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Magnetic properties of magnetite nanoparticles prepared by mechanochemical reaction
Materials Letters 60 (2006) 447 – 450 www.elsevier.com/locate/matlet
Magnetic properties of magnetite nanoparticles prepared by mechanochemical reaction Chun-Rong Lin ⁎, Yuan-Ming Chu, Sheng-Chang Wang Department of Mechanical Engineering, Southern Taiwan University of Technology, 1 Nan-Tai Street, Yung-Kang City, Tainan Hsien 710, Taiwan, Republic of China Received 22 December 2004; accepted 6 September 2005 Available online 23 September 2005
Abstract Nanosized magnetite (Fe3O4) powders were synthesized via a mechanochemical reaction. Ball milling of 1.2FeCl2 + 2FeCl3 + 8.4NaOH led to a mixture of Fe3O4 and NaCl. To avoid agglomeration, the excess NaCl was added to the precursor before ball milling. To prepare different size of particles, the as-milled powders were annealed at temperatures ranging from 100 to 800 °C for 1 h in 10% H2/Ar mixed gas. Single phase Fe3O4 powders were obtained after removing the NaCl from the as-milled or heated powders. The average crystallite size of the powders varied from 12.5 to 46 nm by changing the annealed temperatures and the corresponding saturation magnetization (σS) value ranged from 52 to 66.4 emu/g. The coercivity (Hc) first increases as the crystallite size decreases, reaches a maximum value of 110 Oe at 22.2 nm and then decreases for any further decrease in particle size. © 2005 Elsevier B.V. All rights reserved. PACS: 75.50.Lk; 75.50.Tt; 75.60.Ej; 75.75.+a Keywords: Magnetite; Ball milling; Nanoparticles
1. Introduction Magnetite (Fe3O4) nanoparticles have attracted an increasing interest in the fields of nanoscience and nanotechnology because of the unique and novel physiochemical properties that can be attained according to their particle size (quantum size effect), shape morphology, and engineering form (films/selfassembled nanocrystals and ferrofluids) [1–4]. Various methods of preparing Fe3O4 nanoparticles have been carried out by several techniques, including coprecipitation [5], spray pyrolysis [6], microwave irradiation of ferrous hydroxide [7], microemulsion technique [8,9], hydrothermal preparation technique [10], etc. Recently, mechanochemical processing, a process that makes use of chemical reactions activated by high-energy ball milling, has been successfully used for preparing high quality ferrite nanoparticles [11,12]. In this article, we presented the synthesis of nanosized Fe3O4 particles by the mechanochemical
⁎ Corresponding author. Tel.: + 886 6 2533131x3547; fax: +886 6 2425092. E-mail address: [email protected] (C.-R. Lin). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.009
reaction. Furthermore, the structure and magnetic properties of Fe3O4 nanoparticles were investigated as a function of the annealing treatment. 2. Experiment The Fe3O4 nanoparticles were synthesized by mechanochemical processing. The anhydrous FeCl2 powder, FeCl3 powder, NaCl powder and NaOH pellets were used as raw materials for ball milling. Basically, we prepared nanosized Fe3O4 powders via mechanochemical reaction of FeCl2 + 2FeCl3 + 8NaOH → Fe3O4 + 8NaCl + 4H2O. However, the excess of FeCl2 powder and NaCl powder were added to ensure the formation of single Fe3O4 phase and avoid the aggregation of Fe3O4 particles, respectively. The weight ratio of reactants (FeCl2, FeCl3 and NaOH) and NaCl was 1 : 5. Samples weighing 5 g were milled in a high-energy grinding planetary micro-mill Fritsch, model Pulverisette 4, equipped with steel vials with 45 ml capacity containing seven steel balls, 15 mm in diameter. Two funnels close to the grinding vials were used for cooling the sample all over the grinding process.
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C.-R. Lin et al. / Materials Letters 60 (2006) 447–450
Fig. 1. XRD patterns of as-milled (NaCl unwashed) and washed powder.
Fig. 3. XRD patterns for the as-milled and annealed powders after washing.
The samples were ground under argon gas in order to avoid oxidation. For this purpose, the air initially contained in the grinding jar was outgassed before filling with argon gas. A rotation speed equal to 400 rpm and a ball weight/raw materials weight ratio close to 10 were used. The mixture was ground under these conditions for a period of 72 h. After milling, the powders were pressed in a disk form and annealed at temperatures ranging from 100 to 800 °C for 1 h in 10% H2/Ar mixed gas. Removal of NaCl by-product was carried out by washing the as-milled and heat treated powders with deionized water, using an ultrasonic bath and a centrifuge. The washed powder was dried in an oven (100 °C) for 1 h in Ar gas. The crystal structure and phase purity of all the samples were characterized by the X-ray powder diffraction. X-ray diffraction (XRD) spectra were measured using a rotatinganode X-ray generator (Cu Kα 50 kV, 200 mA) with a graphite (002) monochromator. The particle shape and size of samples were observed by the scanning transmission electron microscope [(STEM), Philips/FEI Tecnai G2 F20]. A vibrating sample magnetometer (VSM) was used to
measure the M–H curves under a magnetic field in the range 0∼1.1 T.
Fig. 2. TEM photograph of the washed Fe3O4 powders after grinding for 72 h.
3. Results and discussion In the mechanochemical processing route, the starting mixture of FeCl2 and FeCl3 powders was prepared in a molar ratio of 1.2 : 2. Fig. 1 shows the XRD patterns of the as-milled (NaCl unwashed) and washed powders after grinding for 72 h. Besides the NaCl peaks detected in the as-milled powder, there are two broad diffraction peaks at 2θ values of 36.6° and 62.9° associated with the Fe3O4 phase. After washing to remove the NaCl only a phase of Fe3O4 peaks remained. The mean crystallite size of the Fe3O4 nanoparticles calculated by using Scherrer's equation [13] from broadening peaks is 12.5 nm. Fig. 2 shows the TEM photograph of the washed sample after grinding for 72 h. The particle size ranges from 10 to 20 nm with an average size of about 14.8 nm. The mean particle size is in agreement with the XRD results except for some agglomerated particles. To study the thermal treatment effect, the milled powders were pressed in a disk form and annealed at temperatures ranging from 100 to 800 °C for 1 h in 10% H2/Ar mixed gas. After removing the NaCl by-product, the XRD patterns of the selected samples are shown in Fig. 3. The diffraction patterns correspond to those of the standard spinel ferrite Fe3O4. The full width at half maximum (FWHM) of the 311 reflection peak decreases with increasing treatment temperature. This means that the crystallinity of Fe3O4 is improved with increasing
Fig. 4. Crystallite size, calculated from XRD peak broadening using Scherrer's formula, as a function of annealing temperature (TA).
C.-R. Lin et al. / Materials Letters 60 (2006) 447–450
Fig. 5. Hysteresis loops measured at 298 K for washed and as-milled (NaCl unwashed) Fe3O4 powders annealed at 100 °C for 1 h.
449
Fig. 7. Coercivity Hc as a function of the crystallite size for washed Fe3O4 powders.
treatment temperature in the temperature range studied. In Fig. 4 the crystallite size, calculated from XRD peak broadening using Scherrer's formula, is plotted as a function of annealing temperature (TA). As seen from the plot, the crystallite size increases significantly when the sample is heated above 500 °C. Room temperature hysteresis loops of the as-milled and washed samples are shown in Fig. 5. The magnetizations, measured at the maximum applied field of 11 kOe, of the as-milled and washed powder are 2.6 and 49.1 emu/g, respectively. Magnetic hysteresis loops for the washed samples annealed at different temperature are shown in Fig. 4. They are typical of soft ferromagnets; however, complete magnetic saturation is not achieved even at 11 kOe indicating superparamagnetic fractions in the material. The saturation magnetization, σs, can be estimated by fitting the experimental σ value to the law of approach to magnetization [14] i.e.,
reduction of the saturation magnetization in Fe3O4 spinel may be attributed to the surface disorder or spin canting at the particles surface [15,16]. The relationship between the crystallite size and coercivity (Hc) is shown in Fig. 7. The Hc value was determined from the M–H hysteresis loop measured at room temperature. As seen from the plot the Hc value increases to show a maximum and dropped. The coercivity maximum is centered around 22.2 nm for a sample annealed at 600 °C. A further increase in crystallite size above 22.2 nm lowers the coercivity. The increase of Hc with increasing crystallites suggests a monodomain behavior of the crystallites, if the crystallite size does not exceed a critical value for single-domain structure. In this connection we estimate the critical size (Dc) of a single-domain particle using formula [17]
r ¼ rð1−a0 =HÞ þ v0 H;
Dc c18
where the second term is the paramagnetic contribution, a0 is a constant and χ0 is the magnetic susceptibility. From our experiments, the obtained σs values versus the annealing temperature are displayed in the inset of the Fig. 6. It can be seen that σs increases with TA, slowly up to 500 °C and then at an accelerated rate above 500°C. The magnetization at room temperature, σs (298 K) = 52∼66.4 emu/g, is significantly less than that of the bulk magnetic particles [14], i.e. σs (bulk) = 92 emu/g. The
where A is the exchange constant, |K| is the magnetocrystalline anisotropy constant, μ0 is the vacuum permeability and Ms is the saturation magnetization. For particle size D N Dc, the particles are multidomains. A value of A = 1∼2 × 10− 11 J/m is typical for most ferromagnets [17]. For magnetite, using |K| = 1.1 × 104 J/m3 and Ms = 4.8 × 105A/m [17], the value of Dc estimated is about 20∼29 nm, which is in agreement with the evaluation from X-ray diffraction, of 22.2 nm for the sample treated at 600 °C.
Fig. 6. Hysteresis loops measured at 298 K for washed Fe3O4 powders annealed at various temperatures for 1 h. Inset: variation with annealing temperature (TA) of the saturation magnetization (σS).
Fig. 8. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization as a function of temperature (applied field H = 50 Oe) for the washed Fe3O4 powders after grinding for 72 h followed by annealing at 600 °C for 1 h.
ðAjKjÞ1=2 ; l0 Ms2
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C.-R. Lin et al. / Materials Letters 60 (2006) 447–450
As a result, the particles exhibit the maximum Hc value of 110 Oe for 22.2 nm particles, which are in the single-domain size range. The decrease in coercivity after 22.2 nm can be rationalized due to the formation of particles in multidomain size range. The changes in zero-field-cooled (ZFC) and field-cooled (FC) magnetization with temperature of the washed Fe3O4 nanoparticles annealed at 600 °C are shown in Fig. 8. For the ZFC magnetization curve, it seems to be consisted of apparently two shoulders at around 36 and 114 K. Furthermore, the ZFC magnetization does not exhibit a defined maximum and ZFC and FC branches of the experimental data remain non-overlapping up to values beyond 390 K. The FC and ZFC curves are decoupled over the whole range of temperatures which suggest occurrence of strong interactions between particles that agglomerate upon annealing. We suppose that the samples have the average blocking temperatures, at which superparamagnetic relaxation starts to appear, above 390 K.
4. Conclusions In this report we have shown that mechanochemical processing can be used for the synthesis of nanosized Fe3O4 particles with diameters ranging from 12.5 to about 46 nm. Nanosized Fe3O4 powder prepared by this way requires the simple processes of ball milling, annealing and washing with water. Magnetic measurements show that the saturation magnetization of the Fe3O4 nanoparticles is significantly less than that of the bulk magnetic particles. The saturation magnetization of the Fe3O4 nanoparticles increases as a function of average crystallite size, whereas, the coercivity increases to show a maximum and dropped.
Acknowledgements We are indebted to the Southern Taiwan University of Technology and the National Science Council of the Republic of China (NSC 92-2112-M-218-001) for the financial support of this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
G.A. Ozin, Adv. Mater. 4 (1992) 612. Y.S. Kang, S. Risbud, J.F. Rabolt, P. Stroeve, Chem. Mater. 8 (1996) 2209. M.P. Pileni, J. Phys. Chem. 105 (2001) 3358. R. Kaiser, G. Miskolczy, J. Appl. Phys 41 (1970) 1064. G. Visalakshi, G. Venkateswaran, S.K. Kulshreshtha, P.N. Moorthy, Mater. Res. Bull. 28 (1993) 829. T. Gonzalez-Catteno, M.P. Morales, Mater. Lett. 18 (1993) 151. D. Dong, P. Hong, Mater. Res. Bull. 30 (1995) 537. V. Chhabra, P. Ayyub, Mater. Lett. 26 (1996) 21. Y. Deng, L. Wang, W. Yang, S. Fu, A. Elaïssari, J. Magn. Magn. Mater 194 (1999) 254. Y. Li, H. Liao, Y. Qian, Mater. Res. Bull 33 (1998) 841. J. Ding, T. Tsuzuki, P.G. McCormick, J. Magn. Magn. Mater. 177–181 (1998) 931. Y. Shi, J. Ding, H. Yin, J. Alloys Compd. 308 (2000) 290. J.I. Langford, A.J.C. Wilson, J. Appl. Crystallogr. 11 (1978) 102. B.D. Cullity, Introduction to Magnetic Materials, Addison Wesley, New York, 1972. J.L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98 (1997) 283. X. Battle, A. Labarta, J. Phys., D, Appl. Phys. 35 (2002) R15. R.C. O'Handley, Modern Magnetic Materials: Principles and Applications, John Wiley & Sons, Inc., New York, 2000.
Magnetic properties of magnetite nanoparticles prepared by mechanochemical reaction Chun-Rong Lin ⁎, Yuan-Ming Chu, Sheng-Chang Wang Department of Mechanical Engineering, Southern Taiwan University of Technology, 1 Nan-Tai Street, Yung-Kang City, Tainan Hsien 710, Taiwan, Republic of China Received 22 December 2004; accepted 6 September 2005 Available online 23 September 2005
Abstract Nanosized magnetite (Fe3O4) powders were synthesized via a mechanochemical reaction. Ball milling of 1.2FeCl2 + 2FeCl3 + 8.4NaOH led to a mixture of Fe3O4 and NaCl. To avoid agglomeration, the excess NaCl was added to the precursor before ball milling. To prepare different size of particles, the as-milled powders were annealed at temperatures ranging from 100 to 800 °C for 1 h in 10% H2/Ar mixed gas. Single phase Fe3O4 powders were obtained after removing the NaCl from the as-milled or heated powders. The average crystallite size of the powders varied from 12.5 to 46 nm by changing the annealed temperatures and the corresponding saturation magnetization (σS) value ranged from 52 to 66.4 emu/g. The coercivity (Hc) first increases as the crystallite size decreases, reaches a maximum value of 110 Oe at 22.2 nm and then decreases for any further decrease in particle size. © 2005 Elsevier B.V. All rights reserved. PACS: 75.50.Lk; 75.50.Tt; 75.60.Ej; 75.75.+a Keywords: Magnetite; Ball milling; Nanoparticles
1. Introduction Magnetite (Fe3O4) nanoparticles have attracted an increasing interest in the fields of nanoscience and nanotechnology because of the unique and novel physiochemical properties that can be attained according to their particle size (quantum size effect), shape morphology, and engineering form (films/selfassembled nanocrystals and ferrofluids) [1–4]. Various methods of preparing Fe3O4 nanoparticles have been carried out by several techniques, including coprecipitation [5], spray pyrolysis [6], microwave irradiation of ferrous hydroxide [7], microemulsion technique [8,9], hydrothermal preparation technique [10], etc. Recently, mechanochemical processing, a process that makes use of chemical reactions activated by high-energy ball milling, has been successfully used for preparing high quality ferrite nanoparticles [11,12]. In this article, we presented the synthesis of nanosized Fe3O4 particles by the mechanochemical
⁎ Corresponding author. Tel.: + 886 6 2533131x3547; fax: +886 6 2425092. E-mail address: [email protected] (C.-R. Lin). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.009
reaction. Furthermore, the structure and magnetic properties of Fe3O4 nanoparticles were investigated as a function of the annealing treatment. 2. Experiment The Fe3O4 nanoparticles were synthesized by mechanochemical processing. The anhydrous FeCl2 powder, FeCl3 powder, NaCl powder and NaOH pellets were used as raw materials for ball milling. Basically, we prepared nanosized Fe3O4 powders via mechanochemical reaction of FeCl2 + 2FeCl3 + 8NaOH → Fe3O4 + 8NaCl + 4H2O. However, the excess of FeCl2 powder and NaCl powder were added to ensure the formation of single Fe3O4 phase and avoid the aggregation of Fe3O4 particles, respectively. The weight ratio of reactants (FeCl2, FeCl3 and NaOH) and NaCl was 1 : 5. Samples weighing 5 g were milled in a high-energy grinding planetary micro-mill Fritsch, model Pulverisette 4, equipped with steel vials with 45 ml capacity containing seven steel balls, 15 mm in diameter. Two funnels close to the grinding vials were used for cooling the sample all over the grinding process.
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C.-R. Lin et al. / Materials Letters 60 (2006) 447–450
Fig. 1. XRD patterns of as-milled (NaCl unwashed) and washed powder.
Fig. 3. XRD patterns for the as-milled and annealed powders after washing.
The samples were ground under argon gas in order to avoid oxidation. For this purpose, the air initially contained in the grinding jar was outgassed before filling with argon gas. A rotation speed equal to 400 rpm and a ball weight/raw materials weight ratio close to 10 were used. The mixture was ground under these conditions for a period of 72 h. After milling, the powders were pressed in a disk form and annealed at temperatures ranging from 100 to 800 °C for 1 h in 10% H2/Ar mixed gas. Removal of NaCl by-product was carried out by washing the as-milled and heat treated powders with deionized water, using an ultrasonic bath and a centrifuge. The washed powder was dried in an oven (100 °C) for 1 h in Ar gas. The crystal structure and phase purity of all the samples were characterized by the X-ray powder diffraction. X-ray diffraction (XRD) spectra were measured using a rotatinganode X-ray generator (Cu Kα 50 kV, 200 mA) with a graphite (002) monochromator. The particle shape and size of samples were observed by the scanning transmission electron microscope [(STEM), Philips/FEI Tecnai G2 F20]. A vibrating sample magnetometer (VSM) was used to
measure the M–H curves under a magnetic field in the range 0∼1.1 T.
Fig. 2. TEM photograph of the washed Fe3O4 powders after grinding for 72 h.
3. Results and discussion In the mechanochemical processing route, the starting mixture of FeCl2 and FeCl3 powders was prepared in a molar ratio of 1.2 : 2. Fig. 1 shows the XRD patterns of the as-milled (NaCl unwashed) and washed powders after grinding for 72 h. Besides the NaCl peaks detected in the as-milled powder, there are two broad diffraction peaks at 2θ values of 36.6° and 62.9° associated with the Fe3O4 phase. After washing to remove the NaCl only a phase of Fe3O4 peaks remained. The mean crystallite size of the Fe3O4 nanoparticles calculated by using Scherrer's equation [13] from broadening peaks is 12.5 nm. Fig. 2 shows the TEM photograph of the washed sample after grinding for 72 h. The particle size ranges from 10 to 20 nm with an average size of about 14.8 nm. The mean particle size is in agreement with the XRD results except for some agglomerated particles. To study the thermal treatment effect, the milled powders were pressed in a disk form and annealed at temperatures ranging from 100 to 800 °C for 1 h in 10% H2/Ar mixed gas. After removing the NaCl by-product, the XRD patterns of the selected samples are shown in Fig. 3. The diffraction patterns correspond to those of the standard spinel ferrite Fe3O4. The full width at half maximum (FWHM) of the 311 reflection peak decreases with increasing treatment temperature. This means that the crystallinity of Fe3O4 is improved with increasing
Fig. 4. Crystallite size, calculated from XRD peak broadening using Scherrer's formula, as a function of annealing temperature (TA).
C.-R. Lin et al. / Materials Letters 60 (2006) 447–450
Fig. 5. Hysteresis loops measured at 298 K for washed and as-milled (NaCl unwashed) Fe3O4 powders annealed at 100 °C for 1 h.
449
Fig. 7. Coercivity Hc as a function of the crystallite size for washed Fe3O4 powders.
treatment temperature in the temperature range studied. In Fig. 4 the crystallite size, calculated from XRD peak broadening using Scherrer's formula, is plotted as a function of annealing temperature (TA). As seen from the plot, the crystallite size increases significantly when the sample is heated above 500 °C. Room temperature hysteresis loops of the as-milled and washed samples are shown in Fig. 5. The magnetizations, measured at the maximum applied field of 11 kOe, of the as-milled and washed powder are 2.6 and 49.1 emu/g, respectively. Magnetic hysteresis loops for the washed samples annealed at different temperature are shown in Fig. 4. They are typical of soft ferromagnets; however, complete magnetic saturation is not achieved even at 11 kOe indicating superparamagnetic fractions in the material. The saturation magnetization, σs, can be estimated by fitting the experimental σ value to the law of approach to magnetization [14] i.e.,
reduction of the saturation magnetization in Fe3O4 spinel may be attributed to the surface disorder or spin canting at the particles surface [15,16]. The relationship between the crystallite size and coercivity (Hc) is shown in Fig. 7. The Hc value was determined from the M–H hysteresis loop measured at room temperature. As seen from the plot the Hc value increases to show a maximum and dropped. The coercivity maximum is centered around 22.2 nm for a sample annealed at 600 °C. A further increase in crystallite size above 22.2 nm lowers the coercivity. The increase of Hc with increasing crystallites suggests a monodomain behavior of the crystallites, if the crystallite size does not exceed a critical value for single-domain structure. In this connection we estimate the critical size (Dc) of a single-domain particle using formula [17]
r ¼ rð1−a0 =HÞ þ v0 H;
Dc c18
where the second term is the paramagnetic contribution, a0 is a constant and χ0 is the magnetic susceptibility. From our experiments, the obtained σs values versus the annealing temperature are displayed in the inset of the Fig. 6. It can be seen that σs increases with TA, slowly up to 500 °C and then at an accelerated rate above 500°C. The magnetization at room temperature, σs (298 K) = 52∼66.4 emu/g, is significantly less than that of the bulk magnetic particles [14], i.e. σs (bulk) = 92 emu/g. The
where A is the exchange constant, |K| is the magnetocrystalline anisotropy constant, μ0 is the vacuum permeability and Ms is the saturation magnetization. For particle size D N Dc, the particles are multidomains. A value of A = 1∼2 × 10− 11 J/m is typical for most ferromagnets [17]. For magnetite, using |K| = 1.1 × 104 J/m3 and Ms = 4.8 × 105A/m [17], the value of Dc estimated is about 20∼29 nm, which is in agreement with the evaluation from X-ray diffraction, of 22.2 nm for the sample treated at 600 °C.
Fig. 6. Hysteresis loops measured at 298 K for washed Fe3O4 powders annealed at various temperatures for 1 h. Inset: variation with annealing temperature (TA) of the saturation magnetization (σS).
Fig. 8. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization as a function of temperature (applied field H = 50 Oe) for the washed Fe3O4 powders after grinding for 72 h followed by annealing at 600 °C for 1 h.
ðAjKjÞ1=2 ; l0 Ms2
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C.-R. Lin et al. / Materials Letters 60 (2006) 447–450
As a result, the particles exhibit the maximum Hc value of 110 Oe for 22.2 nm particles, which are in the single-domain size range. The decrease in coercivity after 22.2 nm can be rationalized due to the formation of particles in multidomain size range. The changes in zero-field-cooled (ZFC) and field-cooled (FC) magnetization with temperature of the washed Fe3O4 nanoparticles annealed at 600 °C are shown in Fig. 8. For the ZFC magnetization curve, it seems to be consisted of apparently two shoulders at around 36 and 114 K. Furthermore, the ZFC magnetization does not exhibit a defined maximum and ZFC and FC branches of the experimental data remain non-overlapping up to values beyond 390 K. The FC and ZFC curves are decoupled over the whole range of temperatures which suggest occurrence of strong interactions between particles that agglomerate upon annealing. We suppose that the samples have the average blocking temperatures, at which superparamagnetic relaxation starts to appear, above 390 K.
4. Conclusions In this report we have shown that mechanochemical processing can be used for the synthesis of nanosized Fe3O4 particles with diameters ranging from 12.5 to about 46 nm. Nanosized Fe3O4 powder prepared by this way requires the simple processes of ball milling, annealing and washing with water. Magnetic measurements show that the saturation magnetization of the Fe3O4 nanoparticles is significantly less than that of the bulk magnetic particles. The saturation magnetization of the Fe3O4 nanoparticles increases as a function of average crystallite size, whereas, the coercivity increases to show a maximum and dropped.
Acknowledgements We are indebted to the Southern Taiwan University of Technology and the National Science Council of the Republic of China (NSC 92-2112-M-218-001) for the financial support of this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
G.A. Ozin, Adv. Mater. 4 (1992) 612. Y.S. Kang, S. Risbud, J.F. Rabolt, P. Stroeve, Chem. Mater. 8 (1996) 2209. M.P. Pileni, J. Phys. Chem. 105 (2001) 3358. R. Kaiser, G. Miskolczy, J. Appl. Phys 41 (1970) 1064. G. Visalakshi, G. Venkateswaran, S.K. Kulshreshtha, P.N. Moorthy, Mater. Res. Bull. 28 (1993) 829. T. Gonzalez-Catteno, M.P. Morales, Mater. Lett. 18 (1993) 151. D. Dong, P. Hong, Mater. Res. Bull. 30 (1995) 537. V. Chhabra, P. Ayyub, Mater. Lett. 26 (1996) 21. Y. Deng, L. Wang, W. Yang, S. Fu, A. Elaïssari, J. Magn. Magn. Mater 194 (1999) 254. Y. Li, H. Liao, Y. Qian, Mater. Res. Bull 33 (1998) 841. J. Ding, T. Tsuzuki, P.G. McCormick, J. Magn. Magn. Mater. 177–181 (1998) 931. Y. Shi, J. Ding, H. Yin, J. Alloys Compd. 308 (2000) 290. J.I. Langford, A.J.C. Wilson, J. Appl. Crystallogr. 11 (1978) 102. B.D. Cullity, Introduction to Magnetic Materials, Addison Wesley, New York, 1972. J.L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98 (1997) 283. X. Battle, A. Labarta, J. Phys., D, Appl. Phys. 35 (2002) R15. R.C. O'Handley, Modern Magnetic Materials: Principles and Applications, John Wiley & Sons, Inc., New York, 2000.