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Ba ratio to obtain single phase BaFe12O19 prepared by ammonium nitrate melt technique
Journal of Alloys and Compounds 428 (2007) 17–21
Finding optimal Fe/Ba ratio to obtain single phase BaFe12O19 prepared by ammonium nitrate melt technique Ugur Topal a,∗ , Husnu Ozkan b , Lev Dorosinskii a a b
˙ TUBITAK-UME, National Metrology Institute, P.K. 54, 41470 Kocaeli, Turkey Middle East Technical University, Physics Department, 06531 Ankara, Turkey Received 2 March 2006; accepted 17 March 2006 Available online 19 April 2006
Abstract In the present work, the magnetic properties and the phase composition of barium ferrite BaFe12 O19 powders prepared by the ammonium nitrate melt technique with Fe/Ba ratios varying from 2 to 13 was investigated. We observed that BaFe2 O4 phase is formed at low values of Fe/Ba ratios (2–6) and ␣-Fe2 O3 phase is formed at high values of Fe/Ba ratio as secondary phases. BaFe2 O4 phase could be easily eliminated from the structure by washing the annealed powders with diluted HCl. HCl-washing is not effective for the elimination of ␣-Fe2 O3 phase. Consequently, we succeed to obtain high quality single phase BaFe12 O19 powders at low Fe/Ba ratios. Highest specific saturation magnetization Ms of the powders with Fe/Ba ratio of 2 was determined to be 66.7 emu/g. © 2006 Elsevier B.V. All rights reserved. Keywords: Permanent magnets; Chemical synthesis; Magnetization; Magnetic measurements; Barium ferrites; BaFe12 O19 ; Fe/Ba ratio
1. Introduction Hard magnetic barium hexaferrite, BaFe12 O19 , has been widely investigated during the last decades due to its large magneto crystalline anisotropy, high Curie temperature, relatively high saturation magnetization and chemical stability. These features make it good for many applications such as permanent magnets, in a wide variety of devices used in information storage media, communications, electrical power generation and distribution. In order to fulfill the properties required for these purposes, such as fine particles for use in high density magnetic recording media, it is necessary to find the best synthesis conditions. Conventional way of producing these materials is the solidstate reaction method in which the oxides or carbonates of barium and iron are mixed and annealed at high temperatures (≥1200 ◦ C). The solid-state reaction method has some inherent disadvantages such as production of chemically inhomogeneous coarse powders as a consequence of high temperature annealing and introduction of impurities during ball milling. To avoid
∗
Corresponding author. Tel.: +90 262 679 50 00; fax: +90 262 679 50 01. E-mail address: [email protected] (U. Topal).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.047
these detrimental effects seen in conventional solid-state reaction method, various chemistry-based synthesis routes have been proposed. These are namely co-precipitation [1,2], glass crystallization [3], hydrothermal synthesis [4,5], sol–gel technique [6], organometallic precursor method [7] and microemulsion synthesis [8,9]. Recently, we have presented a new chemical method to synthesize barium hexaferrite (BaFe12 O19 ), which is called ammonium nitrate melt technique (ANMT) [10]. Our experiments have shown that BaFe12 O19 phase is formed at temperatures as low as 800 ◦ C and phase formation is completed at 850 ◦ C just after 1 h of annealing. On the other hand, impurity phases always exist in the structure even after annealing at 1200 ◦ C. Magnetization measurements have shown that the magnetic parameters, such as saturation magnetization Ms and coercieve field Hc , are better than those of the samples obtained by the solid-state reaction method but they are still quite low compared with the theoretical predictions [11,12]. It is beyond doubt to say that the synthesis process needs to be optimized to eliminate impurity phases and thus to obtain good quality samples. Many studies have shown that finding the optimal Fe/Ba ratio is important if formation of impurity phases should be avoided [13–19]. It is also interesting to note that this optimal ratio is different for different synthesis routes. For instance, optimal Fe/Ba
18
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21
ratios were reported to be around 11 for sol–gel and microwaveinduced combustion methods [16,17], 4 for hydrothermal route [18] and 12 for ball milling technique [19]. It is also quite interesting that good quality thin films, which were synthesized by sol–gel method, were obtained when the Fe/Ba ratio was chosen as 2.28 [20]. In the case of chemical synthesis routes, it has been proposed that deviation from the BaFe12 O19 stoichiometry towards Ba-rich side should be considered due to variations in the solubility of the Fe3+ and Ba2+ in aqueous media [21]. As a second solution for eliminating the undesired impurity phases, the high temperature annealed powders can be washed with HCl. S¨urig et al. [14] pointed out that the barium monoferrite phase (BaFe2 O4 ) dissolves in HCl more easily than the barium hexaferrite (BaFe12 O19 ). They also reported that the etching process is not effective for the removal of ␣-Fe2 O3 phase. In the present work, we investigate the influence of Fe/Ba ratio (from 2 to 13) and HCl-washing on structural and magnetic properties of BaFe12 O19 prepared using ANMT, systematically, in order to find best synthesis conditions. 2. Experimental Required amounts of BaCO3 and Fe2 O3 powders of high purity (99%) were weighed and mixed at different Fe/Ba atomic ratios (2, 3, 4, 6, 8, 9, 10, 10.5, 11, 11.5, 12 and 13). The mixed powder was put into the melted ammonium nitrate in a glass beaker. Here, we must note that powder/ammonium nitrate ratio was 1/3 in weight. The solution was kept on a heater until the liquid disappears and a precipitate is formed at the bottom of the beaker. The precipitate was then heated at 260 ◦ C for 1 day and subsequently at 450 ◦ C for 5 h using a box furnace. This preheating was just to eliminate the organic compounds. Then, the powders were divided into several parts and independently heat treated at 800, 850, 900, 1000, 1100 and 1200 ◦ C for 1 h to find the temperature at which best magnetic and structural properties are obtained. Finally, the powders were washed with HCl. Structural analysis was done by Rigaku-Minflex diffractometer (Cu K␣) utilizing the ICDD qualitative analysis software and by scanning electron microscope (JEOL 6335F Field Emission Gun). The magnetic properties of samples were examined at room temperature using both vibrating sample magnetometer (LDJ Electronics Inc., Model 9600) with the maximum field up to 1.6 T and SQUID magnetometer (Quantum Design, MPMS 5) up to 5 T to crosscheck the results.
Fig. 1. The X-ray diffraction patterns of powders with Fe/Ba ratios of 3 and 10, heat treated at 1100 ◦ C for 1 h.
ples just to show what happens at low and high Fe/Ba ratios. All the peaks were indexed to the BaFe12 O19 phase for the samples with Fe/Ba ratio of 2 and 3. Well-defined sharp peaks indicate the good crystalline quality of these samples. Peaks’ sharpness decreases with increasing the Fe/Ba ratio. If we look closely at the XRD patterns of the samples with the Fe/Ba ratio of 10 and 12, we can still see ␣-Fe2 O3 phase even after HCL-washing. This means that HCl-washing is effective for elimination of the BaFe2 O4 phase, but it is not effective for the elimination of the ␣-Fe2 O3 phase. This is in good agreement with the results of S¨urig et al. [14]. Fig. 3 shows SEM micrographs of the samples heat treated at 1100 ◦ C with the Fe/Ba ratios of: (a) 12, (b) 8 and (c) 2. Typical grain size in all samples is below 1 m. At the same time, it was observed that individual grains are not distributed homogenously, but rather tend to agglomerate forming larger bundles. Smaller grain sizes and more homogenous distribution of grains can be achieved by using ball milling but it is not the aim of this work. Hysteresis loops of the HCl-washed samples prepared at different Fe/Ba ratios and heat treated at 1100 ◦ C are shown in
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of powders with Fe/Ba ratios of 3 and 10, heat treated at 1100 ◦ C for 1 h. Main peaks of BaFe12 O19 phase are clearly seen in the diffraction pattern, but BaFe2 O4 is seen to be the dominant phase in the sample with the Fe/Ba ratio of 3. Also, we did not observe any reflections of ␣-Fe2 O3 phase in this sample. On the other hand, the major phase in the sample with the Fe/Ba ratio of 10 is BaFe12 O19 . Contrary to the case of Fe/Ba ratio equal 3, ␣-Fe2 O3 is seen as impurity phase and there is no trace of BaFe2 O4 phase. Combining all structural analysis for different Fe/Ba ratios (from 2 up to 13), it will be correct to say that while BaFe2 O4 is formed at low Fe/Ba ratios in addition to the BaFe12 O19 phase, the secondary phase at high Fe/Ba ratios is ␣-Fe2 O3 . We have washed all samples with HCl to eliminate impurity phases. Fig. 2 shows the X-ray diffraction patterns of samples with Fe/Ba ratio of 2, 3, 10 and 12. We have selected these sam-
Fig. 2. The X-ray diffraction patterns of HCl-washed samples with Fe/Ba ratio of 2, 3, 10 and 12.
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21
19
Fig. 4. (a) Hysteresis loops of the HCl-washed powders prepared at different Fe/Ba ratios (2, 3, 8 and 12) and heat-treated at 1100 ◦ C. (b) Hysteresis loops of the HCl-washed powders with Fe/Ba ratio of 2 obtained by SQUID magnetometer up to 5 T and VSM up to 1.6 T.
Fig. 3. The SEM micrograph of the samples heat-treated at 1100 ◦ C with Fe/Ba ratios of: (a) 12, (b) 8 and (c) 2.
Fig. 4. As seen, there is considerable difference in the magnetization values of the samples at different Fe/Ba ratios. In good agreement with the structural analysis, the high purity sample with the Fe/Ba ratio of 2 has the highest specific saturation mag-
netization (Ms ). Its Ms value is 66.7 emu/g, which is near to the theoretically estimated value of 72 emu/g [11,12]. Magnetization values decrease upon increasing the Fe/Ba ratio. Low values at high Fe/Ba ratios can be attributed to the presence of the ␣-Fe2 O3 phase. That is, ␣-Fe2 O3 phase deteriorates the magnetization significantly. For instance, while the single-phase sample with the Fe/Ba ratio of 2 has a specific magnetization value of 63 emu/g at 1.6 T, ␣-Fe2 O3 -rich sample with the Fe/Ba ratio of 12 exhibits a value of 36.7 emu/g. Table 1 shows the evolution of magnetic parameters of BaFe12 O19 as a function of Fe/Ba ratio, HCl-washing and calcination temperature. It is clearly seen that magnetization values drastically increase with HCl-washing. For instance, Ms value of the sample with the Fe/Ba ratio of 3 heat-treated at 1100 ◦ C increases from 9.3 to 60.3 emu/g after HCl-washing. On the other hand, it is interesting to note that Ms value for the sample with the Fe/Ba ratio of 11.5 increases just from 31.0 to 36.8 emu/g. In the former situation, elimination of the antiferromagnetic BaFe2 O4 phase as a result of HCl-washing must be responsible for such a large difference. On the other hand, we know from the results of the structural analysis that the sample with Fe/Ba ratio of 11.5 contains only ␣-Fe2 O3 as impurity phase. XRD measurements have also shown that HCl-washing cannot eliminate ␣-Fe2 O3 phase from the structure. As a result, HCl-washing is seen as less
20
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21
Table 1 Effect of Fe/Ba ratio and HCl-washing on magnetic parameters of BaFe12 O19 powders calcined at different temperatures Fe/Ba
Calcination temperature (◦ C)
HCl
Mr (emu/g)
Ms (emu/g) (1.6 T)
Mr /Ms
Hc (Oe)
2 2 3 3 3 3 4 4 4 6 8 8 9 10 10 10.5 11 11.5 11.5 12 13
850 1100 850 850 1100 1100 1100 1200 1300 1100 1100 1100 1100 1100 1100 1100 1100 1100 1100 1100 1100
Yes Yes No Yes No Yes Yes Yes Yes Yes No Yes Yes No Yes Yes Yes No Yes Yes Yes
26.2 38.5 6.6 28.7 5.2 33.1 33.4 30.3 21.8 31.7 20.5 26.7 25.3 18.4 24.9 23.3 23.1 17.6 21.1 21.2 20.5
46.8 63 at 1.6 T, 66.7 at 5 T 11.9 50.8 9.3 60.3 59.2 57.3 55.2 56.8 36.0 46.4 44.3 31.8 43.6 41.2 40.3 31.0 36.8 36.7 36.0
0.559 0.577 0.554 0.565 0.559 0.548 0.564 0.528 0.394 0.558 0.569 0.575 0.571 0.578 0.571 0.565 0.573 0.567 0.573 0.577 0.569
3083 4228 2558 2974 4475 4242 4339 2292 1026 4074 4104 4045 3957 4160 4061 4002 4016 4070 4079 4099 4122
effective on the magnetization values of ␣-Fe2 O3 -rich samples. It is also clearly seen from Table 1 that optimal calcination temperature is 1100 ◦ C. Coercieve fields for all samples are between 4000 Oe and 4500 Oe. Although these values are nearly twice as high as for the samples obtained by the classical ceramic method (Hc ∼ 2200 Oe), they are still below the theoretically estimated ones (Hc ∼ 6700 Oe) [11,12]. The low values of the coercieve field can be attributed to the agglomerations of the particles. Table 1 also reveals that all the magnetic parameters increase with the increase of the calcination temperature up to 1100 ◦ C. While magnetization values (Mr and Ms ) stay constant above 1100 ◦ C, the coercive field starts to decrease. For instance, coercive field decreases down to1026 Oe for the sample with the Fe/Ba ratio of 4 after calcination at 1300 ◦ C. This behavior can be connected with the multi-domain nature of the grains. It is well known that grain sizes increase with increasing calcination temperature. Grains with dimensions above 1 m exhibit multi-domain character [22]. This is also supported by the Mr /Ms values. Mr /Ms values of the single-domain particles were theoretically estimated to be 0.5 [23]. In Table 1, samples calcined below 1300 ◦ C have the Mr /Ms value near 0.5. However, the sample calcined at 1300 ◦ C has Mr /Ms value of 0.394. As a result, calcination at 1300 ◦ C results in particles exhibiting multi-domain character and thus coercieve field becomes lower (see Ref. [10] for details). 4. Conclusion In the present work, we have investigated the role of initial Fe/Ba ratio and HCl-washing on the evolution of magnetic and structural properties of hard magnetic BaFe12 O19 synthesized using the ammonium nitrate melt technique. Our measurements have shown that, while antiferromagnetic BaFe2 O4 is
formed as an impurity phase at small Fe/Ba ratios (Fe/Ba = 2–6) in addition to BaFe12 O19 phase, ␣-Fe2 O3 is the only impurity phase at high Fe/Ba ratios (Fe/Ba = 8–13). These impurity phases were observed to be rather detrimental to the overall magnetization of the BaFe12 O19 . We could succeed to eliminate BaFe2 O4 phase easily by washing the product with diluted HCl. On the other hand, it is impossible to eliminate ␣-Fe2 O3 phase by HCl-washing. Consequently, single crystalline high quality BaFe12 O19 phase can be obtained with ANMT if Fe/Ba ratio is between 2 and 6 and the product is washed with diluted HCl. We achieved a specific saturation magnetization Ms value of 66.7 emu/g, which is close to the theoretical limit, and a coercieve field value of 4500 Oe. References [1] S.E. Jacobo, C. Domingo-Pascual, R. Rodriguez-Clemente, J. Mater. Sci. 32 (1997) 1025. [2] K. Haneda, C. Miyakama, H. Kojima, J. Am. Ceram. Soc. 57 (1974) 354. [3] O. Kabo, T. Ido, H. Yokoyama, IEEE Trans. Magn. 18 (1982) 1122. [4] A. Ataie, M.R. Piramoon, I.R. Harris, C.B. Ponton, J. Mater. Sci. 30 (1995) 5600. [5] H. Kumazawa, Y. Maeda, E. Sada, J. Mater. Sci. Lett. 14 (1995) 68. [6] W. Zhong, W.P. Ding, N. Zhang, J. Magn. Magn. Mater. 168 (1997) 196. [7] F. Licci, T. Besagni, IEEE Trans. Magn. 20 (1984) 1639. [8] V. Pillai, P. Kumar, D.O. Shah, J. Magn. Magn. Mater. 116 (1992) 299. [9] V. Pillai, P. Kumar, M.S. Multani, D.O. Shah, Colloids Surf. A 80 (1993) 69. [10] U. Topal, H. Ozkan, H. Sozeri, J. Magn. Magn. Mater. 284 (2004) 416. [11] H. Kojima, Fundamental properties of hexagonal ferrites, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, North-Holland, New York, 1982. [12] K. Haneda, H. Kojima, J. Appl. Phys. 44 (1973) 3760. [13] J. Huang, H. Zhuang, W. Li, Mater. Res. Bull. 38 (2003) 149.
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21 [14] C. S¨urig, K.A. Hempel, Ch. Sauer, J. Magn. Magn. Mater. 157/158 (1996) 268. [15] R.C. Pullar, A.K. Bhattacharya, Mater. Lett. 57 (2002) 537. [16] G. Mendoza-Suarez, M.C. Cisneros-Morales, M.M. Cisneros-Guerrero, K.K. Johal, H. Mancha-Molinar, O.E. Ayala-Valenzuela, J.I. EscalanteGarcia, Mater. Chem. Phys. 77 (2002) 796. [17] F. Yen-Pei, L. Cheng-Hsiung, P. Ko-Ying, Jpn. J. Appl. Phys. 42 (2003) 2681. [18] S. Che, J. Wang, Q. Chen, J. Phys. Condens. Matter 15 (2003) L335.
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[19] G. Mendoza-Suarez, J.A. Matutes-Aquino, J.I. Escalante-Garcia, H. Mancha-Molinar, D. Rios-Jara, K.K. Johal, J. Magn. Magn. Mater. 223 (2001) 55. [20] W.-T. Liu, J.-M. Wu, Mater. Chem. Phys. 69 (2001) 148. [21] X. Liu, J. Wang, L.M. Gan, S.-Ch. Ng, J. Magn. Magn. Mater. 195 (1999) 452. [22] L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, J. Magn. Magn. Mater. 193 (1999) 288. [23] S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964, pp. 554.
Finding optimal Fe/Ba ratio to obtain single phase BaFe12O19 prepared by ammonium nitrate melt technique Ugur Topal a,∗ , Husnu Ozkan b , Lev Dorosinskii a a b
˙ TUBITAK-UME, National Metrology Institute, P.K. 54, 41470 Kocaeli, Turkey Middle East Technical University, Physics Department, 06531 Ankara, Turkey Received 2 March 2006; accepted 17 March 2006 Available online 19 April 2006
Abstract In the present work, the magnetic properties and the phase composition of barium ferrite BaFe12 O19 powders prepared by the ammonium nitrate melt technique with Fe/Ba ratios varying from 2 to 13 was investigated. We observed that BaFe2 O4 phase is formed at low values of Fe/Ba ratios (2–6) and ␣-Fe2 O3 phase is formed at high values of Fe/Ba ratio as secondary phases. BaFe2 O4 phase could be easily eliminated from the structure by washing the annealed powders with diluted HCl. HCl-washing is not effective for the elimination of ␣-Fe2 O3 phase. Consequently, we succeed to obtain high quality single phase BaFe12 O19 powders at low Fe/Ba ratios. Highest specific saturation magnetization Ms of the powders with Fe/Ba ratio of 2 was determined to be 66.7 emu/g. © 2006 Elsevier B.V. All rights reserved. Keywords: Permanent magnets; Chemical synthesis; Magnetization; Magnetic measurements; Barium ferrites; BaFe12 O19 ; Fe/Ba ratio
1. Introduction Hard magnetic barium hexaferrite, BaFe12 O19 , has been widely investigated during the last decades due to its large magneto crystalline anisotropy, high Curie temperature, relatively high saturation magnetization and chemical stability. These features make it good for many applications such as permanent magnets, in a wide variety of devices used in information storage media, communications, electrical power generation and distribution. In order to fulfill the properties required for these purposes, such as fine particles for use in high density magnetic recording media, it is necessary to find the best synthesis conditions. Conventional way of producing these materials is the solidstate reaction method in which the oxides or carbonates of barium and iron are mixed and annealed at high temperatures (≥1200 ◦ C). The solid-state reaction method has some inherent disadvantages such as production of chemically inhomogeneous coarse powders as a consequence of high temperature annealing and introduction of impurities during ball milling. To avoid
∗
Corresponding author. Tel.: +90 262 679 50 00; fax: +90 262 679 50 01. E-mail address: [email protected] (U. Topal).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.047
these detrimental effects seen in conventional solid-state reaction method, various chemistry-based synthesis routes have been proposed. These are namely co-precipitation [1,2], glass crystallization [3], hydrothermal synthesis [4,5], sol–gel technique [6], organometallic precursor method [7] and microemulsion synthesis [8,9]. Recently, we have presented a new chemical method to synthesize barium hexaferrite (BaFe12 O19 ), which is called ammonium nitrate melt technique (ANMT) [10]. Our experiments have shown that BaFe12 O19 phase is formed at temperatures as low as 800 ◦ C and phase formation is completed at 850 ◦ C just after 1 h of annealing. On the other hand, impurity phases always exist in the structure even after annealing at 1200 ◦ C. Magnetization measurements have shown that the magnetic parameters, such as saturation magnetization Ms and coercieve field Hc , are better than those of the samples obtained by the solid-state reaction method but they are still quite low compared with the theoretical predictions [11,12]. It is beyond doubt to say that the synthesis process needs to be optimized to eliminate impurity phases and thus to obtain good quality samples. Many studies have shown that finding the optimal Fe/Ba ratio is important if formation of impurity phases should be avoided [13–19]. It is also interesting to note that this optimal ratio is different for different synthesis routes. For instance, optimal Fe/Ba
18
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21
ratios were reported to be around 11 for sol–gel and microwaveinduced combustion methods [16,17], 4 for hydrothermal route [18] and 12 for ball milling technique [19]. It is also quite interesting that good quality thin films, which were synthesized by sol–gel method, were obtained when the Fe/Ba ratio was chosen as 2.28 [20]. In the case of chemical synthesis routes, it has been proposed that deviation from the BaFe12 O19 stoichiometry towards Ba-rich side should be considered due to variations in the solubility of the Fe3+ and Ba2+ in aqueous media [21]. As a second solution for eliminating the undesired impurity phases, the high temperature annealed powders can be washed with HCl. S¨urig et al. [14] pointed out that the barium monoferrite phase (BaFe2 O4 ) dissolves in HCl more easily than the barium hexaferrite (BaFe12 O19 ). They also reported that the etching process is not effective for the removal of ␣-Fe2 O3 phase. In the present work, we investigate the influence of Fe/Ba ratio (from 2 to 13) and HCl-washing on structural and magnetic properties of BaFe12 O19 prepared using ANMT, systematically, in order to find best synthesis conditions. 2. Experimental Required amounts of BaCO3 and Fe2 O3 powders of high purity (99%) were weighed and mixed at different Fe/Ba atomic ratios (2, 3, 4, 6, 8, 9, 10, 10.5, 11, 11.5, 12 and 13). The mixed powder was put into the melted ammonium nitrate in a glass beaker. Here, we must note that powder/ammonium nitrate ratio was 1/3 in weight. The solution was kept on a heater until the liquid disappears and a precipitate is formed at the bottom of the beaker. The precipitate was then heated at 260 ◦ C for 1 day and subsequently at 450 ◦ C for 5 h using a box furnace. This preheating was just to eliminate the organic compounds. Then, the powders were divided into several parts and independently heat treated at 800, 850, 900, 1000, 1100 and 1200 ◦ C for 1 h to find the temperature at which best magnetic and structural properties are obtained. Finally, the powders were washed with HCl. Structural analysis was done by Rigaku-Minflex diffractometer (Cu K␣) utilizing the ICDD qualitative analysis software and by scanning electron microscope (JEOL 6335F Field Emission Gun). The magnetic properties of samples were examined at room temperature using both vibrating sample magnetometer (LDJ Electronics Inc., Model 9600) with the maximum field up to 1.6 T and SQUID magnetometer (Quantum Design, MPMS 5) up to 5 T to crosscheck the results.
Fig. 1. The X-ray diffraction patterns of powders with Fe/Ba ratios of 3 and 10, heat treated at 1100 ◦ C for 1 h.
ples just to show what happens at low and high Fe/Ba ratios. All the peaks were indexed to the BaFe12 O19 phase for the samples with Fe/Ba ratio of 2 and 3. Well-defined sharp peaks indicate the good crystalline quality of these samples. Peaks’ sharpness decreases with increasing the Fe/Ba ratio. If we look closely at the XRD patterns of the samples with the Fe/Ba ratio of 10 and 12, we can still see ␣-Fe2 O3 phase even after HCL-washing. This means that HCl-washing is effective for elimination of the BaFe2 O4 phase, but it is not effective for the elimination of the ␣-Fe2 O3 phase. This is in good agreement with the results of S¨urig et al. [14]. Fig. 3 shows SEM micrographs of the samples heat treated at 1100 ◦ C with the Fe/Ba ratios of: (a) 12, (b) 8 and (c) 2. Typical grain size in all samples is below 1 m. At the same time, it was observed that individual grains are not distributed homogenously, but rather tend to agglomerate forming larger bundles. Smaller grain sizes and more homogenous distribution of grains can be achieved by using ball milling but it is not the aim of this work. Hysteresis loops of the HCl-washed samples prepared at different Fe/Ba ratios and heat treated at 1100 ◦ C are shown in
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of powders with Fe/Ba ratios of 3 and 10, heat treated at 1100 ◦ C for 1 h. Main peaks of BaFe12 O19 phase are clearly seen in the diffraction pattern, but BaFe2 O4 is seen to be the dominant phase in the sample with the Fe/Ba ratio of 3. Also, we did not observe any reflections of ␣-Fe2 O3 phase in this sample. On the other hand, the major phase in the sample with the Fe/Ba ratio of 10 is BaFe12 O19 . Contrary to the case of Fe/Ba ratio equal 3, ␣-Fe2 O3 is seen as impurity phase and there is no trace of BaFe2 O4 phase. Combining all structural analysis for different Fe/Ba ratios (from 2 up to 13), it will be correct to say that while BaFe2 O4 is formed at low Fe/Ba ratios in addition to the BaFe12 O19 phase, the secondary phase at high Fe/Ba ratios is ␣-Fe2 O3 . We have washed all samples with HCl to eliminate impurity phases. Fig. 2 shows the X-ray diffraction patterns of samples with Fe/Ba ratio of 2, 3, 10 and 12. We have selected these sam-
Fig. 2. The X-ray diffraction patterns of HCl-washed samples with Fe/Ba ratio of 2, 3, 10 and 12.
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21
19
Fig. 4. (a) Hysteresis loops of the HCl-washed powders prepared at different Fe/Ba ratios (2, 3, 8 and 12) and heat-treated at 1100 ◦ C. (b) Hysteresis loops of the HCl-washed powders with Fe/Ba ratio of 2 obtained by SQUID magnetometer up to 5 T and VSM up to 1.6 T.
Fig. 3. The SEM micrograph of the samples heat-treated at 1100 ◦ C with Fe/Ba ratios of: (a) 12, (b) 8 and (c) 2.
Fig. 4. As seen, there is considerable difference in the magnetization values of the samples at different Fe/Ba ratios. In good agreement with the structural analysis, the high purity sample with the Fe/Ba ratio of 2 has the highest specific saturation mag-
netization (Ms ). Its Ms value is 66.7 emu/g, which is near to the theoretically estimated value of 72 emu/g [11,12]. Magnetization values decrease upon increasing the Fe/Ba ratio. Low values at high Fe/Ba ratios can be attributed to the presence of the ␣-Fe2 O3 phase. That is, ␣-Fe2 O3 phase deteriorates the magnetization significantly. For instance, while the single-phase sample with the Fe/Ba ratio of 2 has a specific magnetization value of 63 emu/g at 1.6 T, ␣-Fe2 O3 -rich sample with the Fe/Ba ratio of 12 exhibits a value of 36.7 emu/g. Table 1 shows the evolution of magnetic parameters of BaFe12 O19 as a function of Fe/Ba ratio, HCl-washing and calcination temperature. It is clearly seen that magnetization values drastically increase with HCl-washing. For instance, Ms value of the sample with the Fe/Ba ratio of 3 heat-treated at 1100 ◦ C increases from 9.3 to 60.3 emu/g after HCl-washing. On the other hand, it is interesting to note that Ms value for the sample with the Fe/Ba ratio of 11.5 increases just from 31.0 to 36.8 emu/g. In the former situation, elimination of the antiferromagnetic BaFe2 O4 phase as a result of HCl-washing must be responsible for such a large difference. On the other hand, we know from the results of the structural analysis that the sample with Fe/Ba ratio of 11.5 contains only ␣-Fe2 O3 as impurity phase. XRD measurements have also shown that HCl-washing cannot eliminate ␣-Fe2 O3 phase from the structure. As a result, HCl-washing is seen as less
20
U. Topal et al. / Journal of Alloys and Compounds 428 (2007) 17–21
Table 1 Effect of Fe/Ba ratio and HCl-washing on magnetic parameters of BaFe12 O19 powders calcined at different temperatures Fe/Ba
Calcination temperature (◦ C)
HCl
Mr (emu/g)
Ms (emu/g) (1.6 T)
Mr /Ms
Hc (Oe)
2 2 3 3 3 3 4 4 4 6 8 8 9 10 10 10.5 11 11.5 11.5 12 13
850 1100 850 850 1100 1100 1100 1200 1300 1100 1100 1100 1100 1100 1100 1100 1100 1100 1100 1100 1100
Yes Yes No Yes No Yes Yes Yes Yes Yes No Yes Yes No Yes Yes Yes No Yes Yes Yes
26.2 38.5 6.6 28.7 5.2 33.1 33.4 30.3 21.8 31.7 20.5 26.7 25.3 18.4 24.9 23.3 23.1 17.6 21.1 21.2 20.5
46.8 63 at 1.6 T, 66.7 at 5 T 11.9 50.8 9.3 60.3 59.2 57.3 55.2 56.8 36.0 46.4 44.3 31.8 43.6 41.2 40.3 31.0 36.8 36.7 36.0
0.559 0.577 0.554 0.565 0.559 0.548 0.564 0.528 0.394 0.558 0.569 0.575 0.571 0.578 0.571 0.565 0.573 0.567 0.573 0.577 0.569
3083 4228 2558 2974 4475 4242 4339 2292 1026 4074 4104 4045 3957 4160 4061 4002 4016 4070 4079 4099 4122
effective on the magnetization values of ␣-Fe2 O3 -rich samples. It is also clearly seen from Table 1 that optimal calcination temperature is 1100 ◦ C. Coercieve fields for all samples are between 4000 Oe and 4500 Oe. Although these values are nearly twice as high as for the samples obtained by the classical ceramic method (Hc ∼ 2200 Oe), they are still below the theoretically estimated ones (Hc ∼ 6700 Oe) [11,12]. The low values of the coercieve field can be attributed to the agglomerations of the particles. Table 1 also reveals that all the magnetic parameters increase with the increase of the calcination temperature up to 1100 ◦ C. While magnetization values (Mr and Ms ) stay constant above 1100 ◦ C, the coercive field starts to decrease. For instance, coercive field decreases down to1026 Oe for the sample with the Fe/Ba ratio of 4 after calcination at 1300 ◦ C. This behavior can be connected with the multi-domain nature of the grains. It is well known that grain sizes increase with increasing calcination temperature. Grains with dimensions above 1 m exhibit multi-domain character [22]. This is also supported by the Mr /Ms values. Mr /Ms values of the single-domain particles were theoretically estimated to be 0.5 [23]. In Table 1, samples calcined below 1300 ◦ C have the Mr /Ms value near 0.5. However, the sample calcined at 1300 ◦ C has Mr /Ms value of 0.394. As a result, calcination at 1300 ◦ C results in particles exhibiting multi-domain character and thus coercieve field becomes lower (see Ref. [10] for details). 4. Conclusion In the present work, we have investigated the role of initial Fe/Ba ratio and HCl-washing on the evolution of magnetic and structural properties of hard magnetic BaFe12 O19 synthesized using the ammonium nitrate melt technique. Our measurements have shown that, while antiferromagnetic BaFe2 O4 is
formed as an impurity phase at small Fe/Ba ratios (Fe/Ba = 2–6) in addition to BaFe12 O19 phase, ␣-Fe2 O3 is the only impurity phase at high Fe/Ba ratios (Fe/Ba = 8–13). These impurity phases were observed to be rather detrimental to the overall magnetization of the BaFe12 O19 . We could succeed to eliminate BaFe2 O4 phase easily by washing the product with diluted HCl. On the other hand, it is impossible to eliminate ␣-Fe2 O3 phase by HCl-washing. Consequently, single crystalline high quality BaFe12 O19 phase can be obtained with ANMT if Fe/Ba ratio is between 2 and 6 and the product is washed with diluted HCl. We achieved a specific saturation magnetization Ms value of 66.7 emu/g, which is close to the theoretical limit, and a coercieve field value of 4500 Oe. References [1] S.E. Jacobo, C. Domingo-Pascual, R. Rodriguez-Clemente, J. Mater. Sci. 32 (1997) 1025. [2] K. Haneda, C. Miyakama, H. Kojima, J. Am. Ceram. Soc. 57 (1974) 354. [3] O. Kabo, T. Ido, H. Yokoyama, IEEE Trans. Magn. 18 (1982) 1122. [4] A. Ataie, M.R. Piramoon, I.R. Harris, C.B. Ponton, J. Mater. Sci. 30 (1995) 5600. [5] H. Kumazawa, Y. Maeda, E. Sada, J. Mater. Sci. Lett. 14 (1995) 68. [6] W. Zhong, W.P. Ding, N. Zhang, J. Magn. Magn. Mater. 168 (1997) 196. [7] F. Licci, T. Besagni, IEEE Trans. Magn. 20 (1984) 1639. [8] V. Pillai, P. Kumar, D.O. Shah, J. Magn. Magn. Mater. 116 (1992) 299. [9] V. Pillai, P. Kumar, M.S. Multani, D.O. Shah, Colloids Surf. A 80 (1993) 69. [10] U. Topal, H. Ozkan, H. Sozeri, J. Magn. Magn. Mater. 284 (2004) 416. [11] H. Kojima, Fundamental properties of hexagonal ferrites, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, North-Holland, New York, 1982. [12] K. Haneda, H. Kojima, J. Appl. Phys. 44 (1973) 3760. [13] J. Huang, H. Zhuang, W. Li, Mater. Res. Bull. 38 (2003) 149.
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