SYNTHESIS AND MAGNETIC-PROPERTIES OF SRZN2-W TYPE HEXAGONAL FERRITES USING A PARTIAL 2ZN2+-]LI+FE3+ SUBSTITUTION - A NEW SERIES OF PERMANENT-MAGNETS MATERIALS

The W-type, SrZn2Fe16O27 (SrZn2-W), hexagonal ferrite powders using a partial 2Zn2+ --> Li+Fe3+ substitution for the zinc cations have been prepared, for the first time, by standard ceramic methods. Such a substitution of lithium, according to the Sr[Zn2(1-x)(LiFe)x]Fe16O27 series, gradually increases the saturation magnetization (M(s)), as large as M(s) almost-equal-to 91 emu/g obtained at room temperature, for a sample with x almost-equal-to 0.5 and sintered at T(s) almost-equal-to 1100-degrees-C for 15 h. Also it favors the formation of W-type ferrite in the reaction by reducing the sintering temperature and the synthesis appears less stringent to atmosphere or calcining times. The product, however, comprises characteristically rather low intrinsic coercivity (H(c)) which can be varied in a wide range from 3530 to 1260 Oe, depending on the amounts of lithium (x) used in the series. Grain sizes are utmost under 1-mu-m. The critical size for the single domain particles was calculated to be 1-mu-m by using the theory of Kittel. Thus there are several reasons to believe that the decreasing coercivity (H(c) < 3500 Oe) observed in those cases is intrinsic due to decrease of magnetocrystalline anisotropy (H(a)) subject to the substitution of lithium in the Sr[Zn2(1-x)(LiFe)x]Fe16O27 ferrites. Moreover, the H(c) decreases rapidly to about zero in by increasing the grain size over 1-mu-m (by sintering the samples at reasonably higher T(s) greater-than-or-equal-to 1200-degrees-C for several hours) as solely governed by for the mechanism of multidomain nucleation formation. We also propose a model, for the distribution of the lithium through the probable sublattice sites (octahedral), which satisfactorily accounts for the variation of M(s) with the substitution of lithium in the ferrite series and the M(s) we calculated using a collinear Gorter's model.