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

M-type Sr_1-xLa_xFe_12-xCo_xO₁₉ 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 4f₂ and 2a octahedral sites, and that some Fe²⁺ is present in the 2a site. On increasing x, the main changes concern the Mossbauer contributions of the 12k, 4f₂ and 2b sites. These changes are interpreted in relation with the Co²⁺-Fe³⁺ substitution at the 4f₂ site, the Fe³⁺-Fe²⁺ valence change at the 2a site and the La³⁺-Sr²⁺ substitution. The temperature dependence of the hyperfine field at the Fe sites of the Sr_0.6La_0.4Fe_11.6Co_0.4O₁₉ 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.