Hexaferrites and phase relations in the iron-rich part of the system Sr–La–Co–Fe–O
Graphical abstract
M-type hexaferrite solid solution series (0≤x≤1 and 0≤y≤0.40) at 1300 °C; M-type contains significant amounts of FeO even at 1200 °C; blue=data from electronprobe microanalyses; SF6=SrFe3+12O19; LCoFf6=LaCo0.4Fe2+0.6Fe3+11O19; S=SrO; L=La2O3; Co=CoO; F=Fe2O3; f=FeO.
Introduction
Ba- and Sr-hexaferrites are the most important ceramic permanent magnets [1]. BaFe12O19 and SrFe12O19 are hexagonal compounds with elongated crystals along c-axis. The high magnetic coercive force, saturation magnetization and remanence enable different applications such as in the generation of microwaves, memories and high frequency electromagnetic devices. For example the saturation magnetization MS of Sr-Hexaferrite at room temperature is about 74.3 A m2/kg (emu/g) and the remanence BR between 400 and 420 mT (4000 and 4200 Gs~Oe) [2], [3]. Besides that the ferrites are economically favourable for production and have an excellent chemical stability [4], [5]. Nevertheless it is necessary to improve the magnetic properties of those materials. Remanence is mainly influenced by the chemical composition and subsequently the position of iron or substituted ions in the magnetic sublattices. Co-substitution of Fe3+ and Sr2+ by other ions is a good approach to reach this aim. Moreover the grain size and shape are very important for anisotropy field and the coercive force. Coupled substitution by La3+ and Co2+ gives the possibility to enhance the magnetic properties of M-type ferrites. Magnets with remanence Br=440–450 mT (4400–4500 Gs~Oe) and coercivity Hci=360–380 kA/m were produced [6], [7], [8], [9], [10], [11].
Aim of this study is to examine the phase relations in the iron rich part of the system Sr–La–Co–Fe–O. First investigations at 1300 °C in this system lead to the conclusion that Fe2+ is already formed at temperatures of 1300 °C in air. Therefore the substitution formula (0≤x≤1 and 0≤y≤x) was determined by [6], [12]. Other authors consider the formation of divalent iron without describing the influence of this ion on the magnetic properties. This leads to the substitution formula Sr1−xLaxCoxFe12−xO19 (0≤x≤1) [13], [14], [15], [16]. This study is focussed on the presence of Fe2+-ions in the M-type hexaferrites, the existence and the extent of solid solutions at different temperatures (1200, 1300 and 1380 °C).
Section snippets
Experimental
To determine the phase relations suitable samples were prepared by mixing Fe2O3 (Riedel-de Haën, >97%), SrCO3 (Aldrich, >98%), La2O3 (Alfa Aesar, >99.99%) and CoCO3 (Alfa Aesar, >99%). Chosen mixtures were homogenized, milled with a vibratory disc mill RETSCH RS 1 (grinding tool:hardened steel) and decalcinated for 20 h at 1000 °C in air. Subsequently samples were cold isostatic pressed into tablets of about 1 cm in diameter. Afterwards small samples (about 0.5 cm3) were synthesized in platinum
Results and discussion
In this study samples were equilibrated at 1200, 1300 and 1380 °C. All samples are multi-phase. The complete subsolidus phase relations in the Fe-rich part of all systems were examined. Due to recrystallization chemical analyses of the melts were not suitable. Therefore their compositions were not measured. The microprobe analyses and the phase relations are presented in quaternary systems (Fig. 1). In order to display a quinary system in three-dimensions FeO and CoO were plotted together on one
Conclusions
The M-type hexaferrites and their substitution mechanism in the system Sr–La–Co–Fe–O were examined in air. At 1200, 1300 and 1380 °C extensive solid solution ranges were observed. Divalent iron is always incorporated in the crystal structure. The resulting substitution principle Sr2++Fe3+↔La3++(Co2+, Fe2+) yields to a higher dimensional range of the solid solution because of the different amounts of the incorporation of CoO and FeO. CoO occupies the majority of the small cation positions in the
Acknowledgments
The authors are grateful for the titrations experiment that were accomplished by Mrs. Daniela Seifert (F.H. Jena). This work was supported by a grant (GO-606/7-3) from the Deutsche Forschungsgemeinschaft (DFG), Germany.
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