Influences of La3+ substitution on the structure and magnetic properties of M-type strontium ferrites

M-type strontium ferrites with substitution of Sr2+ by rare-earth La 31, according to the formula Sr1-xLaxFe12O19, are prepared by the ceramic process. Influences of the substituted amount of La3+ on structure and magnetic properties of Sr1-xLaxFe12O19 compounds have systematically been investigated by XRD, VSM and Mossbauer spectrum. When the substituted amount x is below 0.30, X-ray diffraction shows that the samples are single M-type hexagonal ferrites. It is found that the suitable amount of La3+ substitution may remarkably increase saturation magnetization sigma(S) and intrinsic coercivity H-cJ. With the La3+ addition for the same sintering temperature, sigma(S) and H-cJ increase at first, then decrease gradually. However, the substituted amount x at the maximum value of H-cJ is bigger than that of sigma(S). Mossbauer spectroscopy of Fe-57 in Sr1 xLaxFe12O19 has shown that the substitution of Sr2+ by La3+ is associated with a valence change of Fe3+ to Fe2+ at 2a or 4f(2) site, The magnetic properties are reflected in the Mossbauer spectra which indicate that the magnetic hyperfine field (H-hf) is detected with the highest value at x = 0.20. The different exchange paths between the iron sublattices are discussed according to the increased hyperfine fields of the 12k- and 2b-site. The Curie temperature of Sr1-xLaxFe12O19 decreases linearly with increasing La3+ substitution. (C) 2002 Elsevier Science B.V. All rights reserved.