Magnetic and Mossbauer investigation of PbFe12-xGaxO19 hexagonal ferrites

Magnetic and Mo ssbauer measurements have been performed on polycrystalline samples of Ga substituted M-type hexagonal ferrites having compositions PbFe12-xGaxO19 (0 < x < 11). The saturation magnetisation (sigma(S)) at 0 K falls with increasing x, from 20 mu(B) for x = 0 to 4.32 mu(B) for x = 6. The Curie temperature tends to zero at x approximate to 9, indicating that the compounds with x > 9 are paramagnetic at all temperatures. The magnetic moments of the non substituted Fe3+ ions in the spin-up predominant sublattice do not have the same temperature dependence, because of the presence of non magnetic Ga3+ ions in the neighbouring lattice sites. For x < 3, the Mossbauer data show that there is no significant deviation from collinear magnetic order. The decrease of sigma(S)(0) for x < 3 agrees with the collinear model and the distribution of gallium in all the five available sublattices. At x > 3, the faster decline of the saturation magnetisation can be attributed to the onset of non collinear magnetic order. (C) 2002 Kluwer Academic Publishers.