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J. Phys. Chem. C 2007, 111, 5866-5870
Synthesis and Magnetic Properties of BaFe12O19 Hexaferrite Nanoparticles by a Reverse Microemulsion Technique Ping Xu, Xijiang Han,* and Maoju Wang Chemistry Laboratory Center, Department of Chemistry, Harbin Institute of Technology, Harbin 15000, China ReceiVed: December 27, 2006; In Final Form: March 1, 2007
BaFe12O19 hexaferrite nanoparticles, containing cetyltrimethylammonium chloride (CTAC), n-hexanol, and cyclohexane, were synthesized by a reverse microemulsion technique with a combination of (NH4)2CO3 and NH3‚H2O as precipitator. Barium ferrite nanoparticles with 30 nm diameter and uniform flaky structure were proved to be single magnetic domains, which have magnetic properties comparable to some of the best ever reported for fine barium ferrite powders by chemical methods. Heat-treatment conditions can significantly influence the formation of pure BaFe12O19 hexaferrite phase, where quenching and nonprecalcination would produce intermediates of R-Fe2O3 and BaFe2O4, as detected by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analyses, resulting in lower magnetic properties. High magnetocrystalline anisotropy constant K and energy barrier EA calculated from Stoner-Wohlfarth theory may also account for the high coercivity for pure BaFe12O19. The variation of electrical conductivity during the formation and reaction of microemulsion droplets suggests nonpercolating microemulsion conducting systems. Transmission electron microscopic (TEM) images of the microemulsion droplets from a microemulsion system with R ) V(water):V(oil) ) 1:8 displayed microemulsion droplets about 100 nm, containing a barium ferrite precursor “core” of about 30 nm in size, with collision and coalescence being discovered.
Introduction Materials consisting of nanometer particles have attracted broad interest in fundamental sciences and technological applications due to the novel and/or enhanced physicochemical properties of nanoparticles.1,2 M-type barium ferrite with hexagonal molecular structure (BaFe12O19) is a well-known high-performance permanent magnetic material, owing to its fairly large magnetocrystalline anisotropy, high Curie temperature, relatively large magnetization, excellent chemical stability, and corrosion resistivity.3-5 Barium ferrite recently has been extensively studied for advanced recording applications, such as disk drivers and video recorders.6,7 In addition, the hexagonal ferrite materials are suitable microwave-absorbing materials due to a significant value of permeability (>1), high value of magnetization, and planar anisotropic behavior in microwave frequencies,8,9 while conventional spinel-type ferrites do not function well in the gigahertz range due to a drop in the complex permeability µr as given by Snoek’s limit.10,11 The magnetic characteristics of ferrites are strongly affected when the particle size approaches the critical diameter, below which each particle is a single magnetic domain. The technological applications require barium ferrite particles to be single magnetic domains, with good chemical homogeneity and narrow particle size distribution;12 thus several nonconventional techniques, such as the coprecipitation method,13,14 the ammonium nitrate melt technique,15 the hydrothermal technique,16-18 and the glass crystallization method,6,19 have been applied to prepare ultrafine barium ferrite particles with high qualities. A reverse microemulsion is defined as a thermodynamically stable isotropic dispersion of an aqueous phase in a continuous oil phase, stabilized by an interfacial layer of surfactant molecules.20,21 * Corresponding author: tel +86-451-86413702; fax +86-45186418750; e-mail
[email protected].
Reverse micelles that exist in a microemulsion system, which are essentially nanometer aqueous droplets with certain compositions, are known to represent an excellent medium for the synthesis of nanoparticles with uniform morphologies and excellent homogeneities. The reverse microemulsion technique is widely used to prepare spinel ferrites and other materials with good physical and chemical properties;22-27 however, this technique has seldom been applied to synthesize M-type barium ferrites. As reported by Pillai et al. in 1992,20 ultrafine barium ferrite particles ( EA(b). The coercivity is considered as a measure of the magnetic field strength that is required to achieve changes of magnetization direction of a material. Lowering the anisotropy of a material will lower the activation energy barrier and a lower applied field required for spin reversal, that is, a lower coercivity, which is another explanation for the higher coercivity obtained for sample a. Conclusions M-type barium hexaferrite nanoparticles were synthesized by a reverse microemulsion technique, containing cetyltrimethylammonium chloride (CTAC), n-hexanol, and cyclohexane, with a combination of (NH4)2CO3 and NH3‚H2O as precipitator. Effect of different heat-treatment conditions on the formation of pure BaFe12O19 hexaferrite phase was investigated, and XRD and FT-IR results show that quenching and nonprecalcination would produce intermediates such as R-Fe2O3 and BaFe2O4 in the barium ferrite powders. Under proper calcination conditions, pure barium hexaferrite nanoparticles with a 30 nm diameter
and uniform flaky structure were obtained, with single magnetic domain, 64.3 emu‚g-1 of saturation magnetization and 5483.3 Oe of intrinsic coercivity. The pure barium hexaferrite also has larger magnetocrystalline anisotropy constant K and energy barrier EA, calculated from Stoner-Wohlfarth theory. The variation of electrical conductivity during the formation and reaction of microemulsion droplets suggests nonpercolating microemulsion conducting systems. TEM images of the microemulsion droplets after reaction show microemulsion droplets of about 100 nm, containing a barium ferrite precursor “core” of about 30 nm in size from a microemulsion system with R ) V(water):V(oil) ) 1:8. Acknowledgment. This research was supported by the National Natural Science Foundation of China (20676024). References and Notes (1) Service, R. F. Science 1996, 271, 920. (2) Rondinone, A. J.; Samia, A. C. S.; Zhang, Z. J. J. Phys. Chem. B 2000, 104, 7919. (3) Wang, S.; Ng, W. K.; Ding, J. Scr. Mater. 2000, 42, 861. (4) Paul, K. B. M. Physica B 2007, 388, 337. (5) Yu, H. F.; Liu, P. C. J. Alloys Compd. 2006, 416, 222. (6) Kubo, O.; Ido, T.; Yokoyama, H. IEEE Trans. Magn. 1982, 18 (6), 1122. (7) Liu, X. X.; Bai, J. M.; Wei, F. L.; Yang, Z.; Morisako, A.; Matsunori, M. J. Magn. Magn. Mater. 2000, 212, 273. (8) Capraro, S.; Chatelon, J. P.; Le Beree, M.; Joisten, H.; Rouiller, T.; Bayard, B.; Barbier, D.; Rousseau, J. J. J. Magn. Magn. Mater. 2004, 272-276, e1805. (9) Qiu, J. X.; Gu, M. Y. J. Alloys Compd. 2006, 415, 209. (10) Kagotani, T.; Fujiwara, D.; Sugimoto, S.; Inomata, K.; Homma, M. J. Magn. Magn. Mater. 2004, 272-276, e1813. (11) Nakamura, T.; Tsutaoka, T.; Hatakeyama, K. J. Magn. Magn. Mater. 1994, 138, 319. (12) Yu, H. F.; Huang, K. C. J. Magn. Magn. Mater. 2003, 260, 455. (13) Jacobo, S. E.; Blesa, M. A.; Domingo-Pascual, C.; RodriguezClemente, R. J. Mater. Sci. 1997, 32, 1025. (14) Janasi, S. R.; Emura, M.; Landgraf, F. J. G.; Rodrigues, D. J. Magn. Magn. Mater. 2002, 238, 168. (15) Topal, U.; Ozkan, H.; Topal, K. G. J. Alloys Compd. 2006, 422, 276. (16) Li, X. H.; Zhang, D. H.; Chen, J. S. J. Am. Chem. Soc. 2006, 128, 8382. (17) Ataie, A.; Piramoon, M. R.; Harris, I. R.; Ponton, C. B. J. Mater. Sci. 1995, 30, 5600. (18) Lin, C. H.; Shih, Z. W.; Chin, T. S.; Wang, M. L.; Yu, Y. C. IEEE Trans. Magn. 1990, 26 (1), 15. (19) Lucchini, E.; Meriani, S.; Slokar, G. J. Mater. Sci. 1983, 18, 1331. (20) Pallai, V.; Kumar, P.; Shah. D. O. J. Magn. Magn. Mater. 1992, 116, L299. (21) Liu, X. Y.; Wang, J.; Gan, L. M.; Ng, S. C.; Ding, J. J. Magn. Magn. Mater. 1998, 184, 344. (22) Vestal, C. R.; Zhang, Z. J. Nano Lett. 2003, 3 (12), 1739. (23) Liu, C.; Zou, B. S.; Rondinone, A. J.; Zhang, Z. J. J. Phys. Chem. B 2000, 104, 1141. (24) Rondinone, A. J.; Liu, C.; Zhang, Z. J. J. Phys. Chem. B 2001, 105, 7967. (25) Vestal, C. R.; Zhang, Z. J. J. Am. Chem. Soc. 2003, 125, 9828. (26) Han, M.; Vestal, C. R.; Zhang, Z. J. J. Phys. Chem. B 2004, 108, 583. (27) Lopez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446. (28) Pallai, V.; Kumar, P.; Hou, M. J.; Shah. D. O. AdV. Colloid Interface Sci. 1995, 55, 241. (29) Huang, J. G.; Zhuang, H. R.; Li, W. L. Mater. Res. Bull. 2003, 38, 149. (30) Mendoza-Suarez, G.; Cisneros-Morales, M. C.; Cisneros-Guerrero, M. M.; Johal, K. K.; Mancha-Molinar, H.; Ayala-Valenzuela, O. E.; Escalante-Garcia, J. I. Mater. Chem. Phys. 2002, 11, 796. (31) Clausse, M.; Peyrelasse, J.; Boned, C.; Heil, J.; Nicoles-Morgantini, L.; Zradba, A. In Solution Properties of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1983. (32) Jiao, J.; Burgess, D. J. J. Colloid Interface Sci. 2003, 264, 509. (33) Mun, S.; McClements, D. J. Langmuir 2006, 22, 1551. (34) Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164. (35) Stoner, E. C.; Wohlfarth, E. P. Philos Trans. R. Soc.London, A 1948, 240, 599.
