Effects of La3+ and Li1+ co-substitution on structural and magnetic properties of M-type strontium hexaferrite have been studied by using crystallographic and magnetic measurements. Stoichiometric Sr1-xLaxFe12-x/2Lix/2O19 (x=0, 0.25, 0.4, 0.5, and 0.6) samples have been prepared by the conventional ceramic method using La2O3, SrCO3, Fe2O3, and Li2CO3 powders with repeated sinterings at 1250 °C for 24 hours. XRD patterns showed that when x > 0.6, LaFeO3, SrFeO3-δ, and other spinel ferrite coexisted along with the M-type hexaferrite. Lattice parameters deduced from the GSAS refinement shows that both a and c decrease with increasing x, but the change in c is more outstanding (Table 1). Variations of the saturation magnetization Ms and the coercivity Hc with x was obtained from room temperature VSM measurement as in Table 1. Whereas the Ms decreases slightly with x, the Hc increases with the La3+ - Li1+ co-substitution throughout the doping range examined. Effective magnetic anisotropy constants K are calculated as well from the hysteresis curves using the law of approach to saturation method. K shows slight decrease with increasing x as listed in Table 1.
XPS scan shows the Li 1s peak at ~56 eV (Fig. 1(a)). Apparently, monovalent Li1+ is non-magnetic. In view of the facts that Li1+ in the lithium ferrite (LiFe5O8) preferentially occupies the octahedral site and that the M-type hexaferrite has the spinel S-block in its structure, the decreasing trend of Ms with x strongly suggests that Li+1 ions preferentially replace Fe3+ ions either at 12k or 2a sites (rather than the 4f2 site in R-block). Non-magnetic substitution at those sites is known to give rise to negative contribution to Ms. Finally, Mössbauer spectra show gradual decrease in 2a intensity, in support of the octahedral 2a site preference of Li1+ in M-type hexaferrite (Fig. 1(b)).
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Table 1. Various structural and magnetic parameters for Sr1-xLaxFe12-x/2Lix/2O19 (x=0, 0.25, 0.4, 0.5, and 0.6)

Figure 1. (a) A comparison of Li 1s XPS scan for Sr1-xLaxFe12-x/2Lix/2O19 (x=0.25 and 0.5) and (b) 57Fe Mössbauer spectrum for Sr0.4La0.6Fe11.7Li0.3O19 taken at room temperature.