Mossbauer investigation of Sr-Fe-O hexaferrites with La-Co addition

M-type Sr1-xLaxFe12-xCoxO19 ferrite powders with x = 0 to 0.4 were investigated using Mossbauer spectrometry in the 77 K-550 K temperature range. The Mossbauer spectra were fitted by assuming that Co2+ is present in both 4f(2) and 2a octahedral sites, and that some Fe2+ is present in the 2a site. On increasing x, the main changes concern the Mbssbauer contributions of the 12k, 4f(2) and 2b sites. These changes are interpreted in relation with the Co2+-Fe3+ substitution at the 4f(2) site, the Fe3+-Fe2+ valence change at the 2a site and the La3+-Sr2+ substitution. The temperature dependance of the hyperfine field at the Fe sites of the Sr0.6La0.4Fe11.6Co0.4O19 compound is similar to that of the unsubstituted compound. However, the curvature of the temperature dependence of the hyperfine field at the 12k site slightly increases with x. This reflects the increase of the curvature of the temperature dependence of the magnetization of the substituted compounds.