Effect of pelletization on magnetic properties of BaFe12O19

https://doi.org/10.1016/j.jallcom.2009.07.072Get rights and content

Abstract

Barium hexaferrite has been synthesized by three different routes, namely oxidation in nitric acid, solid state reaction and co-precipitation. It was shown that pelletizing before the sintering favors the formation of BaFe12O19 phase when metallic oxides are used as starting materials (i.e. through the first two methods) by increasing the reaction rates of the precursors. Thus, improvement in the saturation magnetization was observed in these samples, whereas the coercive field remained nearly the same with and without pelletizing. Magnetic interactions investigated using the Stoner–Wohlfarth model showed that demagnetizing-like interactions become stronger in the pelletized samples prepared by oxidation in nitric acid and solid state reaction routes. However, this interaction was suppressed in the pelletized sample prepared using co-precipitation technique by reducing the fraction of antiferromagnetic Ba2Fe6O11 phase.

Introduction

Barium hexaferrite, which is widely used a permanent magnet, may be the most extensively investigated material among the family of hard ferrites having the general formula MeFe12O19, where Me = Ba, Sr, Pb. These ferrites constitute an important part in the permanent magnet market due to the low price and wide availability of raw materials, high saturation magnetization and high chemical stability. During the search for better magnetic properties, various techniques have been developed to get single domain BaFe12O19 particles in the last decades. Aerosol pyrolysis [1], chemical co-precipitation [2], hydrothermal reaction [3], sol–gel [4], oxidation technique using nitric acid (HNO3) [5] and self propagating high temperature synthesis [6] are some examples. Synthesis in an ammonium nitrate melt can also be preferred to solid state reaction route for magnetic materials [7] as well as for ceramic superconductors [8], [9]. It is well known that synthesis method strongly determines the homogeneity, average particle size and shape, thus the magnetic characteristics of the final product. In order to synthesize fine particles with good magnetic properties, there are some essential factors, which should be carefully taken into account depending on the preparation method followed. Calcination temperature and time, for example, is of primary importance during the synthesis, which determines the average particle size. The higher the calcination temperature is the larger is the resulting particle size. The Ba:Fe ratio is another factor, which affects the phase purity of the samples. It may vary depending on the synthesis route followed. For instance, single phase samples were obtained when it is around 1:11 in the sol–gel technique [10], when ball milling was used optimum ratio is 1:12 [11], 1:10 in the co-precipitation route [2], and it is 1:4 in the hydrothermal method [12]. Pre-heating between 400 °C and 500 °C for several hours was reported as a key factor to prevent formation of α-Fe2O3 phase in the sol–gel technique [13]. In this way, samples with coercive field of 5950 Oe and having magnetization of 70 emu/g were synthesized. After the calcination, washing the powders with HCl can eliminate the impurity phases like barium monoferrite (BaFe2O4) which dissolves in HCl more easily than the barium hexaferrite. In this way, it is possible to improve phase purity and, thus magnetic properties of the samples [14].

In the present study, we add one more to the above list; it is pelletization. If it is performed during the preparation of this hexaferrite, it can improve the BaFe12O19 phase, especially, when metallic oxides are used as starting materials. Barium hexaferrite samples were prepared with three different methods; solid state reaction (SSR), oxidation in nitric acid and co-precipitation. Pelletizing before calcination enhanced the BaFe12O19 phase in the samples prepared through the first two routes and decreased the effect of demagnetizing-like interactions in the sample synthesized by co-precipitation technique.

Section snippets

Experimental

Starting materials, BaCO3 and Fe2O3, were used to prepare barium hexaferrite through oxidation in nitric acid and solid state reaction routes. In the former method [5], appropriate amounts of powders were mixed in nitric acid solution using magnetic stirrer while heating at 100 °C. Metallic oxides are converted to nitrates during this process. Presintering is necessary to remove gasses like NO and NO2 at 450 °C for 4 h [13]. Precursor was divided into two groups, first one was sintered in powder

Results and discussion

Fig. 1 shows the analysis of the XRD patterns of the samples, both pelletized and powder, sintered at temperatures between 900 °C and 1200 °C. The peaks corresponding to the hard BaFe12O19 phase and impurity phases (α-Fe2O3, BaFe2O4 and Ba2Fe6O11) were marked. It is clearly seen that the fraction of the hard phase is considerably higher in the pelletized samples prepared by oxidation in nitric acid route, see Table 1. At 900 °C, for example, BaFe12O19 is the major phase in the pelletized sample

Conclusion

It has been shown that, especially, when metallic oxides are used as starting materials, pelletization before sintering improves the fraction BaFe12O19 phase significantly. This is achieved by increasing the reaction rate of the initial powders. In this way, considerable enhancement has been observed in saturation magnetization of the samples prepared by oxidation in nitric acid and SSR routes. When the BaFe12O19 precursor is obtained by chemically as in the case of co-precipitation route,

Acknowledgements

The author thanks to Dr. F. Fıçıcıoğlu for his help during the preparation of the samples by co-precipitation technique. Cem Berk and Orhan İpek are also acknowledged for taking SEM micrographs of the samples.

References (18)

  • W. Zhong et al.

    J. Magn. Magn. Mater.

    (1997)
  • H. Sozeri

    J. Magn. Magn. Mater.

    (2009)
  • H. Sozeri et al.

    Mat. Chem. Phys.

    (2009)
  • H. Sozeri et al.

    J. Alloys Compd.

    (2007)
  • G. Mendoza-Suarez et al.

    J. Magn. Magn. Mater.

    (2001)
  • W. Zhong et al.

    J. Magn. Magn. Mater.

    (1997)
  • C. Surig et al.

    J. Magn. Magn. Mater.

    (1996)
  • J. Ding et al.

    J. Magn. Magn. Mater.

    (1995)
  • Z.X. Tang et al.

    IEEE Trans. Magn.

    (1989)
There are more references available in the full text version of this article.

Cited by (43)

  • Estimation of iron ion distribution at various sites contributing to saturation magnetization in barium hexaferrite at different sintering temperatures

    2021, Journal of Physics and Chemistry of Solids
    Citation Excerpt :

    As grain size increases, an increase in domain wall motion or domain size [44] is expected and this could cause a decrease in coercive field. It can also noticed from Table 2 that as the sintering temperature increases, MS increases from 54.9 emu.g−1 at 950 ∘C to 61.7 emu.g−1 at 1150∘C and then slenderly decrease to 60.4 emu.g−1 at 1250∘C. On comparing with literature [12,31,32,46], it has been observed that the obtained values of saturation magnetization of the present work are in agreeable range. Increase in the values of MS (Table 2) might be attributed to increase in grain size or reduction of grain boundaries and distributional changes in ferric ions in various sites of barium hexaferrite [47].

View all citing articles on Scopus
View full text