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Barium hexaferrite monodispersed nanoparticles prepared by the ceramic method
Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
Barium hexaferrite monodispersed nanoparticles prepared by the ceramic method G. Benitoa, M.P. Moralesa, J. Requenaa, V. Raposoa,b, M. Va! zqueza,b, J.S. Moyaa,* b
a Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Instituto de Magnetismo Aplicado, RENFE, UCM, Apdo, correos 155, 28230 Madrid, Spain
Received 30 November 2000; received in revised form 20 April 2001
Abstract Barium hexaferrite particles between 1 mm and 100 nm in diameter have been prepared by solid state reaction from different iron oxide precursors. The characteristics of the iron oxide precursor in terms of particle size and its distribution seem to be essential to allow us to prepare the hexaferrite at a significantly reduced temperature as well as to obtain particles with reduced size. Thus, when a goethite sample consisting of uniform particles of 200 nm length is used as iron oxide precursor, pure and uniform hexaferrite particles of around 100 nm diameter are obtained by heating up to 6751C for 1 h the mixture of iron oxide and barium carbonate. Magnetic measurements of this sample showed a saturation magnetisation of around 60 emu/g at room temperature and coercivities of around 4500 Oe, in agreement with the single magnetic domain character of the particles. r 2001 Elsevier Science B.V. All rights reserved. PACS: 81.20.Zx; 81.05.Ys; 75.50.Tt; 75.50.Ss Keywords: Barium ferrite; Magnetic nanoparticles; Magnetic properties
1. Introduction Hexagonal barium ferrite is widely used as permanent magnet due to its excellent magnetic properties such as high Curie temperature, magnetic anisotropy and coercivity [1]. Recently BaFe12O19 fine particles have become a very interesting particulate media for perpendicular magnetic recording [2]. This material offers many *Corresponding author. Tel.: +34-1-334-9000; fax: +34-91372-0623. E-mail address: [email protected] (J.S. Moya).
advantages in terms of high density recording and chemical stability. The typical method to obtain ferrimagnetic hexagonal oxide particles in general is the solid state reaction. This method consists in heating the mixtures of precursors at temperatures as high as 10001C giving rise to particles larger than 1 mm. In order to reduce the particle size other methods has been investigated such as aerosol pyrolysis [3], Sol–gel [4], chemical coprecipitation [5], crystalglass [6], etc. However, these methods are more complex and more expensive than the ceramic method. Therefore, the aim of this paper has been
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 2 8 8 - 8
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G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
to develop an improved method to synthesise barium hexaferrite nanoparticles based on the solid state reaction but using temperatures lower than used conventionally. It has been previously reported that the formation mechanism of barium hexaferrite particles by the solid state reaction takes place in two main steps, the decarboxilation together with the formation of monoferrite and the diffusion of Ba into the iron oxide [7]: BaCO3 þFe2 O3 -BaFe2 O4 þCO2 ; BaFe2 O4 þ5Fe2 O3 -BaO 6Fe2 O3 : Based on that mechanism which involved diffusion processes, an increase in the specific surface area of the precursors, i.e. a reduction in the particle size, would led to the formation of barium hexaferrite at lower temperatures with smaller particles. In this sense, different iron oxide precursors with different particle sizes have been used in the preparation of barium hexaferrite by the solid state reaction. The effects of the nature and the particle size of the precursor on the formation temperature of the hexaferrite and the final particle morphology and size have been investigated. Samples were characterised by X-ray diffraction, TEM and thermal analysis. The magnetic properties of the samples were studied by using a Vibrating Sample Magnetometer at room temperature.
2. Experimental Two goethite samples with different particle size were synthesised and labelled according to their long dimension as: (G1000) and (G200). Sample G1000 was prepared from ferrihydrite, obtained by adding 180 ml of 5 M NaOH to 100 ml of 1 M Fe(NO3)3 solution [8]. The suspension was then diluted to 1 l with distilled water and heating at 701C for 60 h in a closed flask. Sample G200 was synthesised by mixing 1.5 M FeSO4 and 0.75 M NaCO3 in 500 ml of water [9]. The solution was heated at 401C for 5 h passing through a strong air flow. The resulting precipitates were, in both cases, several times washed, filtered, and dried at 601C
for 24 h. Specific surface area of these iron oxide precursors was obtained by nitrogen adsorption in a Micromeritics Flowsorb II 2300 applying the single point BET method. Samples were previously degassed at 1201C for 1 h. Barium hexaferrite powders were prepared using the ceramic technique. The mixture of BaCO3 (Merck, Germany), consisting in whiterite with >99% purity and average particle size of 0.97 mm, and the corresponding iron oxide precursor was milled for 48 h in a plastic ball mill with 35 g of steel milling balls (s ¼ 2 mm). After drying at 601C for 24 h, the powder was passed through a 100 mm mesh and heated at temperatures between 6751C and 10001C for 1 h. Samples were named according to their precursor, HG200 and HG1000. For comparison purpose, a barium hexaferrite sample was synthesised from a commercial precursor (COM) consisting of Fe2O3 (Prolabo, France) with >99% purity and named as HCOM. In this case, the precursor was hematite with an average particle size larger than 1000 nm. Thermogravimetry and Differential thermal analysis (DTA) of the powder mixture were carried out by using a Seiko TG/DTA 320U in the temperature range 20–10001C in static air with platinum crucibles. Structural characterisation of the iron oxide precursors and the hexaferrite samples was performed on a Siemens D-501 DACO X-ray powder diffractometer with Cu Ka radiation. In order to clarify the formation mechanism of barium ferrite particles, X-ray diffraction of the mixture of barium carbonate and sample G200 quenched at temperatures between 4251C and 8501C was carried out. The mixture was heated up to certain temperatures, chosen from the TG curve, under equal conditions and the power supply was then switched off. Particle size and morphology of the iron oxide precursors and the hexaferrite samples were examined by direct observation in a JEOL 2000 FXII transmission electron microscope (TEM), and the average mean diameter and the standard deviation were calculated by counting around 100 particles. Magnetic properties of the hexaferrite samples were measured by using a Vibrating Sample
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Magnetometer with a maximum applied field of 15 kOe. At such a field, magnetisation starts its approach to saturation. Magnetisation values at
the maximum applied field are taken as the saturation magnetisation (Ms). Coercivity (Hc) and squareness ratio (Mr/Ms) were obtained from the hysteresis loops at room temperature.
3. Results and discussion Two goethite samples with different particle size have been synthesised by different methods (Fig. 1 and Table 1). Sample G200 consists of spindle goethite particles of around 240 nm in length and a standard deviation of around 40 nm, leading to a degree of polidispersity (standard deviation/mean size) smaller than 0.2 and therefore indicating that the solid is monodispersed [10] (Fig. 1a). Sample G1000 consists of rod-like goethite particles of around 1000 nm in length with a very wide size distribution (s ¼ 500) (Fig. 1b). Particle width is similar for both samples and around 70 nm but again, the data dispersion is greater for sample G1000 with a standard deviation of 50 nm against 15 nm for sample G200 (Table 1). A picture of the commercial hematite sample (named as COM) has been included in Fig. 1c for comparison. It consists of aggregated spherical particles larger than 1 mm. In spite of the fact that the long dimension of sample G1000 and the mean diameter of sample COM are similar, an important difference should be pointed out between these samples in relation to the specific surface area. Thus, samples G200 and G1000 have a specific surface area of 45 and 69 m2/ g while sample COM has a much lower specific surface area of 5 m2/g (Table 1), as consequence of the different particle morphology. Thermal analysis (Fig. 2) and X-ray diffraction (Fig. 3) have followed the evolution with
Table 1 Specific surface area and particle size by TEM of the iron oxide precursors. Standard deviations are included between brackets Sample
Fig. 1. TEM pictures of the iron oxide precursors: (a) G200, (b) G1000 and (c) COM.
G200 G1000 COM
Iron phase
Goethite Goethite Hematite
SBET (m2/g)
44 69 5
Particle size (nm) Length
Width
240 (40) 970 (530) >1000
60 (15) 76 (50) F
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G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
Fig. 2. DTA/TG curves of the mixture of BaCO3 and samples G200, G1000 and COM.
Fig. 3. X-ray diffraction of the mixture of BaCO3 and sample G200 quenched at different temperatures between 4001C and 8501C.
temperature of the powder mixture of the iron oxide precursors and barium carbonate. In the case of the goethite samples, G200 and G1000, TG curves between 201C and 10001C show two main steps. The first step takes place between 1001C and 3001C and DTA curve shows an endothermic peak in this temperature range, which can be assigned to the dehydration of goethite. Accordingly, X-ray diffraction of the powder mixture, goethite and barium carbonate, quenched at 4001C reveals the formation of hematite [11] (Fig. 3). The rest of the peaks were assigned to carbonate which did not suffer any transformation at this temperature. A slight difference was found between samples G200 and G1000 in relation to the temperature of the endothermic peak, which is lower for sample G200 in accordance with a smaller particle size [12]. The second step observed in the TG curves takes place at around 6001C for the samples obtained from goethite and at around 8001C for sample COM (Fig. 2). This step could be assigned to the decarboxilation of BaCO3, which has been reported to take place at 10551C for pure carbonate [13] and at around 8001C for the mixture of carbonate and an iron oxide [14]. Surprisingly, in the presence of goethite particles a reduction of the decarboxilation temperature from 8001C down to around 6001C is observed while the partial weight loss is around the same for the three samples and similar to the theoretical one (3.8%). XRD of the powder mixture for sample G200 after heating at 6501C shows peaks assigned to barium monoferrite and hematite (Fig. 3). Therefore, decarboxilation and formation of barium monoferrite seem to take place at the same time. In conclusion, the temperatures at which these reactions occur are lower for the samples prepared from goethite in agreement with the higher specific surface area for these samples in comparison to the commercial iron precursor. Since the formation of monoferrite has been proved to be based on a diffusion process [7], a higher specific surface area of the precursors is expected to help the reaction to take place at lower temperature. On the other hand, it can be observed from Fig. 2 that the temperature range is shorter for sample G200 probably due to the narrower particle size distribution.
G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
At higher temperatures, an exothermic peak at 9001C is clearly observed only in the case of the sample prepared from the commercial precursor. This peak has been previously assigned to the barium diffusion into the hematite and therefore, to the beginning of the barium hexaferrite formation which is not completed until 10001C [7]. For samples G200 and G1000, a slight exothermic peak is also observed in the DTA at around 8251C. However, X-ray diffraction of these samples heated at 8001C reveals that hexaferrite has been already formed at this temperature and therefore the origin of this peak seems to be different compared to that in the case of sample COM. Based on the TG results, the mixtures from goethite samples were heated at 6751C for 1 h, which is the temperature of the end of the decarboxilation process for samples G200 and G1000 (Fig. 2). X-ray diffraction of the resulting samples shows that the reaction is completed for sample G200, while for sample G1000 significant amounts of hematite and monoferrite are still present (Fig. 4). In the last case, pure hexaferrite is obtained only after heating up to 7501C for 1 h. It can be concluded that, once the decarboxilation
Fig. 4. X-ray diffraction of samples HG200 and HG1000 heated at 6751C for 1 h.
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has finished, the formation of hexaferrite begins to take place, according to previous results [7]. Additionally, the transformation from monoferrite to hexaferrite is quicker as the specific surface area of the precursors increases from sample COM to samples G200 and G1000 (Table 1). In relation to
Fig. 5. TEM pictures of the barium hexaferrite samples: (a) HG200 heated at 6751C, (b) HG1000 heated at 7501C and (c) HCOM heated at 10001C.
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Table 2 Preparation conditions, particle size by TEM and magnetic properties of the barium ferrite particles (Ms=Saturation magnetisation, Mr =Ms =Squareness ratio, Hc =Coercivity). Standard deviations are included between brackets Sample
HG200 HG1000 HCOM
Formation temperature (1C) 675 750 1000
Particle size (nm) 100 (20) 460 (230) >1000
the last two samples, particle size distribution seems an important factor, which also affects the monoferrite–hexaferrite transformation. The formation of sample HG200 is quicker than sample HG1000 probably due to the monodispersed character of sample G200 (Table 1). The morphology and particle size of the hexaferrite samples, HG200, HG1000 and HCOM show significant differences when observed by TEM (Fig. 5 and Table 2). Sample HG200 obtained at 6751C (Fig. 5a) consists of pseudohexagonal particles of around 100 nm diameter, while HG1000 (Fig. 5b) obtained at 7501C consists of particles of 460 nm mean diameter. The differences between these samples are not only the particle size but also the size distribution. Thus, standard deviation for sample HG200 is 18 nm while for sample HG1000 the standard deviation is 250 nm (Table 2). This result shows that sample HG200 is a monodispersed solid of hexaferrite. As far as we know, no data have been reported in the literature about the formation of barium hexaferrite monodispersed particles by ceramic method. On the other hand, the reduction in particle size from the goethite precursor (240 nm) to the final hexaferrite particle (100 nm) can be explained by two main factors. First of all, a volume contraction is taking place due to the transformation from goethite to hematite [15] and secondly, there is a re crystallisation process where the hexaferrite particles grow topotactically in the (0 0 1) direction from the (1 1 1) plane of hematite [7]. A schematic representation of this transformation is shown in Fig. 6. Magnetic behaviour of these pure hexaferrite samples is presented in Fig. 7 and the hysteresis
Magnetic properties Ms (emu/g)
Mr =Ms
Hc (Oe)
60 47 48
0.50 0.51 0.26
4500 3200 350
Fig. 6. Schematic representation of the transformation from hematite to barium hexaferrite.
Fig. 7. Hysteresis loops at room temperature of the barium hexaferrite samples HG200, HG1000 and HCOM.
parameters are included in Table 2. Hysteresis loops for samples HG200, HG1000 and HCOM showed significant differences in the saturation magnetisation (Ms) and coercivity values (Hc). The maximum Ms value is 60 emu/g for sample
G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
HG200, which is lower than the theoretical one calculated for single crystals of barium hexaferrite particles, i.e. 72 emu/g [16]. This can be explained partially by the lack of full saturation reached at the maximum applied field. Anyway the smallness in size of the particles can also contribute to that reduced value. Several theories, including surface effects, spin canting and sample inhomogeneity, have been proposed to account for the relatively low magnetisation in fine particles [17]. Thus, particles of similar size prepared by different methods have been reported to give rise to lower or similar Ms values [5,6], even when higher temperatures have been used in the synthesis. On the other hand, samples HG1000 and HCOM have similar Ms values, 47 and 48 emu/g, respectively. The low Ms value for these samples in comparison to sample HG200, could be a consequence of the wide particle size distribution where there is an important fraction of small particles contributing to reduce Ms due to surface effects. The squareness ratio, Mr =Ms ; for samples HG200 and HG1000 is around 0.5 (Table 2) that is close to the expected value for randomly packed single domain particles if a coherent magnetisation rotation reversal mechanism is assumed [18]. According to that, coercivity values are higher for these two samples in comparison to sample HCOM. The maximum coercivity is obtained for the sample with the smallest particle size, HG200 (4500 Oe) (Table 2). Values up to 6000 Oe have been reported for barium hexaferrite particles of similar size prepared by aerosol pyrolysis and sol–gel [3,4] but in those cases it was necessary to heat the sample up to 9001C, 2001C higher than the temperature used to prepare sample HG200. On the other hand, the squareness ratio for sample HCOM is only 0.26 (Table 2), which can be attributed to the existence of a multidomain structure [18]. This sample consists of particles larger than 1 mm together with a very wide size distribution (Fig. 5c), which means that there is an important fraction of particles over the critical diameter reported for single domain behaviour for barium ferrite, 900 nm [1]. The multidomain behaviour and therefore the mechanism of magnetisation reversal by domain wall motion explain
71
the reduced coercivity, 350 Oe, observed for this sample (Table 2).
4. Conclusions It can be concluded that uniform and monodomain particles of BaFe12O19 can be prepared by solid state reaction at a temperature lower than 7001C whenever an iron oxide-hydroxide of small particle size and narrow size distribution is used as precursor. In this work, pure hexaferrite particles have been obtained after heating at 6751C for 1 h a mixture of barium carbonate and acicular goethite particles of 200 nm in length. The sample consists of pseudo-hexagonal particles of around 100 nm in diameter and a very narrow size distribution (s ¼ 20). Magnetic properties of this sample show a saturation magnetisation of around 60 emu/g and a coercivity of 4500 Oe, according to a single domain behaviour of the particles.
Acknowledgements This investigation has been supported by the Regional Government of Madrid (CAM), Spain, Reference No. 07N/0057/1998, and the Ministry of Education of Spain under contract No. 950300-OP.
References [1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Vol. 3, North-Holland, New York, 1982 (chapter 5). [2] H. Pfeiffer, R.W. Chantrell, P. Go. rnet, W. Schu. ppel, E. Sinn, M. Ro. sler, J. Magn. Magn. Mater. 125 (1993) 373. [3] T. Gonz!alez-Carren* o, M.P. Morales, C.J. Serna, Mater. Lett. 43 (2000) 97. [4] W. Zhong, W.P. Ding, N. Zhang, J.M. Hong, Q.J. Yan, Y.W. Du, J. Magn. Magn. Mater. 168 (1997) 196. [5] K. Haneda, C. Miyakawa, H. Kojima, J. Am. Ceram. Soc. 57 (1974) 354. [6] L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, J. Magn. Magn. Mater. 193 (1999) 288. [7] H.P. Steier, J. Requena, J.S. Moya, J. Mater. Res. 14 (1999) 3647.
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[8] R.M. Cornell and U. Schwertmann, The iron oxides, VCH Publishers, New York, 1996, p. 489. [9] N. Sugita, M. Maekawa, Y. Ohta, K. Okinaka, N. Nagai, IEEE Trans. Magn. 31 (1995) 2854. [10] R. Hunter, Foundations of Colloid Science, Clarendon Press, Oxford, 1987, p. 127. [11] L.A. P!erez-Maqueda, J.M. Criado, C. Real, J. Subrt, J. Bohacek, J. Mater. Chem. 9 (1999) 1839. [12] R.C. Mackenzie, Differential Thermal Analysis I, Academic Press, New York, 1982, p. 274. [13] C. Duval, Inorganic Thermogravimetric Analysis, Elsevier, London, 1963, p. 533.
[14] G.C. Bye, C.R. Howard, J. Appl. Chem. Biotechnol. 21 (1971) 319. [15] H. Naono, K. Nakai, T. Sueyoshi, H. Yagi, J. Colloid Interface Sci. 120 (1987) 439. [16] B.T. Shirk, W.R. Buessem, J. Appl. Phys. 40 (1969) 1294. [17] K.V.P.M. Shafi, Y. Koltypin, A. Gedanken, R. Prozorov, J. Balogh, J. Lendvai, I. Felner, J. Phys. Chem. B 101 (1997) 6409. [18] E.C. Stoner, E.P. Wohlfarth, Phil. Trans. Roy. Soc. London 240 (1948) 599.
Barium hexaferrite monodispersed nanoparticles prepared by the ceramic method G. Benitoa, M.P. Moralesa, J. Requenaa, V. Raposoa,b, M. Va! zqueza,b, J.S. Moyaa,* b
a Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Instituto de Magnetismo Aplicado, RENFE, UCM, Apdo, correos 155, 28230 Madrid, Spain
Received 30 November 2000; received in revised form 20 April 2001
Abstract Barium hexaferrite particles between 1 mm and 100 nm in diameter have been prepared by solid state reaction from different iron oxide precursors. The characteristics of the iron oxide precursor in terms of particle size and its distribution seem to be essential to allow us to prepare the hexaferrite at a significantly reduced temperature as well as to obtain particles with reduced size. Thus, when a goethite sample consisting of uniform particles of 200 nm length is used as iron oxide precursor, pure and uniform hexaferrite particles of around 100 nm diameter are obtained by heating up to 6751C for 1 h the mixture of iron oxide and barium carbonate. Magnetic measurements of this sample showed a saturation magnetisation of around 60 emu/g at room temperature and coercivities of around 4500 Oe, in agreement with the single magnetic domain character of the particles. r 2001 Elsevier Science B.V. All rights reserved. PACS: 81.20.Zx; 81.05.Ys; 75.50.Tt; 75.50.Ss Keywords: Barium ferrite; Magnetic nanoparticles; Magnetic properties
1. Introduction Hexagonal barium ferrite is widely used as permanent magnet due to its excellent magnetic properties such as high Curie temperature, magnetic anisotropy and coercivity [1]. Recently BaFe12O19 fine particles have become a very interesting particulate media for perpendicular magnetic recording [2]. This material offers many *Corresponding author. Tel.: +34-1-334-9000; fax: +34-91372-0623. E-mail address: [email protected] (J.S. Moya).
advantages in terms of high density recording and chemical stability. The typical method to obtain ferrimagnetic hexagonal oxide particles in general is the solid state reaction. This method consists in heating the mixtures of precursors at temperatures as high as 10001C giving rise to particles larger than 1 mm. In order to reduce the particle size other methods has been investigated such as aerosol pyrolysis [3], Sol–gel [4], chemical coprecipitation [5], crystalglass [6], etc. However, these methods are more complex and more expensive than the ceramic method. Therefore, the aim of this paper has been
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 2 8 8 - 8
66
G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
to develop an improved method to synthesise barium hexaferrite nanoparticles based on the solid state reaction but using temperatures lower than used conventionally. It has been previously reported that the formation mechanism of barium hexaferrite particles by the solid state reaction takes place in two main steps, the decarboxilation together with the formation of monoferrite and the diffusion of Ba into the iron oxide [7]: BaCO3 þFe2 O3 -BaFe2 O4 þCO2 ; BaFe2 O4 þ5Fe2 O3 -BaO 6Fe2 O3 : Based on that mechanism which involved diffusion processes, an increase in the specific surface area of the precursors, i.e. a reduction in the particle size, would led to the formation of barium hexaferrite at lower temperatures with smaller particles. In this sense, different iron oxide precursors with different particle sizes have been used in the preparation of barium hexaferrite by the solid state reaction. The effects of the nature and the particle size of the precursor on the formation temperature of the hexaferrite and the final particle morphology and size have been investigated. Samples were characterised by X-ray diffraction, TEM and thermal analysis. The magnetic properties of the samples were studied by using a Vibrating Sample Magnetometer at room temperature.
2. Experimental Two goethite samples with different particle size were synthesised and labelled according to their long dimension as: (G1000) and (G200). Sample G1000 was prepared from ferrihydrite, obtained by adding 180 ml of 5 M NaOH to 100 ml of 1 M Fe(NO3)3 solution [8]. The suspension was then diluted to 1 l with distilled water and heating at 701C for 60 h in a closed flask. Sample G200 was synthesised by mixing 1.5 M FeSO4 and 0.75 M NaCO3 in 500 ml of water [9]. The solution was heated at 401C for 5 h passing through a strong air flow. The resulting precipitates were, in both cases, several times washed, filtered, and dried at 601C
for 24 h. Specific surface area of these iron oxide precursors was obtained by nitrogen adsorption in a Micromeritics Flowsorb II 2300 applying the single point BET method. Samples were previously degassed at 1201C for 1 h. Barium hexaferrite powders were prepared using the ceramic technique. The mixture of BaCO3 (Merck, Germany), consisting in whiterite with >99% purity and average particle size of 0.97 mm, and the corresponding iron oxide precursor was milled for 48 h in a plastic ball mill with 35 g of steel milling balls (s ¼ 2 mm). After drying at 601C for 24 h, the powder was passed through a 100 mm mesh and heated at temperatures between 6751C and 10001C for 1 h. Samples were named according to their precursor, HG200 and HG1000. For comparison purpose, a barium hexaferrite sample was synthesised from a commercial precursor (COM) consisting of Fe2O3 (Prolabo, France) with >99% purity and named as HCOM. In this case, the precursor was hematite with an average particle size larger than 1000 nm. Thermogravimetry and Differential thermal analysis (DTA) of the powder mixture were carried out by using a Seiko TG/DTA 320U in the temperature range 20–10001C in static air with platinum crucibles. Structural characterisation of the iron oxide precursors and the hexaferrite samples was performed on a Siemens D-501 DACO X-ray powder diffractometer with Cu Ka radiation. In order to clarify the formation mechanism of barium ferrite particles, X-ray diffraction of the mixture of barium carbonate and sample G200 quenched at temperatures between 4251C and 8501C was carried out. The mixture was heated up to certain temperatures, chosen from the TG curve, under equal conditions and the power supply was then switched off. Particle size and morphology of the iron oxide precursors and the hexaferrite samples were examined by direct observation in a JEOL 2000 FXII transmission electron microscope (TEM), and the average mean diameter and the standard deviation were calculated by counting around 100 particles. Magnetic properties of the hexaferrite samples were measured by using a Vibrating Sample
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Magnetometer with a maximum applied field of 15 kOe. At such a field, magnetisation starts its approach to saturation. Magnetisation values at
the maximum applied field are taken as the saturation magnetisation (Ms). Coercivity (Hc) and squareness ratio (Mr/Ms) were obtained from the hysteresis loops at room temperature.
3. Results and discussion Two goethite samples with different particle size have been synthesised by different methods (Fig. 1 and Table 1). Sample G200 consists of spindle goethite particles of around 240 nm in length and a standard deviation of around 40 nm, leading to a degree of polidispersity (standard deviation/mean size) smaller than 0.2 and therefore indicating that the solid is monodispersed [10] (Fig. 1a). Sample G1000 consists of rod-like goethite particles of around 1000 nm in length with a very wide size distribution (s ¼ 500) (Fig. 1b). Particle width is similar for both samples and around 70 nm but again, the data dispersion is greater for sample G1000 with a standard deviation of 50 nm against 15 nm for sample G200 (Table 1). A picture of the commercial hematite sample (named as COM) has been included in Fig. 1c for comparison. It consists of aggregated spherical particles larger than 1 mm. In spite of the fact that the long dimension of sample G1000 and the mean diameter of sample COM are similar, an important difference should be pointed out between these samples in relation to the specific surface area. Thus, samples G200 and G1000 have a specific surface area of 45 and 69 m2/ g while sample COM has a much lower specific surface area of 5 m2/g (Table 1), as consequence of the different particle morphology. Thermal analysis (Fig. 2) and X-ray diffraction (Fig. 3) have followed the evolution with
Table 1 Specific surface area and particle size by TEM of the iron oxide precursors. Standard deviations are included between brackets Sample
Fig. 1. TEM pictures of the iron oxide precursors: (a) G200, (b) G1000 and (c) COM.
G200 G1000 COM
Iron phase
Goethite Goethite Hematite
SBET (m2/g)
44 69 5
Particle size (nm) Length
Width
240 (40) 970 (530) >1000
60 (15) 76 (50) F
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G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
Fig. 2. DTA/TG curves of the mixture of BaCO3 and samples G200, G1000 and COM.
Fig. 3. X-ray diffraction of the mixture of BaCO3 and sample G200 quenched at different temperatures between 4001C and 8501C.
temperature of the powder mixture of the iron oxide precursors and barium carbonate. In the case of the goethite samples, G200 and G1000, TG curves between 201C and 10001C show two main steps. The first step takes place between 1001C and 3001C and DTA curve shows an endothermic peak in this temperature range, which can be assigned to the dehydration of goethite. Accordingly, X-ray diffraction of the powder mixture, goethite and barium carbonate, quenched at 4001C reveals the formation of hematite [11] (Fig. 3). The rest of the peaks were assigned to carbonate which did not suffer any transformation at this temperature. A slight difference was found between samples G200 and G1000 in relation to the temperature of the endothermic peak, which is lower for sample G200 in accordance with a smaller particle size [12]. The second step observed in the TG curves takes place at around 6001C for the samples obtained from goethite and at around 8001C for sample COM (Fig. 2). This step could be assigned to the decarboxilation of BaCO3, which has been reported to take place at 10551C for pure carbonate [13] and at around 8001C for the mixture of carbonate and an iron oxide [14]. Surprisingly, in the presence of goethite particles a reduction of the decarboxilation temperature from 8001C down to around 6001C is observed while the partial weight loss is around the same for the three samples and similar to the theoretical one (3.8%). XRD of the powder mixture for sample G200 after heating at 6501C shows peaks assigned to barium monoferrite and hematite (Fig. 3). Therefore, decarboxilation and formation of barium monoferrite seem to take place at the same time. In conclusion, the temperatures at which these reactions occur are lower for the samples prepared from goethite in agreement with the higher specific surface area for these samples in comparison to the commercial iron precursor. Since the formation of monoferrite has been proved to be based on a diffusion process [7], a higher specific surface area of the precursors is expected to help the reaction to take place at lower temperature. On the other hand, it can be observed from Fig. 2 that the temperature range is shorter for sample G200 probably due to the narrower particle size distribution.
G. Benito et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 65–72
At higher temperatures, an exothermic peak at 9001C is clearly observed only in the case of the sample prepared from the commercial precursor. This peak has been previously assigned to the barium diffusion into the hematite and therefore, to the beginning of the barium hexaferrite formation which is not completed until 10001C [7]. For samples G200 and G1000, a slight exothermic peak is also observed in the DTA at around 8251C. However, X-ray diffraction of these samples heated at 8001C reveals that hexaferrite has been already formed at this temperature and therefore the origin of this peak seems to be different compared to that in the case of sample COM. Based on the TG results, the mixtures from goethite samples were heated at 6751C for 1 h, which is the temperature of the end of the decarboxilation process for samples G200 and G1000 (Fig. 2). X-ray diffraction of the resulting samples shows that the reaction is completed for sample G200, while for sample G1000 significant amounts of hematite and monoferrite are still present (Fig. 4). In the last case, pure hexaferrite is obtained only after heating up to 7501C for 1 h. It can be concluded that, once the decarboxilation
Fig. 4. X-ray diffraction of samples HG200 and HG1000 heated at 6751C for 1 h.
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has finished, the formation of hexaferrite begins to take place, according to previous results [7]. Additionally, the transformation from monoferrite to hexaferrite is quicker as the specific surface area of the precursors increases from sample COM to samples G200 and G1000 (Table 1). In relation to
Fig. 5. TEM pictures of the barium hexaferrite samples: (a) HG200 heated at 6751C, (b) HG1000 heated at 7501C and (c) HCOM heated at 10001C.
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Table 2 Preparation conditions, particle size by TEM and magnetic properties of the barium ferrite particles (Ms=Saturation magnetisation, Mr =Ms =Squareness ratio, Hc =Coercivity). Standard deviations are included between brackets Sample
HG200 HG1000 HCOM
Formation temperature (1C) 675 750 1000
Particle size (nm) 100 (20) 460 (230) >1000
the last two samples, particle size distribution seems an important factor, which also affects the monoferrite–hexaferrite transformation. The formation of sample HG200 is quicker than sample HG1000 probably due to the monodispersed character of sample G200 (Table 1). The morphology and particle size of the hexaferrite samples, HG200, HG1000 and HCOM show significant differences when observed by TEM (Fig. 5 and Table 2). Sample HG200 obtained at 6751C (Fig. 5a) consists of pseudohexagonal particles of around 100 nm diameter, while HG1000 (Fig. 5b) obtained at 7501C consists of particles of 460 nm mean diameter. The differences between these samples are not only the particle size but also the size distribution. Thus, standard deviation for sample HG200 is 18 nm while for sample HG1000 the standard deviation is 250 nm (Table 2). This result shows that sample HG200 is a monodispersed solid of hexaferrite. As far as we know, no data have been reported in the literature about the formation of barium hexaferrite monodispersed particles by ceramic method. On the other hand, the reduction in particle size from the goethite precursor (240 nm) to the final hexaferrite particle (100 nm) can be explained by two main factors. First of all, a volume contraction is taking place due to the transformation from goethite to hematite [15] and secondly, there is a re crystallisation process where the hexaferrite particles grow topotactically in the (0 0 1) direction from the (1 1 1) plane of hematite [7]. A schematic representation of this transformation is shown in Fig. 6. Magnetic behaviour of these pure hexaferrite samples is presented in Fig. 7 and the hysteresis
Magnetic properties Ms (emu/g)
Mr =Ms
Hc (Oe)
60 47 48
0.50 0.51 0.26
4500 3200 350
Fig. 6. Schematic representation of the transformation from hematite to barium hexaferrite.
Fig. 7. Hysteresis loops at room temperature of the barium hexaferrite samples HG200, HG1000 and HCOM.
parameters are included in Table 2. Hysteresis loops for samples HG200, HG1000 and HCOM showed significant differences in the saturation magnetisation (Ms) and coercivity values (Hc). The maximum Ms value is 60 emu/g for sample
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HG200, which is lower than the theoretical one calculated for single crystals of barium hexaferrite particles, i.e. 72 emu/g [16]. This can be explained partially by the lack of full saturation reached at the maximum applied field. Anyway the smallness in size of the particles can also contribute to that reduced value. Several theories, including surface effects, spin canting and sample inhomogeneity, have been proposed to account for the relatively low magnetisation in fine particles [17]. Thus, particles of similar size prepared by different methods have been reported to give rise to lower or similar Ms values [5,6], even when higher temperatures have been used in the synthesis. On the other hand, samples HG1000 and HCOM have similar Ms values, 47 and 48 emu/g, respectively. The low Ms value for these samples in comparison to sample HG200, could be a consequence of the wide particle size distribution where there is an important fraction of small particles contributing to reduce Ms due to surface effects. The squareness ratio, Mr =Ms ; for samples HG200 and HG1000 is around 0.5 (Table 2) that is close to the expected value for randomly packed single domain particles if a coherent magnetisation rotation reversal mechanism is assumed [18]. According to that, coercivity values are higher for these two samples in comparison to sample HCOM. The maximum coercivity is obtained for the sample with the smallest particle size, HG200 (4500 Oe) (Table 2). Values up to 6000 Oe have been reported for barium hexaferrite particles of similar size prepared by aerosol pyrolysis and sol–gel [3,4] but in those cases it was necessary to heat the sample up to 9001C, 2001C higher than the temperature used to prepare sample HG200. On the other hand, the squareness ratio for sample HCOM is only 0.26 (Table 2), which can be attributed to the existence of a multidomain structure [18]. This sample consists of particles larger than 1 mm together with a very wide size distribution (Fig. 5c), which means that there is an important fraction of particles over the critical diameter reported for single domain behaviour for barium ferrite, 900 nm [1]. The multidomain behaviour and therefore the mechanism of magnetisation reversal by domain wall motion explain
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the reduced coercivity, 350 Oe, observed for this sample (Table 2).
4. Conclusions It can be concluded that uniform and monodomain particles of BaFe12O19 can be prepared by solid state reaction at a temperature lower than 7001C whenever an iron oxide-hydroxide of small particle size and narrow size distribution is used as precursor. In this work, pure hexaferrite particles have been obtained after heating at 6751C for 1 h a mixture of barium carbonate and acicular goethite particles of 200 nm in length. The sample consists of pseudo-hexagonal particles of around 100 nm in diameter and a very narrow size distribution (s ¼ 20). Magnetic properties of this sample show a saturation magnetisation of around 60 emu/g and a coercivity of 4500 Oe, according to a single domain behaviour of the particles.
Acknowledgements This investigation has been supported by the Regional Government of Madrid (CAM), Spain, Reference No. 07N/0057/1998, and the Ministry of Education of Spain under contract No. 950300-OP.
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