The effect of La-substitution on magnetic properties of nanosized Sr1-xLaxTi0.05Zn0.2(Fe3+)11.75-δ(Fe2+)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr1-xLaxTi0.05Zn0.2Fe11.753+O19, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters ("a'' and "c'') decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 degrees C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth. (C) 2009 Elsevier B.V. All rights reserved.