| Summary: | Hexaferrites are still a very important component of the permanent magnets market because of their low price combined with reasonably attractive magnetic properties. In such a context, any improvement of the magnet data would be of great importance. In recent years, work has been carried out on improving the magnetic properties by a combined substitution of La and Co into the Sr-ferrite composition. This improvement was largely associated with an increase of coercivity and the underlying magneto-crystalline anisotropy. The possibility of replacing Sr by La and the subsequent improvement of the magnetic properties of the M-type ferrite stimulated our interest in studying other rare-earth ion substitutions. This thesis focuses on the effects of rare-earth element substitutions on the magnetic properties of strontium hexaferrite, SrM (SrFe12O19b)y chemical co-precipitation and by hydrothermal synthesis,a s well as on the properties-microstructure relationship. In order to produce the nanosized SrM powder, chemical co-precipitation was employed and subsequently proved to be an effective route for producing nanosized single domain SrM powder, where the SrM phase crystallises from the mainly amorphous co-precipitates through an exothermic reaction after calcination at >650°C. A very high intrinsic coercivity of 518. OkA/m (6509 Oe) with magnetisation at I IOOkA/m of 67 J/T. kg was obtained for the sample calcined at 850°C for 2h in air. The coercivity is close to the theoretical limit and is one of the highest values reported so far for isotropic SrM particles. Nanosized SrM powders with Sm and La-Zn additives were also studied. It was found that Sm doping increased slightly the coercivity of SrM and exhibited higher values than those of the corresponding samples with La additives. However, the values of magnetisation decreased slightly with the increase of the Sm/Sr ratio, which can be correlated with the increased proportion of weakly magnetic SrFeO3_pxh ase in the Sm doped SrM. Sm doping slightly increasedt he ferritization temperatureo f SrM. In the case of the La-Zn substitution, single phase Sri_,(,L aZn),,F e12_XO19 nanosized powders were produced successfully for all x, where x varied from 0 to 0.4. La-Zn substitution caused a decrease of the Curie temperature. The values of magnetization, remanence and coercivity decreased with La-Zn content. Unlike the results of La-Zn doped SrM particles prepared by conventional ceramic methods and the results of La-Zn doped SrM film prepared by rf sputtering, no improvement of the magnetisation was observed, which suggested that the properties of these materials are strongly processing-dependent and that the site preference of the Zn 2+ cation is not identical in all cases but would be affected by the processing route. Mainly single phase RE (RE=Sm, Nd, Pr and La) substituted SrM plate-like particles with the magnetoplumbite structure could be produced by hydrothermal synthesis and subsequent calcination. Under the particular hydrothermal conditions, RE elements did not substitute totally into the SrM structure and this resulted in incomplete reactions between Fe 3+ and Sr2+, indicated by traces of a-Fe2O3 and RE203 in all the RE substituted SrM samples. The presence of a-Fe2O3 and RE203 increased with the increase of the RE/Sr ratio. High temperature calcination homogenizes the materials and promotes the substitution of RE elements, resulting generally in the disappearance of a-Fe2O3 and RE203 after calcinations at >_ 1100°C. A higher calcination temperature was required to obtain the SrM single phase for a high RE/Sr ratio. The SrFe03_xp hase was found to be present in most of the RE substituted samples after calcination above 1100°C. Except for the La substitution, the other RE elements reduced the rate of grain growth during the calcination. Generally, the magnetization values of the RE substituted samples were almost the same as that of SrM. On the other hand, their intrinsic coercivities generally increased upon RE doping. Thus, an appropriate amount of RE substitution resulted in a useful increase in the intrinsic coercivity (18% for Sm, 14% for Pr, 11 % for Nd and 5% for La) without causing any significant deterioration in the saturation magnetisation or in the remanence. The improvement in the coercivity as a result of the RE substitutions is discussed in terms of the extrinsic effect associated with the microstructure and the intrinsic effect associated with an increase in the magnetocrystalline anisotropy. Finally, anisotropic SrM magnets with Sm substitution, which is observed to have the largest enhancement of coercivity among the other elements, were studied. All the magnets with Sm additions exhibit a bigger coercivity and remanence than those of the SrM magnet and the coercivity of the magnets increases with increasing Sm/Sr ratio. The average grain size of the samples decreases with increasing Sm/Sr ratio. EDX quantitative analysis suggests that the solubility of Sm 3+ in the SrM-type structure is very small and that the Sm3+ preferably goes into SrFe03_Xw, hich is probably located around the SrM grain boundaries. The coercivity mechanism of the magnets is nucleation controlled. The formation and the distribution of the SrFe03_x phase around the SrM grain boundraies probably provides the inhibition of SrM grain growth, the reduction of the reverse domain nucleation at the grain surface and the isolation of the SrM grains. All these factors would contribute to the improvements of the coercivity of the magnets with Sm additions.
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