.3Sr0.7Fe10CoTiO19. Moessbauer spectra have been analysed with 7 sub-patterns which correspond to the 4fVI, 2b, 12k, 4fIV, 2a, α-Fe and paramagnetic Fe. The Moessbauer results show that Co and Ti substitutions suppress the reduction process. This behavior between Ba0.5Sr0.5Fe10O19 and Ba0.3Sr0.7Fe10CoTiO19 is explicable with the site preference of Co2+ ions and Ti4+ ions. Moessbauer analysis revealed that α-Fe, which influences coercivity and specific saturation magnetization of particles, is introduced from the 12k site mainly, and from 4fVI