Hexaferrites and phase relations in the iron-rich part of the system Sr–La–Co–Fe–O

doi:10.1016/j.jssc.2009.07.024

Journal of Solid State Chemistry

Volume 182, Issue 10, October 2009, Pages 2725-2732

https://doi.org/10.1016/j.jssc.2009.07.024 Get rights and content

Abstract

The iron rich part of the system was examined in the temperature range of 1200–1380 °C in air, with focus on the solid solutions of M-type hexaferrites. Samples of suitable compositions were studied by electronprobe microanalysis (EPMA). Substituted Sr-hexaferrites in the system Sr–La–Co–Fe–O do not follow the 1:1 substitution mechanism of La/Co in M-type ferrites. Due to the presence and limited Co²⁺-incorporation Fe³⁺-ions are reduced to Fe²⁺ within the crystal lattice to obtain charge balance. In all examined M-type ferrites divalent iron is formed, even at 1200 °C. The substitution principle Sr²⁺+Fe³⁺↔La³⁺+(Fe²⁺, Co²⁺) yields to the general substitution formula for the M-type hexaferrite ${Sr}^{2 +}_{1 - x} {La}^{3 +}_{x} {Fe}^{2 +}_{x - y} {Co}^{2 +}_{y} {Fe}^{3 +}_{12 - x} O^{19}$ (0≤x≤1 and 0≤y≤x). In addition Sr/La-perovskite_SS (_SS=solid solution), Co/Fe-spinel_SS, hematite and magnetite are formed. Sr-hexaferrite exhibits at 1200 °C a limited solid solution with small amounts of Fe²⁺ (SrFe₁₂O₁₉↔Sr_0.3La_0.7Co_0.5Fe²⁺_0.2Fe_11.3O₁₉). At 1300 and 1380 °C a continuous solid solution series of the M-type hexaferrite is stable. SrFe₁₂O₁₉ and LaCo_0.4Fe²⁺_0.6Fe₁₁O₁₉ are the end members at 1300 °C. The maximum Fe²⁺O content is about 13 mol% in the M-type ferrite at 1380 °C (LaCo_0.1Fe²⁺_0.9Fe₁₁O₁₉).

Graphical abstract

M-type hexaferrite solid solution series $Sr_{1 - x} La_{x} {Fe}^{2 +}_{x - y} {Co}^{2 +}_{y} {Fe}^{3 +}_{12 - x} O^{19}$ (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=LaCo_0.4Fe²⁺_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 M_S of Sr-Hexaferrite at room temperature is about 74.3 A m²/kg (emu/g) and the remanence B_R 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 B_r=440–450 mT (4400–4500 Gs~Oe) and coercivity H_ci=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 ${Sr}^{2 +}_{1 - x} {La}^{3 +}_{x} {Fe}^{2 +}_{x - y} {Co}^{2 +}_{y} {Fe}^{3 +}_{12 - x} O^{19}$ (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₋_xLa_xCo_xFe₁₂₋_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.

References (22)

J. Töpfer et al.
J. Eur. Ceram. Soc.
(2005)
D. Ravinder et al.
J. Alloys Compd.
(2004)
P. Tenaud et al.
J. Alloys Compd.
(2004)
A. Richter, Private Communication, Department of Applied Mineralogy, University of Erlangen-Nuremberg,...
W. Hart, in: Proceedings of the Conference Iron Oxides for Hard/Soft Ferrites, Gorham-Intertec, Pittsburgh,...
B.T. Shirk et al.
J. Appl. Phys.
(1969)
J. Smith et al.
Ferrites
(1959)
H. Stablein
A. Wesselsky, Diploma Thesis, Institute of Geology and Mineralogy, Department of Applied Mineralogy,...
A. Wesselsky, M. Göbbels, S. Schwarzer, J. Töpfer, in: Ninth International Conference Ferrites, ICF-9, 2004, pp....

Y. Ogata et al.

IEEE Trans. Magn.

(1999)

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