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2SnxO19, x=0.0–2.0
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 814–818
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Preparation, characterization and magnetic properties of the doped barium hexaferrites BaFe12 2xCox/2Znx/2SnxO19, x=0.0–2.0 Ying Liu a, Michael G.B. Drew b, Yue Liu a,n, Jingping Wang c, Milin Zhang c a
College of Chemistry and Life Science, Shenyang Normal University, 253 Huanghe Beidajie, Shenyang, Liaoning Province 110034, PR China School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK c Harbin Engineering University, Harbin 150001, PR China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 October 2009 Received in revised form 5 November 2009 Available online 12 November 2009
A series of powders of M-typed barium hexaferrites doped with Co, Zn and Sn of general formula BaFe12–2xCox/2Znx/2SnxO19 (x =0–2.0) were prepared by the co-precipitation/molten salt method. The structures, particle morphology and magnetic properties of the products were characterized by X-ray powder diffraction, vibrating sample magnetometer and ESEM/EDX. The results show that the crystallinity of the samples decreases with increase in the doping amount x. When x is less than 0.6, it is possible to obtain perfectly crystallized hexagonal BaFe12–2xCox/2Znx/2SnxO19, where the diameters of the particles are around 500 nm. The saturation magnetization of pure barium ferrite BaFe12O19 produced with this method is 71.9 A m2 kg 1 at room temperature and the intrinsic coercivity (Hc) is 367.8 kA m 1. The doped barium hexaferrite powder obtained when x is between 0.3 and 0.4 exhibits high saturation magnetization and a temperature dependence of coercivity close to zero. & 2009 Elsevier B.V. All rights reserved.
Keywords: Co-precipitation/molten salt method Barium ferrite Doping Saturation magnetization Coercivity
1. Introduction It is well known that the magnetic properties of barium ferrite, BaFe12O19 (BaM) can be optimised for particular purposes through doping [1–15]. Barium ferrite and its doped materials have been used as chip inductors, and microwave absorbers in the GHz range, because they have greater permeability and higher magnetic resonance frequency than spinel ferrites. BaM is a hard ferrite with a hexagonal magneto-plumbite structure belonging to space group P63/mmc. Due to its high saturation magnetization (Ms), great coercivity (Hc), high magnetocrystalline anisotropy and excellent chemical stability, BaM is of much current scientific and technological interest as an important material used extensively in magnetic permanent, recording media. Advanced barium ferrite materials offer higher signal-to-noise ratio and are capable of supporting high densities. The magnetic recording media require high coercivity since keeping the recording information for a considerable time depends on magnetic permanence. The information is recorded by the metastable states of BaM. The magnetic stability of metastable states is related to the timedependent behavior of the material. This time-dependent behavior is known as the magnetic relaxation. Thermal activation is needed to overcome energy barriers between the metastable states. On the other hand, the extremely high-uniaxial anisotropy
n
Corresponding author. E-mail address: [email protected] (Y. Liu).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.11.009
of pure barium ferrite precludes its application in the field of magnetic recording. If the coercivity is too high, the magnetic head will be subject to magnetic saturation. So if these kinds of materials are used as longitudinal instead of perpendicular magnetic recording media [1], pure BaM is usually required to be doped with other cations in order to reduce its magnetocrystalline anisotropy. Cation doping is one way to tailor the BaM in order to meet the differing requirements of these specific uses. In the spinel structure only 1/8 of the tetrahedral holes were occupied. In magneto-plumbite, there are only 4 holes of fIV and 4 holes of fVI type in a unit cell and all of them are occupied by Fe3 + . These two kinds of holes have opposite spins to the Fe3 + in the other holes. Half of the 12k holes are occupied by Fe3 + for a molecule with formula BaFe12O19. Different substituting cations have been reported to preferentially occupy specific sites when doping barium ferrite. If the Fe3 + ions in different holes are selectively substituted by other cations, the magnetic properties can be altered. Co2 + [2,3] and Sn4 + ions have been shown to have strong preference for the 2b, 4fVI and 12k sites [4,5]. For Co–Sn substituted barium ferrite, BaFe12 2xCoxSnxO19, the 2b, 4fVI positions are filled and such occupation decreases markedly the magnetocrystalline anisotropy of the BaM compound [6] since these sub-lattices show the highest contributions to the anisotropy. For Zn–Sn substituted barium ferrite, BaFe12 2xZnxSnxO19, Zn2 + ions have a preference of substituting for Fe3 + ions in 4fIV tetrahedral sites [7–9] and could yield an increase of saturation magnetization Ms. Substitutions in the 2a and 12k positions normally cause a decrease in Ms but an increase is reported as a
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consequence of substitution in the 4fVI and 4fIV sites. So, Zn2 + substitution could help to increase saturation magnetization Ms and Sn4 + substitution could decrease the temperature dependence of coercivity dHc/dT. But in most literature reports, it is found that doping decreases Ms. Cr3 + ions preferentially occupy the octahedral 2a, 4fVI and 12k sites. With Cr3 + ions entering the Fe3 + crystallographic 2a and 12k sites, saturation magnetization dramatically falls. But the increase of coercivity in that case is attributed to its finer grains [10]. In addition to one and two component doping, three component doping such as BaFe9.6Co0.8Ti0.8M0.8O19 was also investigated [11]. In addition to altering the magnetic properties by substituting Fe3 + with other cations, the other holes are also candidates for occupation by the doping ions so that the properties of BaM are optimised but it is, of course, necessary for the required symmetry and charge balance to be maintained. The doped ions can also alter the super-exchange between the two sub-lattices of Fe3 + that causes the magnetic moments to align in an anti-parallel fashion in the ferrimagnetic barium ferrite [9,12]. Dilute level of impurities can have a noticeable effect on Hc and Ms values. Both the electrical and magnetic properties of substituted BaM ferrites are strongly dependent on the synthesis conditions [2,13–20]. Structure similarity is another consideration in doping. For example, BaFe2O4 and Fe3O4 are similar in crystal structure. BaFe2O4 could be considered as a substance in which the Fe2 + in Fe3O4 had been replaced by Ba2 + even though Ba2 + would replace O2 . In any attempted synthesis of barium ferrite, BaFe2O4 may be obtained if an excess of Ba2 + compared with the stoichiometric ratio of Ba/Fe in BaFe12O19 is used. In the same synthesis if Fe2 + and Fe3 + are used instead of only Fe3 + , Fe3O4 may be obtained if an excess of Fe2 + is used [12]. The same strategy can also be used in doping to alter the magnetic properties of barium ferrite. So there are a variety of ways to modify the magnetic properties of barium ferrite to satisfy the different applications. In this paper, three component doped BaFe12–2xCox/2Znx/2 SnxO19 ultrafine particles with 0rx r2 were prepared by the co-precipitation/molten salt approach. The coercivity of the product was found to be intermediate between those of BaFe12 2xCoxSnxO19 and BaFe12 2xZnxSnxO19 for the same doping level. The temperature dependence of coercivity Hc was also investigated.
2. Experimental BaFe12–2xCox/2Znx/2SnxO19 ultrafine particles with 0 rxr2 were prepared via the chemical co-precipitation/molten salt route. All the starting materials (FeCl3, BaCl2, CoCl2, SnCl2 and anhydrous Na2CO3) were analytical grade. An aqueous solution of the metallic chlorides Ba2 + , Fe3 + , Co2 + , Zn2 + and Sn4 + , in the ratio required for the ferrite was prepared, except that the mole ratio of Fe and dopant Co–Zn–Sn over Ba was chosen to be 11:1, and added to an aqueous solution of 60% in excess of anhydrous Na2CO3 and stirred for 1 h at 70 1C. The solution was then cooled and filtered off, washed thoroughly with deionized water until no Cl could be detected, and dried at a temperature of 80 1C. Appropriate amounts of the intermediate and molten salt KCl were mixed with 1:1 weight ratios. The co-precipitate/molten salt mixtures were heated at 450 1C for 2 h. The heat treatment at a low temperature for 2 h in the beginning is a very important technique step and will be discussed in detail in a separate paper in preparation. The temperature was then raised from 450 to 950 1C at a rate of 15 1C/min, kept at 950 1C for 4 h, then cooled. The product was washed with hot deionized water several times until the water became Cl free. The Co–Zn–Sn substituted barium ferrite hexagonal particles were then dried at a
815
temperature of 80 1C. BaFe12 2xCoxSnxO19 and BaFe12 2xZnx SnxO19 were prepared similarly. The crystalline phases of the samples were identified by powder X-ray diffractograms with CuKa radiation using a Y-500 X-ray powder diffractometer. The magnetic measurements were measured at 30 and 188 1C using a vibrating vample vagnetometer (VSM-7300, Lakeshore) with an applied field of up to 10 kOe. The microstructure was observed using ESEM/EDX (Environmental scanning electron microscopy/electron dispersive X-ray analysis, FEI/Philips XL-30).
3. Result and discussion 3.1. Structure and particle morphology of the samples In the preparation of BaFe12O19 using this co-precipitation/ molten salt method, the optimum ratio of Ba:Fe was found to be 1:11 [12] and this ratio was used for all preparations of BaFe12–2x Cox/2Znx/2SnxO19(x= 0.2 2.0). The XRD patterns are shown in Fig. 1. The X-ray diffraction patterns of all the samples were well matched with single phase M-type barium ferrite (JCPDS card no. 39-1433), thus providing good evidence that the doped ions replaced Fe3 + . The XRD diffraction peaks were broader and weaker as x increased, indicating that the crystallinity decreased and the particle grains became smaller as Fe3 + was substituted by Zn2 + , Co2 + and Sn4 + , presumably as a consequence of the fact that the symmetry of the crystal lattice was reduced when Fe3 + was substituted by the three different kinds of ions. Fig. 2(1–4) shows SEM representative micrographs of the samples with different amounts of substitution. As shown in the figure, the phases were perfect crystalline hexaferrite when x =0.2 (Fig. 2-1) and 1.0 (Fig. 2-2). The diameters of most of the grains were about 500 nm. As the amount of substituent x was increased to 1.4 and 1.8, the sizes of the grains did not change much, but the plates became thinner and the crystallinity was decreased. This result can be explained as follows: the reactants can be made more homogeneous by the co-precipitation/molten approach, and the calcination temperature can be decreased by the use of the molten reagent KCl [16]. So the perfect crystalline phase can be achieved at lower temperature than from the more usual synthesis methods. The radii of Zn2 + , Sn4 + , [9] Co2 + are only slightly different from Fe3 + so their substitution in small amounts will not affect the crystal structure and morphology of the samples significantly but the lattice will be distorted and the crystallinity reduced when the amount is increased. The observation that the grain size decreases with increasing doping concentration x was also noted in previous work with substituents such as Zn2 + –Sn4 + [8], Co2 + –Ti4 + [6] and Cr3 + [10]. EDS diagrams for BaFe12O19 and BaFe10.0Zn0.5Co0.5Sn1.0O19 samples are shown in Fig. 3. Zn2 + , Co2 + and Sn4 + have been detected in the EDS diagram for BaFe10.0Zn0.5Co0.5Sn1.0O19, and the XRD patterns show that it is crystallized as a single phase, thus proving that dopants Zn2 + , Co2 + and Sn4 + replaced Fe3 + in the lattice.
3.2. Magnetic properties Hysteresis loops for the samples BaFe12O19, BaFe11.6Zn0.1 Co0.1Sn0.2O19 and BaFe10.0Zn0.5Co0.5Sn1.0O19 are given in Fig. 4. As shown in the figure, Hc decreases rapidly with increasing x, thus is 367.8 kA m 1 at x =0.0, 141.0 kA m 1 at x =0.2 and 22.4 kA m 1 at x= 1.0. The hard pure ferrite became softer as more Fe3 + was substituted by Zn2 + , Co2 + and Sn4 + .
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Fig. 1. XRD partterns of the samples BaFe12–2xCox/2Znx/2SnxO19 prepared by the co-precipitation/molten salt method. M: barium ferrite. (1) BaFe12O19: JCPDS 39-1433; (2) BaFe12O19; (3) BaFe11.6Zn0.1Co0.1Sn0.2O19; (4) BaFe10.8Zn0.3Co0.3Sn0.6O19; (5) BaFe10.4Zn0.4Co0.4Sn0.8O19; (6) BaFe10.0Zn0.5Co0.5Sn1.0O19; (7) BaFe9.6Zn0.6Co0.6Sn1.2O19; (8) BaFe9.2Zn0.7Co0.7Sn1.4O19; (9) BaFe8.4Zn0.9Co0.9Sn1.8O19; (10) BaFe8Zn1.0Co1.0Sn2.0O19.
Fig. 2. SEM images of BaFe12–2xCox/2Znx/2SnxO19 samples under parallel conditions: (1) BaFe11.6Zn0.1Co0.1Sn0.2O19; (2) BaFe10.0Zn0.5Co0.5Sn1.0O19; (3) BaFe9.2Zn0.7Co0.7Sn1.4O19; (4) BaFe8.4Zn0.9Co0.9Sn1.8O19.
Magnetic properties versus the amount x of substituents for samples BaFe12–2xCox/2Znx/2SnxO19, BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19 are shown in Fig. 5. BaFe12–2xCox/2Znx/2 SnxO19, BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19 were produced under the same conditions and compared with the same amount of substituents in Fig. 5. Hc values were obtained from the hysteresis loop data. The zero-field saturation
magnetizations Ms were determined by the law of approach to saturation (LAS): MðHÞ ¼ Ms ð1-a1 =H-a2 =H2 -a3 =H3 Þ þ wp H
ð1Þ
where Ms is the zero-field saturation magnetization, a1 the inhomogeneity parameter, a2 the anisotropy parameter and wp the high-field differential susceptibility. wp in Eq. 1 can be safely
ARTICLE IN PRESS Y. Liu et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 814–818
817
Fig. 3. EDS diagrams for BaFe12O19 (upper) and BaFe10.0Zn0.5Co0.5Sn1.0O19 (lower) samples.
neglected since the curves in Fig. 4 level out for high magnetic field [9]. a3 can be neglected since the magnetic field is high. So, Eq. 2 was used to calculate Ms in this study: MðHÞ ¼ Ms ð1-a1 =H-a2 =H2 Þ
Fig. 4. Hysteresis loops for the samples BaFe12O19, BaFe11.6Zn0.1Co0.1Sn0.2O19 and BaFe10.0Zn0.5Co0.5Sn1.0O19.
Fig. 5. Magnetic properties versus amount x of substituents for samples BaFe12–2x Cox/2Znx/2SnxO19, BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19.
ð2Þ
As shown in Fig. 5, the Ms of BaFe12–2xCox/2Znx/2SnxO19 decreases slowly, while Hc decreases sharply for x o0.6. Ms decreases linearly, while Hc does not change much in the range 0.6 oxo1.2. Most reports in the literature show Hc decreasing with increasing doping amounts of Co2 + , Zn2 + or Sn4 + . The Ms of BaFe12–2xCoxSnxO19 first increases then decreases as x increases (Fig. 5). This is partly because Co2 + and Sn4 + ions have shown a preference for the 2b, 4fVI and 12k sites [4,5]. The fact that Zn2 + ions have a preference for substituting Fe3 + ions in 2b, 4fIV sites [6] might account for the decrease of Ms for BaFe12–2xZnxSnxO19 as x increases. The values of Ms are greatly dependent on the synthesis conditions. For example as reported by Mendoza-Suarez et al. [6] who prepared the barium ferrites via the sol–gel route, the Ms values of Co–Ti doped BaM were lower than those of Zn–Ti doped Ba–M when x was small, but the reverse was true when x was higher. Thus, the Ms value of Co–Ti doped BaM became higher when x was greater than 0.4 (heat treated at 950 1C) or greater than 0.6 (heat treated at 1000 1C), indicating that lower temperature treatment extended the region of x in which Co2 + doped BaM possesses higher Ms than Zn2 + doped BaM. In our coprecipitation/molten salt route, the calcine temperature can be decreased by using the molten reagent KCl. So it is reasonable that the Ms value of BaFe12–2xCoxSnxO19 is consistently higher than that of BaFe12–2xZnxSnxO19 as shown in Fig. 5, indicating that the region of higher Ms for Co2 + doped BaM was further extended to smaller x. It is worth noting that Zn is a diamagnetic material and Co is a ferromagnetic material, and above the Curie temperature Co becomes paramagnetic. Although Zn is diamagnetic and Co is ferromagnetic below Curie temperature, these facts do not allow us to predict the properties of the complex material with Zn2 + and/or Co2 + substitution in the lattice. However, there is an effect of temperature when doping occurs which is not fully understood although it could be concluded that any method such as the molten-salt method which allows a lower synthesis temperature might contribute in an increase of Ms for Co2 + doped BaM. This would suggest, on the other hand, that higher temperature
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while when x= 0.4, Ms = 67.0 A m2 kg 1, Hc = 65.5 kA m 1 at room temperature.
4. Conclusions
Fig. 6. The effect of varying the amount x of substituents on the average temperature coefficient of Hc for the samples BaFe12–2xCox/2Znx/2SnxO19, x= 0.0–0.6.
processes that introduce dopants Co2 + –Sn4 + in the lattice would result in lower Ms. It is also interesting to note that the value of Ms for BaFe12–2x Cox/2Znx/2SnxO19 was intermediate between values for BaFe12–2x ZnxSnxO19 and BaFe12–2xCoxSnxO19 at the same level of doping. This result for Ms indicates that relatively simple methods can be used to tailor BaM to make it suitable for different uses. Comprehending complex word with simplicity is one of the goal that science research endeavors to reach [21]. The effect of varying the amount x of substituent on the average temperature coefficient of Hc for the samples BaFe12–2x Cox/2Znx/2SnxO19 was shown in Fig. 6. It was obtained by Eq. (3):
DHc =DT ¼ ½Hc ð303 KÞ-Hc ð85 KÞ=218 K
ð3Þ
Hc (303 K) and Hc (85 K) are the values of Hc at temperatures of 303 and 85 K, respectively. With the increase of doping concentration x, the temperature dependence of coercivity DHc/DT showed a decrease from positive to negative, approaching zero, the point at which the coercivity is independent of temperature, at values of x between 0.3 and 0.4. The variation in DHc/DT shows that the temperature stability of coercivity can be controlled by the doping amount. Fang et al. [7] obtained a low positive temperature coefficient of coercivity of 1.9 Oe/K with BaFe12-2x ZnxSnxO19 for x =1.1 prepared using the co-precipitation method. It is clear that when substituting Fe3 + by the three components Zn2 + –Co2 + –Sn4 + , the temperature coefficient of coercivity can be further reduced. BaFe12–2xCox/2Znx/2SnxO19 has a higher Ms at room temperature and lower temperature coefficient of coercivity. Thus when x =0.3, Ms = 68.4 A m2 kg 1, and Hc =80.8 kA m 1;
BaFe12–2xCox/2Znx/2SnxO19 ultrafine particles with 0 rx r2 were prepared though the chemical co-precipitation/molten salt route. The symmetry and crystallinity decreased as x increased. The value of Ms of BaFe12–2xCox/2Znx/2SnxO19 was intermediate between those of BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19 at the same level of doping. This result for Ms indicates that simple methods can be used to tailor BaM to make it suitable for different uses. The temperature dependence of coercivity approaches 0 when x lies between 0.3 and 0.4, which indicates that the temperature stability of coercivity can be controlled by the doping amount. It also provides information for time-dependent behavior since thermal activation is needed to overcome energy barriers between the metastable states.
Acknowledgements This work was supported by Shenyang Normal University. References [1] Y. Li, R. Liu, Z. Zhang, C. Xiong, Mater. Chem. Phys. 64 (2000) 256. [2] K.K. Mallick, P. Shepherd, R.J. Green, J. Magn. Magn. Mater. 312 (2007) 418. [3] M. Be´gard, K. Bobzin, G. Bolelli, A. Hujanen, P. Lintunen, D. Lisjak, S. Gyergyek, L. Lusvarghi, Surf. Coatings Technol. 203 (2009) 3312. [4] X.Z. Zhou, A.H. Morrish, Z. Yang, H. Zeng, J. Appl. Phys. 75 (1994) 5556. [5] H.C. Frang, C.K. Ong, X.Y. Zang, Y. Li, X.Z. Wang, Z. Yang, J. Magn. Magn. Mater. 191 (1999) 277. [6] G. Mendoza-Suarez, L.P. Rivas-Vazquez, J.C. Corral-Huacuz, A.F. Fuentes, J.I. Escalante-Garcia, Physica B 339 (2003) 110. [7] H.C. Fang, C.K. Ong, X.Y. Zhang, Y. Li, X.Z. Wang, Z. Yang, J. Magn. Magn. Mater. 191 (1999) 277. [8] C.K. Ong, H.C. Fang, Z. Yang, Y. Li, J. Magn. Magn. Mater. 213 (2000) 413. [9] H.C. Fang, Z. Yang, C.K. Ong, Y. Li, C.S. Wang, J. Magn. Magn. Mater. 187 (1998) 129. [10] S. Ounnunkad, P. Winotai, J. Magn. Magn. Mater. 301 (2006) 292. [11] Y. Li, R. Liu, Z. Zhang, C. Xiong, Mater. Chem. Phys. 64 (2000) 256. [12] Y. Liu, M.G.B. Drew, J. Wang, M. Zhang, Y. Liu, J. Magn. Magn. Mater. 322 (2010) 366–374. [13] J. Slama, A. Gruskova, M. Papanova, D. Kevicka, R. Dosoudil, V.J. Warik, A. Gonzalez, G. Mendoza, J. Magn. Magn. Mater. 272–276 (2004) 385. [14] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Mater. Chem. Phys. 86 (2004) 132. [15] Y. Liu, J. Wang, M. Hu, M. Zhang, T. Xia, Chin. J. Appl. Chem. 24 (2007) 1182. [16] J. Wang, Y. Liu, M. Hu, M. Zhang, Chin. J. Aeronaut. 16 (2006) S206. [17] Y. Liu, Z. Ning, Y. Tan, J. Wang, M. Zhang, Chin. J. Appl. Chem. 24 (2007) 468. [18] Z. Ning, Y. Chen, Y. Liu, M. Zhang, S. Qi, C. Shu, Chin. Rare Earths 28 (2007) 43. [19] Y. Liu, J. Wang, Y. Tan, M. Zhang, J. Meng, J. Funct. Mater. 37A (2006) 974. [20] J. Wang, Y. Liu, M. Zhang, Y. Qiao, T. Xia, Chem. Res. Chin. Univ. 24 (2008) 525. [21] Y. Liu, Y. Liu, M. G. B. Drew, Structural Chemistry, in press. Available from: hhttp://dx.doi.org/10.1007/s11224-009-9519-8i.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation, characterization and magnetic properties of the doped barium hexaferrites BaFe12 2xCox/2Znx/2SnxO19, x=0.0–2.0 Ying Liu a, Michael G.B. Drew b, Yue Liu a,n, Jingping Wang c, Milin Zhang c a
College of Chemistry and Life Science, Shenyang Normal University, 253 Huanghe Beidajie, Shenyang, Liaoning Province 110034, PR China School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK c Harbin Engineering University, Harbin 150001, PR China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 October 2009 Received in revised form 5 November 2009 Available online 12 November 2009
A series of powders of M-typed barium hexaferrites doped with Co, Zn and Sn of general formula BaFe12–2xCox/2Znx/2SnxO19 (x =0–2.0) were prepared by the co-precipitation/molten salt method. The structures, particle morphology and magnetic properties of the products were characterized by X-ray powder diffraction, vibrating sample magnetometer and ESEM/EDX. The results show that the crystallinity of the samples decreases with increase in the doping amount x. When x is less than 0.6, it is possible to obtain perfectly crystallized hexagonal BaFe12–2xCox/2Znx/2SnxO19, where the diameters of the particles are around 500 nm. The saturation magnetization of pure barium ferrite BaFe12O19 produced with this method is 71.9 A m2 kg 1 at room temperature and the intrinsic coercivity (Hc) is 367.8 kA m 1. The doped barium hexaferrite powder obtained when x is between 0.3 and 0.4 exhibits high saturation magnetization and a temperature dependence of coercivity close to zero. & 2009 Elsevier B.V. All rights reserved.
Keywords: Co-precipitation/molten salt method Barium ferrite Doping Saturation magnetization Coercivity
1. Introduction It is well known that the magnetic properties of barium ferrite, BaFe12O19 (BaM) can be optimised for particular purposes through doping [1–15]. Barium ferrite and its doped materials have been used as chip inductors, and microwave absorbers in the GHz range, because they have greater permeability and higher magnetic resonance frequency than spinel ferrites. BaM is a hard ferrite with a hexagonal magneto-plumbite structure belonging to space group P63/mmc. Due to its high saturation magnetization (Ms), great coercivity (Hc), high magnetocrystalline anisotropy and excellent chemical stability, BaM is of much current scientific and technological interest as an important material used extensively in magnetic permanent, recording media. Advanced barium ferrite materials offer higher signal-to-noise ratio and are capable of supporting high densities. The magnetic recording media require high coercivity since keeping the recording information for a considerable time depends on magnetic permanence. The information is recorded by the metastable states of BaM. The magnetic stability of metastable states is related to the timedependent behavior of the material. This time-dependent behavior is known as the magnetic relaxation. Thermal activation is needed to overcome energy barriers between the metastable states. On the other hand, the extremely high-uniaxial anisotropy
n
Corresponding author. E-mail address: [email protected] (Y. Liu).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.11.009
of pure barium ferrite precludes its application in the field of magnetic recording. If the coercivity is too high, the magnetic head will be subject to magnetic saturation. So if these kinds of materials are used as longitudinal instead of perpendicular magnetic recording media [1], pure BaM is usually required to be doped with other cations in order to reduce its magnetocrystalline anisotropy. Cation doping is one way to tailor the BaM in order to meet the differing requirements of these specific uses. In the spinel structure only 1/8 of the tetrahedral holes were occupied. In magneto-plumbite, there are only 4 holes of fIV and 4 holes of fVI type in a unit cell and all of them are occupied by Fe3 + . These two kinds of holes have opposite spins to the Fe3 + in the other holes. Half of the 12k holes are occupied by Fe3 + for a molecule with formula BaFe12O19. Different substituting cations have been reported to preferentially occupy specific sites when doping barium ferrite. If the Fe3 + ions in different holes are selectively substituted by other cations, the magnetic properties can be altered. Co2 + [2,3] and Sn4 + ions have been shown to have strong preference for the 2b, 4fVI and 12k sites [4,5]. For Co–Sn substituted barium ferrite, BaFe12 2xCoxSnxO19, the 2b, 4fVI positions are filled and such occupation decreases markedly the magnetocrystalline anisotropy of the BaM compound [6] since these sub-lattices show the highest contributions to the anisotropy. For Zn–Sn substituted barium ferrite, BaFe12 2xZnxSnxO19, Zn2 + ions have a preference of substituting for Fe3 + ions in 4fIV tetrahedral sites [7–9] and could yield an increase of saturation magnetization Ms. Substitutions in the 2a and 12k positions normally cause a decrease in Ms but an increase is reported as a
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consequence of substitution in the 4fVI and 4fIV sites. So, Zn2 + substitution could help to increase saturation magnetization Ms and Sn4 + substitution could decrease the temperature dependence of coercivity dHc/dT. But in most literature reports, it is found that doping decreases Ms. Cr3 + ions preferentially occupy the octahedral 2a, 4fVI and 12k sites. With Cr3 + ions entering the Fe3 + crystallographic 2a and 12k sites, saturation magnetization dramatically falls. But the increase of coercivity in that case is attributed to its finer grains [10]. In addition to one and two component doping, three component doping such as BaFe9.6Co0.8Ti0.8M0.8O19 was also investigated [11]. In addition to altering the magnetic properties by substituting Fe3 + with other cations, the other holes are also candidates for occupation by the doping ions so that the properties of BaM are optimised but it is, of course, necessary for the required symmetry and charge balance to be maintained. The doped ions can also alter the super-exchange between the two sub-lattices of Fe3 + that causes the magnetic moments to align in an anti-parallel fashion in the ferrimagnetic barium ferrite [9,12]. Dilute level of impurities can have a noticeable effect on Hc and Ms values. Both the electrical and magnetic properties of substituted BaM ferrites are strongly dependent on the synthesis conditions [2,13–20]. Structure similarity is another consideration in doping. For example, BaFe2O4 and Fe3O4 are similar in crystal structure. BaFe2O4 could be considered as a substance in which the Fe2 + in Fe3O4 had been replaced by Ba2 + even though Ba2 + would replace O2 . In any attempted synthesis of barium ferrite, BaFe2O4 may be obtained if an excess of Ba2 + compared with the stoichiometric ratio of Ba/Fe in BaFe12O19 is used. In the same synthesis if Fe2 + and Fe3 + are used instead of only Fe3 + , Fe3O4 may be obtained if an excess of Fe2 + is used [12]. The same strategy can also be used in doping to alter the magnetic properties of barium ferrite. So there are a variety of ways to modify the magnetic properties of barium ferrite to satisfy the different applications. In this paper, three component doped BaFe12–2xCox/2Znx/2 SnxO19 ultrafine particles with 0rx r2 were prepared by the co-precipitation/molten salt approach. The coercivity of the product was found to be intermediate between those of BaFe12 2xCoxSnxO19 and BaFe12 2xZnxSnxO19 for the same doping level. The temperature dependence of coercivity Hc was also investigated.
2. Experimental BaFe12–2xCox/2Znx/2SnxO19 ultrafine particles with 0 rxr2 were prepared via the chemical co-precipitation/molten salt route. All the starting materials (FeCl3, BaCl2, CoCl2, SnCl2 and anhydrous Na2CO3) were analytical grade. An aqueous solution of the metallic chlorides Ba2 + , Fe3 + , Co2 + , Zn2 + and Sn4 + , in the ratio required for the ferrite was prepared, except that the mole ratio of Fe and dopant Co–Zn–Sn over Ba was chosen to be 11:1, and added to an aqueous solution of 60% in excess of anhydrous Na2CO3 and stirred for 1 h at 70 1C. The solution was then cooled and filtered off, washed thoroughly with deionized water until no Cl could be detected, and dried at a temperature of 80 1C. Appropriate amounts of the intermediate and molten salt KCl were mixed with 1:1 weight ratios. The co-precipitate/molten salt mixtures were heated at 450 1C for 2 h. The heat treatment at a low temperature for 2 h in the beginning is a very important technique step and will be discussed in detail in a separate paper in preparation. The temperature was then raised from 450 to 950 1C at a rate of 15 1C/min, kept at 950 1C for 4 h, then cooled. The product was washed with hot deionized water several times until the water became Cl free. The Co–Zn–Sn substituted barium ferrite hexagonal particles were then dried at a
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temperature of 80 1C. BaFe12 2xCoxSnxO19 and BaFe12 2xZnx SnxO19 were prepared similarly. The crystalline phases of the samples were identified by powder X-ray diffractograms with CuKa radiation using a Y-500 X-ray powder diffractometer. The magnetic measurements were measured at 30 and 188 1C using a vibrating vample vagnetometer (VSM-7300, Lakeshore) with an applied field of up to 10 kOe. The microstructure was observed using ESEM/EDX (Environmental scanning electron microscopy/electron dispersive X-ray analysis, FEI/Philips XL-30).
3. Result and discussion 3.1. Structure and particle morphology of the samples In the preparation of BaFe12O19 using this co-precipitation/ molten salt method, the optimum ratio of Ba:Fe was found to be 1:11 [12] and this ratio was used for all preparations of BaFe12–2x Cox/2Znx/2SnxO19(x= 0.2 2.0). The XRD patterns are shown in Fig. 1. The X-ray diffraction patterns of all the samples were well matched with single phase M-type barium ferrite (JCPDS card no. 39-1433), thus providing good evidence that the doped ions replaced Fe3 + . The XRD diffraction peaks were broader and weaker as x increased, indicating that the crystallinity decreased and the particle grains became smaller as Fe3 + was substituted by Zn2 + , Co2 + and Sn4 + , presumably as a consequence of the fact that the symmetry of the crystal lattice was reduced when Fe3 + was substituted by the three different kinds of ions. Fig. 2(1–4) shows SEM representative micrographs of the samples with different amounts of substitution. As shown in the figure, the phases were perfect crystalline hexaferrite when x =0.2 (Fig. 2-1) and 1.0 (Fig. 2-2). The diameters of most of the grains were about 500 nm. As the amount of substituent x was increased to 1.4 and 1.8, the sizes of the grains did not change much, but the plates became thinner and the crystallinity was decreased. This result can be explained as follows: the reactants can be made more homogeneous by the co-precipitation/molten approach, and the calcination temperature can be decreased by the use of the molten reagent KCl [16]. So the perfect crystalline phase can be achieved at lower temperature than from the more usual synthesis methods. The radii of Zn2 + , Sn4 + , [9] Co2 + are only slightly different from Fe3 + so their substitution in small amounts will not affect the crystal structure and morphology of the samples significantly but the lattice will be distorted and the crystallinity reduced when the amount is increased. The observation that the grain size decreases with increasing doping concentration x was also noted in previous work with substituents such as Zn2 + –Sn4 + [8], Co2 + –Ti4 + [6] and Cr3 + [10]. EDS diagrams for BaFe12O19 and BaFe10.0Zn0.5Co0.5Sn1.0O19 samples are shown in Fig. 3. Zn2 + , Co2 + and Sn4 + have been detected in the EDS diagram for BaFe10.0Zn0.5Co0.5Sn1.0O19, and the XRD patterns show that it is crystallized as a single phase, thus proving that dopants Zn2 + , Co2 + and Sn4 + replaced Fe3 + in the lattice.
3.2. Magnetic properties Hysteresis loops for the samples BaFe12O19, BaFe11.6Zn0.1 Co0.1Sn0.2O19 and BaFe10.0Zn0.5Co0.5Sn1.0O19 are given in Fig. 4. As shown in the figure, Hc decreases rapidly with increasing x, thus is 367.8 kA m 1 at x =0.0, 141.0 kA m 1 at x =0.2 and 22.4 kA m 1 at x= 1.0. The hard pure ferrite became softer as more Fe3 + was substituted by Zn2 + , Co2 + and Sn4 + .
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Fig. 1. XRD partterns of the samples BaFe12–2xCox/2Znx/2SnxO19 prepared by the co-precipitation/molten salt method. M: barium ferrite. (1) BaFe12O19: JCPDS 39-1433; (2) BaFe12O19; (3) BaFe11.6Zn0.1Co0.1Sn0.2O19; (4) BaFe10.8Zn0.3Co0.3Sn0.6O19; (5) BaFe10.4Zn0.4Co0.4Sn0.8O19; (6) BaFe10.0Zn0.5Co0.5Sn1.0O19; (7) BaFe9.6Zn0.6Co0.6Sn1.2O19; (8) BaFe9.2Zn0.7Co0.7Sn1.4O19; (9) BaFe8.4Zn0.9Co0.9Sn1.8O19; (10) BaFe8Zn1.0Co1.0Sn2.0O19.
Fig. 2. SEM images of BaFe12–2xCox/2Znx/2SnxO19 samples under parallel conditions: (1) BaFe11.6Zn0.1Co0.1Sn0.2O19; (2) BaFe10.0Zn0.5Co0.5Sn1.0O19; (3) BaFe9.2Zn0.7Co0.7Sn1.4O19; (4) BaFe8.4Zn0.9Co0.9Sn1.8O19.
Magnetic properties versus the amount x of substituents for samples BaFe12–2xCox/2Znx/2SnxO19, BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19 are shown in Fig. 5. BaFe12–2xCox/2Znx/2 SnxO19, BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19 were produced under the same conditions and compared with the same amount of substituents in Fig. 5. Hc values were obtained from the hysteresis loop data. The zero-field saturation
magnetizations Ms were determined by the law of approach to saturation (LAS): MðHÞ ¼ Ms ð1-a1 =H-a2 =H2 -a3 =H3 Þ þ wp H
ð1Þ
where Ms is the zero-field saturation magnetization, a1 the inhomogeneity parameter, a2 the anisotropy parameter and wp the high-field differential susceptibility. wp in Eq. 1 can be safely
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Fig. 3. EDS diagrams for BaFe12O19 (upper) and BaFe10.0Zn0.5Co0.5Sn1.0O19 (lower) samples.
neglected since the curves in Fig. 4 level out for high magnetic field [9]. a3 can be neglected since the magnetic field is high. So, Eq. 2 was used to calculate Ms in this study: MðHÞ ¼ Ms ð1-a1 =H-a2 =H2 Þ
Fig. 4. Hysteresis loops for the samples BaFe12O19, BaFe11.6Zn0.1Co0.1Sn0.2O19 and BaFe10.0Zn0.5Co0.5Sn1.0O19.
Fig. 5. Magnetic properties versus amount x of substituents for samples BaFe12–2x Cox/2Znx/2SnxO19, BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19.
ð2Þ
As shown in Fig. 5, the Ms of BaFe12–2xCox/2Znx/2SnxO19 decreases slowly, while Hc decreases sharply for x o0.6. Ms decreases linearly, while Hc does not change much in the range 0.6 oxo1.2. Most reports in the literature show Hc decreasing with increasing doping amounts of Co2 + , Zn2 + or Sn4 + . The Ms of BaFe12–2xCoxSnxO19 first increases then decreases as x increases (Fig. 5). This is partly because Co2 + and Sn4 + ions have shown a preference for the 2b, 4fVI and 12k sites [4,5]. The fact that Zn2 + ions have a preference for substituting Fe3 + ions in 2b, 4fIV sites [6] might account for the decrease of Ms for BaFe12–2xZnxSnxO19 as x increases. The values of Ms are greatly dependent on the synthesis conditions. For example as reported by Mendoza-Suarez et al. [6] who prepared the barium ferrites via the sol–gel route, the Ms values of Co–Ti doped BaM were lower than those of Zn–Ti doped Ba–M when x was small, but the reverse was true when x was higher. Thus, the Ms value of Co–Ti doped BaM became higher when x was greater than 0.4 (heat treated at 950 1C) or greater than 0.6 (heat treated at 1000 1C), indicating that lower temperature treatment extended the region of x in which Co2 + doped BaM possesses higher Ms than Zn2 + doped BaM. In our coprecipitation/molten salt route, the calcine temperature can be decreased by using the molten reagent KCl. So it is reasonable that the Ms value of BaFe12–2xCoxSnxO19 is consistently higher than that of BaFe12–2xZnxSnxO19 as shown in Fig. 5, indicating that the region of higher Ms for Co2 + doped BaM was further extended to smaller x. It is worth noting that Zn is a diamagnetic material and Co is a ferromagnetic material, and above the Curie temperature Co becomes paramagnetic. Although Zn is diamagnetic and Co is ferromagnetic below Curie temperature, these facts do not allow us to predict the properties of the complex material with Zn2 + and/or Co2 + substitution in the lattice. However, there is an effect of temperature when doping occurs which is not fully understood although it could be concluded that any method such as the molten-salt method which allows a lower synthesis temperature might contribute in an increase of Ms for Co2 + doped BaM. This would suggest, on the other hand, that higher temperature
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while when x= 0.4, Ms = 67.0 A m2 kg 1, Hc = 65.5 kA m 1 at room temperature.
4. Conclusions
Fig. 6. The effect of varying the amount x of substituents on the average temperature coefficient of Hc for the samples BaFe12–2xCox/2Znx/2SnxO19, x= 0.0–0.6.
processes that introduce dopants Co2 + –Sn4 + in the lattice would result in lower Ms. It is also interesting to note that the value of Ms for BaFe12–2x Cox/2Znx/2SnxO19 was intermediate between values for BaFe12–2x ZnxSnxO19 and BaFe12–2xCoxSnxO19 at the same level of doping. This result for Ms indicates that relatively simple methods can be used to tailor BaM to make it suitable for different uses. Comprehending complex word with simplicity is one of the goal that science research endeavors to reach [21]. The effect of varying the amount x of substituent on the average temperature coefficient of Hc for the samples BaFe12–2x Cox/2Znx/2SnxO19 was shown in Fig. 6. It was obtained by Eq. (3):
DHc =DT ¼ ½Hc ð303 KÞ-Hc ð85 KÞ=218 K
ð3Þ
Hc (303 K) and Hc (85 K) are the values of Hc at temperatures of 303 and 85 K, respectively. With the increase of doping concentration x, the temperature dependence of coercivity DHc/DT showed a decrease from positive to negative, approaching zero, the point at which the coercivity is independent of temperature, at values of x between 0.3 and 0.4. The variation in DHc/DT shows that the temperature stability of coercivity can be controlled by the doping amount. Fang et al. [7] obtained a low positive temperature coefficient of coercivity of 1.9 Oe/K with BaFe12-2x ZnxSnxO19 for x =1.1 prepared using the co-precipitation method. It is clear that when substituting Fe3 + by the three components Zn2 + –Co2 + –Sn4 + , the temperature coefficient of coercivity can be further reduced. BaFe12–2xCox/2Znx/2SnxO19 has a higher Ms at room temperature and lower temperature coefficient of coercivity. Thus when x =0.3, Ms = 68.4 A m2 kg 1, and Hc =80.8 kA m 1;
BaFe12–2xCox/2Znx/2SnxO19 ultrafine particles with 0 rx r2 were prepared though the chemical co-precipitation/molten salt route. The symmetry and crystallinity decreased as x increased. The value of Ms of BaFe12–2xCox/2Znx/2SnxO19 was intermediate between those of BaFe12–2xZnxSnxO19 and BaFe12–2xCoxSnxO19 at the same level of doping. This result for Ms indicates that simple methods can be used to tailor BaM to make it suitable for different uses. The temperature dependence of coercivity approaches 0 when x lies between 0.3 and 0.4, which indicates that the temperature stability of coercivity can be controlled by the doping amount. It also provides information for time-dependent behavior since thermal activation is needed to overcome energy barriers between the metastable states.
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