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Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 1251–1255
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles Zhenfa Zi a,b, Yuping Sun a,, Xuebin Zhu a, Zhaorong Yang a, Jianming Dai a, Wenhai Song a a b
Key Laboratory of Materials Physics, Institute of Solid State Physics, and High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, PR China Department of Physics and Electronic Engineering, Hefei Teachers College, Hefei 230061, PR China
a r t i c l e in fo
abstract
Article history: Received 29 June 2008 Received in revised form 4 September 2008 Available online 18 November 2008
CoFe2O4 ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group Fd3m O7h . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe2O4. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co2+ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe2O4 samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Vv 75.75.+a 76.80.+y Keywords: Cobalt ferrite Nanoparticle Mono-domain Superparamagnetic
1. Introduction Magnetic nanoparticles have attracted much attention in the past decades because of their potential applications in highdensity magnetic recording, magnetic fluids, data storage, spintronics, solar cells, sensors, and catalysis [1–5]. Among the magnetic nanoparticles, cobalt ferrite (CoFe2O4) has been widely studied due to high electromagnetic performance [6], excellent chemical stability, mechanical hardness, and high cubic magnetocrystalline anisotropy [7]. These properties make it a promising candidate for many applications in commercial electronics such as video, audio tapes, high-density digital recording media [8–10], and magnetic fluids [11]. The cubic-spinel-structured CoFe2O4 ferrite represents a wellknown and important class of iron oxide materials. The O2 ions form fcc close packing, and the Co2+ and Fe3+ occupy either tetrahedral or octahedral interstitial sites [12]. These two antiparallel sublattices, which are coupled by superexchange interactions through the O2 ions, form the ferrimagnetic structure. Most of the magnetic properties of CoFe2O4 ferrite strongly depend on the size and shape of the nanoparticles [6,13], which are closely related to the method of preparation. Several routes have been used to prepare cobalt ferrite, including the sol–gel
Corresponding author. Tel.: +86 551 5592757; fax: +86 551 5591434.
E-mail address: [email protected] (Y. Sun). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.11.004
method [14], microemulsion with oil in water micelles [15], reverse micelles method [16], chemical coprecipitation method [17], combustion reaction method [18,19], and forced hydrolysis in a polyol medium [20]. However, most of these methods cannot be economically applied on a large scale because they require expensive and often toxic reagents, complicated synthetic steps, high reaction temperatures, and long reaction times. This not only results in waste of energy but also harms our environment. As is well known, the chemical coprecipitation method is usually used to synthesize magnetic oxides due to its simplicity and good control of grain size. However, in previous reports it was always observed that there exist some undesirable intermediate phases, which led to poor magnetic properties and irregular shape for the derived CoFe2O4 ferrite particles. In this paper, singlephase highly monodisperse CoFe2O4 ferrite nanoparticles with relatively homogeneous size were prepared by means of a modified chemical coprecipitation method.
2. Experimental details The CoFe2O4 ferrite particles were prepared by the chemical coprecipitation method by digestion. The starting materials were high-purity Co(CH3COO)2 4H2O and Fe(NO3)3 9H2O. Sodium hydroxide, NaOH, was used as the precipitation agent. The reaction is described by Co2þ þ 2Fe3þ þ 8OH ! CoFe2 O4 þ 4H2 O
(1)
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First, Co(CH3COO)2 4H2O and Fe(NO3)3 9H2O with Co:Fe atomic ratio of 1:2 were dissolved in deionized water by gentle heating. Then the aqueous mixture was slowly poured into a well-stirred NaOH solution and stirred for several minutes. The digestion was performed at 110 1C for 120 min. During digestion, the particles grew and evolved into a spherical structure. After digestion, the gelatinous precipitate was filtered and washed several times using deionized water until the pH value of the solution became neutral. Finally, the gelatinous precipitate was dried at 80 1C in air to form a powder sample. In order to prepare monodisperse CoFeO4 nanoparticles, the powder was dispersed by ultrasonic in ethanol, centrifuged, rewashed with ethanol, and dried. The crystal structure was examined by Philips X’pert PRO X-ray diffractometer (XRD) with CuKa radiation at room temperature and transmission electron microscopy (TEM, JEM-200CX) equipped with a selected-area electron diffractometer (SAED) on a JEOL TEM (400 kV). The morphology of grains was investigated by field-emission scanning electronic microscopy (FE-SEM), and the compositions were examined by energy-dispersive spectroscopy (EDS) in the FE-SEM. Magnetization measurements from room temperature to 800 K were performed using a vibratingsample magnetometer (VSM) as a component in physical properties measurement system (PPMS; 1.8 KpTp1000 K, 0 TpHp9 T).
3. Results and discussion The room-temperature powder XRD patterns of as-prepared CoFe2O4 ferrite particles post-heat treated at 800 K are shown in Fig. 1. All the peaks can be indexed to a single-phase spinel structure. No additional and intermediate phase is observed within the sensitivity of experimental measurements. The average crystallite size of as-prepared CoFe2O4 ferrite particles was estimated through analysis using the classical Scherrer formula [21] Dhkl ¼ kl/b cos y, where Dhkl is the crystallite size derived from the (3 11) peak of the XRD profiles, k the sphere shape factor (0.89), y the angle of the diffraction, b the difference between the full-width at half-maximum (FWHM) of the peak of the sample and the standard SiO2 used to calibrate the intrinsic width associated with the instruments, and l the wavelength of X-ray (1.54056 A˚). The obtained average crystallite size of as-prepared CoFe2O4 ferrite particles is about 8.56 nm. Based on the peak
Fig. 1. XRD patterns of the as-prepared CoFe2O4 ferrite particles (a) and post-heat treated at 800 K (b). Inset: crystal structure sketch map of CoFe2O4 ferrite composed of tetrahedral (A) and octahedral (B) sites.
central positions obtained from the high-intensity low-angle (3 11) and high-intensity high-angle (4 4 0) peaks, the ferrite lattice parameter was estimated as 8.385 A˚, which is in good agreement with the bulk value of 8.377 A˚ [22]. A typical FE-SEM image of the as-prepared CoFe2O4 ferrite particles is shown in Fig. 2(a). It is clearly seen that the grains are regular spherical particles without agglomeration despite the dipolar interaction between the particles. Most of particles are of rather homogenous grain sizes from 20 to 30 nm. A quantitative EDS analysis is used to determine the composition of CoFe2O4 particles. The results in Fig. 2(b) show that the molar ratio of Co, Fe, and O is about 1:1.98:3.96, which is consistent with the nominal composition. The TEM image and SAED pattern of CoFe2O4 ferrite particles are shown in Figs. 2(c) and (d), respectively. TEM observation reveals that the as-prepared CoFe2O4 ferrite particles are monodisperse and the particle size is about 25 nm. It is found this size is approximately three times larger than that estimated by the Scherrer formula. This implies that our CoFe2O4 ferrite particles are polycrystalline. The SAED pattern is also used to identify the crystallite structure of the CoFe2O4 particles. The rings in the electron diffraction pattern are indexed with their respective hkl from the PDF database. The lattice parameter calculated was 8.38770.002 A˚, which is consistent with the result estimated from XRD. Fig. 3 is the diameter distribution of the CoFe2O4 ferrite particles according to Fig. 2(a). The obtained CoFe2O4 particles show a Gaussian distribution with a maximum at grain size of 25 nm. This implies that the sample has a narrow grain size distribution and the diameters of most CoFe2O4 ferrite particles are estimated to be between 20 and 30 nm. The magnetic behavior of the CoFe2O4 ferrite particles will vary with different Co2+ site occupations since the Co2+ ion is highly anisotropic [23]. To investigate the magnetic properties of the synthesized CoFe2O4 ferrite particles, the temperature dependence of magnetization M(T) and the applied magnetic field dependence of magnetization M(H) are measured. The asprepared CoFe2O4 ferrite sample was heated from room temperature to 800 K and then cooled to room temperature, while magnetic data were collected in a magnetic field of 100 Oe. The measured M(T) curve of the CoFe2O4 ferrite particles is shown in Fig. 4. With increasing temperature, a large drop in magnetization occurred. This indicates a sharp ferrimagnetic to paramagnetic transition, and the Curie temperature TC derived by the peak of dM(T)/dT as shown in the inset of Fig. 4 is 677.0 K, which is slightly lower than that of the bulk. This can be explained by the finitesize-scaling effect [24]. When the sample was subsequently cooled to room temperature, it showed a different transition at 668.6 K. This phenomenon may be attributed to the fact that the structure of the as-prepared CoFe2O4 ferrite sample is unstable and can be altered in the process of increasing-temperature measurement. Heating may change the distribution of metal ions between tetrahedral and octahedral sites of ferrite, which has been confirmed by neutron diffraction and experiments [25,26]. In ideal inverted CoFe2O4 ferrite, all the cobalt is on the octahedral sites, hence none on the tetrahedral sites of the spinel lattice. Heating changed the distribution of metal ions between tetrahedral and octahedral sites, which means the different magnetic transition observed in our sample may be the result of site redistribution. In order to verify this hypothesis, the as-prepared CoFe2O4 ferrite sample was annealed at 800 K by simulating experimental measuring process. XRD result of the sample post-heat treated at 800 K is shown in Fig. 1. The lattice parameter for annealed CoFe2O4 ferrite sample was estimated as 8.37470.002 A˚, which is a little lesser than the value of the as-prepared CoFe2O4 ferrite sample (8.38570.002 A˚). In general, there existed enhanced
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Fig. 2. (a) FE-SEM image of as-prepared CoFe2O4 ferrite particles, (b) EDS image analysis of the CoFe2O4 particles, (c) typical TEM image, and (d) SAED pattern of the CoFe2O4 particles.
CoFe2O4
16
H = 100 Oe
0.0
8
4
dM/dT (emu/g)
M (emu/g)
12
-0.2 -0.4 677.0 K 668.6 K
-0.6 300
0 300
400
400
500 600 T (K)
700
500
800
600
700
800
T (K) Fig. 3. Particle diameter distribution of CoFe2O4 ferrite particles synthesized by the modified chemical coprecipitation method.
lattice parameters in the case of nanoparticles due to interface structure with a large volume fraction [27,28]. This reduced lattice parameter in our sample could be attributed to an increased degree of inversion [29]. Nanosized as-prepared CoFe2O4 ferrite particles, generally have some antisite defects. In the spinel lattice, only one eighth of the tetrahedral sites are occupied by Fe3+ ions and half of the octahedral sites are occupied by Co2+ and Fe3+ ions. This large fraction of empty interstitial sites makes the spinel structure a very open structure conducive to cation migration [30]. After the sample was annealed, the relatively larger Co2+ ions would remigrate to their conventionally preferred octahedral sites from the larger tetrahedral sites. In such a case,
Fig. 4. Magnetization versus temperature curve for the CoFe2O4 particles. Arrows indicate direction of temperature change. The inset is the temperature dependence of the first derivative of M(T), dM(T)/dT.
the Co2+ ions need not expand the lattice, which results in the reduced lattice parameter in annealed CoFe2O4 nanoparticles. There have been some reports on extracting the information related to cation distribution in spinel ferrites using the integrated intensity ratios of X-ray diffraction peaks. The integrated intensity of (2 2 0) reflection depends exclusively on the cations occupying the tetrahedral sites and the intensity of (2 2 2) reflection depends on the cations occupying the octahedral sites [31]. Therefore, the decrease in the intensity ratio of I(2 2 0)/I(2 2 2) in Fig. 1 means a reduction in the concentration of Co2+ ions that fill the tetrahedral sites, and as a result an increase of an equal number of Co2+ ions in
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octahedral sites, which could otherwise have filled the octahedral sites. This further supports that heating changed the distribution of metal ions between tetrahedral and octahedral sites in our sample. This result can be used to qualitatively explain the change of TC in increasing- and decreasing-temperature measuring processes. Since the superexchange interaction between Fe3+ ions in tetrahedral and octahedral sites is stronger than that between Co2+ ions in tetrahedral and Fe3+ ions in octahedral sites [32], the increase of the degree of inversion causes the average exchange interaction to decrease, which in turn results in a decrease in TC. This explains why the as-prepared CoFe2O4 nanoparticles with a low inversion degree has a higher TC, while the sample measured in the process of increasing temperature with a high inversion degree has a lower TC. The M(H) loop for the as-prepared CoFe2O4 ferrite particles at 300 K is shown in Fig. 5. It is clear that the value of saturation magnetization, Ms, comes out to be 61.77 emu/g at room temperature, which is smaller than the bulk value (74.08 emu/g) [33]. The remanence magnitude, Mr, can be extracted from the hysteresis loop at the intersections of the loop with the vertical magnetization axis. The Mr value of 14.39 emu/g is obtained from the inset in Fig. 5. For nanosized ferrite particles, the surface areas are larger and thus the surface energy and surface tension are high. This results in changes in cationic preferences and leads to an increased degree of antisite defects and thus lesser magnetizations [34,35]. The coercivity for a ferromagnet or ferrimagnet can be reflected by the coercivity field, Hc. This value refers to the intensity of the magnetic field required to reduce the magnetization of the magnetic sample to zero, after the magnetization of the sample has reached saturation. The obtained Hc value (0.519 kOe) for our sample is much lower than that in the literature [17]. Generally there are two reasons for reduced coercivity. On the one hand, the CoFe2O4 ferrite particles may have multi-domain structure. The formation of multi-domains and the easy movement of the domain walls can result in a decrease of coercivity. The critical size of a mono-domain particle is estimated using the formula [36] Dm ¼ 9sw =2pM2s , where sw ¼ ð2kB T C jK 1 j=aÞ1=2 is the wall density energy, |K1| the magnetocrystalline anisotropy constant, TC the Curie temperature, Ms the saturation magnetization, kB the Boltzmann constant, and a the lattice constant. For D4Dm, the particles exist in a multi-domain structure, while for DoDm, the particles exist in a mono-domain structure. For
80 CoFe2O4 300 K
0 20 M (emu/g)
M (emu/g)
40
-40
Mr
0 Hc
-20 -1000 -500
-80 -40
-20
0
0 H (Oe)
20
500
1000
40
H (kOe) Fig. 5. Room-temperature M–H hysteresis loop of the CoFe2O4 particles. The inset contains the minor loop.
CoFe2O4 ferrite, TC ¼ 677 K, a ¼ 8.385 108 cm, |K1| ¼ 3.8 106 erg/cm3, and Ms ¼ 310 Gs, the estimated value of Dm is about 430 nm, which is far larger the average diameter of the obtained CoFe2O4 ferrite particles. So the grains exhibit a mono-domain behavior. On the other hand, the decrease in coercivity for our sample may be attributed to a typical particle-size-dependent behavior [37]. For DoDm, the particles exist in a mono-domain structure. As the particle size decreases in succession, the coercivity decreases to some extent, which could be characteristically described as a transformation from ferrimagnetic to superparamagnetic state.
4. Conclusions A modified chemical coprecipitation method by digestion was used to prepare CoFe2O4 ferrite nanoparticles. The XRD results showed that the derived sample was in pure phase with space group of Fd3m O7h . FE-SEM and TEM results showed that the grains were regular sphere-shaped nanoparticles with sizes from 20 to 30 nm. The magnetic properties, including TC, roomtemperature Ms, Mr, and Hc, were investigated. The shift of TC can be explained as due to changed Co2+ ion redistribution in tetrahedral and octahedral sites on heating. The reduction of coercivity can be understood by the transformation from ferrimagnetic to superparamagnetic state.
Acknowledgements This work was financially supported by the National Key Basic Research Program of China under Contract no. 2007CB925002, National Nature Science Foundation of China under Contract nos. 10774146, 10774147, 50672099, and 50701042, and Director’s Fund of Hefei Institutes of Physical Science, Chinese Academy of Sciences. References [1] Y. Yin, A.P. Alivisatos, Nature (London) 437 (2005) 664. [2] M.A. El-Sayed, Acc. Chem. Res. 37 (2004) 326. [3] B.Y. Geng, J.Z. Ma, X.W. Liu, Q.B. Du, M.G. Kong, L.D. Zhang, Appl. Phys. Lett. 90 (2007) 043120. [4] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [5] A.H. Lu, E.L. Salabas, F. Schu¨th, Angew. Chem. 46 (2007) 1222. [6] M. Grigorova, H.J. Blythe, V. Blaskov, V. Rusanov, V. Petkov, V. Masheva, D. ˜ oz, M. Mikhov, J. Magn. Magn. Mater. 183 Nihtianova, L.M. Martinez, J.S. Mun (1998) 163. [7] H. Shenker, Phys. Rev. 107 (1957) 1246. [8] L. Piraux, J.M. George, J.F. Despres, C. Leroy, E. Ferain, R. Legras, K. Ounadjela, A. Fert, Appl. Phys. Lett. 65 (1994) 2484. [9] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, V.G. Harris, Appl. Phys. Lett. 80 (2002) 2341. [10] K.E. Mooney, J.A. Nelson, M.J. Wagner, Chem. Mater. 16 (2004) 3155. [11] R. Arulmurugan, G. Vaidyanathan, S. Sendhilnathan, B. Jeyadevan, J. Magn. Magn. Mater. 298 (2006) 83. [12] A.R. West, in: Basic Solid State Chemistry, 1998, p. 356. [13] M. Rajendran, R.C. Pullar, A.K. Bhattacharya, D. Das, S.N. Chintalapudi, C.K. Majumdar, J. Magn. Magn. Mater. 232 (2001) 71. [14] F.X. Cheng, Z.Y. Peng, C.S. Liao, Z.G. Xu, S. Gao, C.H. Yan, D.J. Wang, J. Wang, Solid State Commun. 107 (1996) 471. [15] N. Moumen, P. Veillet, M.P. Pileni, J. Magn. Magn. Mater. 149 (1995) 67. [16] A.T. Ngo, P. Bonville, M.P. Pileni, J. Eur. Phys. B 9 (1999) 583. [17] Y.C. Mattei, O.P. Perez, M.S. Tomar, F. Roman, P.M. Voyles, W.G. Stratton, J. Appl. Phys. 103 (2008) 07E512. [18] A.F. Ju´nior, V. Zapf, P. Egan, J. Appl. Phys. 101 (2007) 09M506. [19] C.H. Yan, Z.G. Xu, F.X. Cheng, Solid State Commun. 111 (1999) 287. [20] S. Ammar, A. Helfen, N. Jouini, J. Mater. Chem. 11 (2001) 186. [21] M.I. Mendelson, J. Am. Ceram. Soc. 52 (1969) 443. [22] T. Meron, Y. Rosenberg, Y. Lereah, J. Magn. Magn. Mater. 292 (2005) 11. [23] S.C. Li, L.M. Liu, V.T. John, C.J. O’Connor, V.G. Harris, IEEE Trans. Magn. 37 (2001) 2350. [24] Z.X. Tang, C.M. Sorensen, K.J. Klabbunde, Phys. Rev. Lett. 67 (1991) 3602. [25] J.M. Hastings, L.M. Corliss, Phys. Rev. 104 (1956) 128.
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S.E. Harrison, W.S. Osmond, P.W. Teale, Phys. Rev. 106 (1957) 865. L. Wang, F.S. Li, J. Magn. Magn. Mater. 223 (2001) 233. C. Upadhyay, H.C. Verma, S. Anand, J. Appl. Phys. 95 (2004) 5746. S.D. Shenoy, P.A. Joy, M.R. Anantharaman, J. Magn. Magn. Mater. 269 (2004) 217. G. Hu, J.H. Choi, C.B. Eom, V.G. Harris, Y. Suzuki, Phys. Rev. B 62 (2000) R779. V. Sepelak, D. Baabe, D. Mienert, D. Schultze, F. Krumeich, F.J. Litterst, K.D. Becker, J. Magn. Magn. Mater. 257 (2003) 377. [32] F.K. Lotgering, Philips Res. Rep. 20 (1965) 320.
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[33] M.P.G. Sandoval, A.M. Beesley, M.M. Yoshida, L.F. Cobas, J.A.M. Aquino, J. Alloys Compd. 369 (2004) 190. [34] S. Calvin, E.E. Carpenter, B. Ravel, V.G. Harris, Phys. Rev. B 66 (2002) 224405. [35] D.J. Fatemi, V.G. Harris, V.M. Browning, J.P. Kirkland, J. Appl. Phys. 83 (1998) 6867. [36] J. Smit, H.P.J. Wijn, Les Ferrites, Dunod, Paris, 1961. [37] M.V. Vasic, B. Antic, A. Kremenovic, A.S. Nikolic, M. Stoiljkovic, N. Bibic, V. Spasojevic, P. Colomban, Nanotechnology 17 (2006) 4877.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles Zhenfa Zi a,b, Yuping Sun a,, Xuebin Zhu a, Zhaorong Yang a, Jianming Dai a, Wenhai Song a a b
Key Laboratory of Materials Physics, Institute of Solid State Physics, and High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, PR China Department of Physics and Electronic Engineering, Hefei Teachers College, Hefei 230061, PR China
a r t i c l e in fo
abstract
Article history: Received 29 June 2008 Received in revised form 4 September 2008 Available online 18 November 2008
CoFe2O4 ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group Fd3m O7h . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe2O4. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co2+ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe2O4 samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Vv 75.75.+a 76.80.+y Keywords: Cobalt ferrite Nanoparticle Mono-domain Superparamagnetic
1. Introduction Magnetic nanoparticles have attracted much attention in the past decades because of their potential applications in highdensity magnetic recording, magnetic fluids, data storage, spintronics, solar cells, sensors, and catalysis [1–5]. Among the magnetic nanoparticles, cobalt ferrite (CoFe2O4) has been widely studied due to high electromagnetic performance [6], excellent chemical stability, mechanical hardness, and high cubic magnetocrystalline anisotropy [7]. These properties make it a promising candidate for many applications in commercial electronics such as video, audio tapes, high-density digital recording media [8–10], and magnetic fluids [11]. The cubic-spinel-structured CoFe2O4 ferrite represents a wellknown and important class of iron oxide materials. The O2 ions form fcc close packing, and the Co2+ and Fe3+ occupy either tetrahedral or octahedral interstitial sites [12]. These two antiparallel sublattices, which are coupled by superexchange interactions through the O2 ions, form the ferrimagnetic structure. Most of the magnetic properties of CoFe2O4 ferrite strongly depend on the size and shape of the nanoparticles [6,13], which are closely related to the method of preparation. Several routes have been used to prepare cobalt ferrite, including the sol–gel
Corresponding author. Tel.: +86 551 5592757; fax: +86 551 5591434.
E-mail address: [email protected] (Y. Sun). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.11.004
method [14], microemulsion with oil in water micelles [15], reverse micelles method [16], chemical coprecipitation method [17], combustion reaction method [18,19], and forced hydrolysis in a polyol medium [20]. However, most of these methods cannot be economically applied on a large scale because they require expensive and often toxic reagents, complicated synthetic steps, high reaction temperatures, and long reaction times. This not only results in waste of energy but also harms our environment. As is well known, the chemical coprecipitation method is usually used to synthesize magnetic oxides due to its simplicity and good control of grain size. However, in previous reports it was always observed that there exist some undesirable intermediate phases, which led to poor magnetic properties and irregular shape for the derived CoFe2O4 ferrite particles. In this paper, singlephase highly monodisperse CoFe2O4 ferrite nanoparticles with relatively homogeneous size were prepared by means of a modified chemical coprecipitation method.
2. Experimental details The CoFe2O4 ferrite particles were prepared by the chemical coprecipitation method by digestion. The starting materials were high-purity Co(CH3COO)2 4H2O and Fe(NO3)3 9H2O. Sodium hydroxide, NaOH, was used as the precipitation agent. The reaction is described by Co2þ þ 2Fe3þ þ 8OH ! CoFe2 O4 þ 4H2 O
(1)
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First, Co(CH3COO)2 4H2O and Fe(NO3)3 9H2O with Co:Fe atomic ratio of 1:2 were dissolved in deionized water by gentle heating. Then the aqueous mixture was slowly poured into a well-stirred NaOH solution and stirred for several minutes. The digestion was performed at 110 1C for 120 min. During digestion, the particles grew and evolved into a spherical structure. After digestion, the gelatinous precipitate was filtered and washed several times using deionized water until the pH value of the solution became neutral. Finally, the gelatinous precipitate was dried at 80 1C in air to form a powder sample. In order to prepare monodisperse CoFeO4 nanoparticles, the powder was dispersed by ultrasonic in ethanol, centrifuged, rewashed with ethanol, and dried. The crystal structure was examined by Philips X’pert PRO X-ray diffractometer (XRD) with CuKa radiation at room temperature and transmission electron microscopy (TEM, JEM-200CX) equipped with a selected-area electron diffractometer (SAED) on a JEOL TEM (400 kV). The morphology of grains was investigated by field-emission scanning electronic microscopy (FE-SEM), and the compositions were examined by energy-dispersive spectroscopy (EDS) in the FE-SEM. Magnetization measurements from room temperature to 800 K were performed using a vibratingsample magnetometer (VSM) as a component in physical properties measurement system (PPMS; 1.8 KpTp1000 K, 0 TpHp9 T).
3. Results and discussion The room-temperature powder XRD patterns of as-prepared CoFe2O4 ferrite particles post-heat treated at 800 K are shown in Fig. 1. All the peaks can be indexed to a single-phase spinel structure. No additional and intermediate phase is observed within the sensitivity of experimental measurements. The average crystallite size of as-prepared CoFe2O4 ferrite particles was estimated through analysis using the classical Scherrer formula [21] Dhkl ¼ kl/b cos y, where Dhkl is the crystallite size derived from the (3 11) peak of the XRD profiles, k the sphere shape factor (0.89), y the angle of the diffraction, b the difference between the full-width at half-maximum (FWHM) of the peak of the sample and the standard SiO2 used to calibrate the intrinsic width associated with the instruments, and l the wavelength of X-ray (1.54056 A˚). The obtained average crystallite size of as-prepared CoFe2O4 ferrite particles is about 8.56 nm. Based on the peak
Fig. 1. XRD patterns of the as-prepared CoFe2O4 ferrite particles (a) and post-heat treated at 800 K (b). Inset: crystal structure sketch map of CoFe2O4 ferrite composed of tetrahedral (A) and octahedral (B) sites.
central positions obtained from the high-intensity low-angle (3 11) and high-intensity high-angle (4 4 0) peaks, the ferrite lattice parameter was estimated as 8.385 A˚, which is in good agreement with the bulk value of 8.377 A˚ [22]. A typical FE-SEM image of the as-prepared CoFe2O4 ferrite particles is shown in Fig. 2(a). It is clearly seen that the grains are regular spherical particles without agglomeration despite the dipolar interaction between the particles. Most of particles are of rather homogenous grain sizes from 20 to 30 nm. A quantitative EDS analysis is used to determine the composition of CoFe2O4 particles. The results in Fig. 2(b) show that the molar ratio of Co, Fe, and O is about 1:1.98:3.96, which is consistent with the nominal composition. The TEM image and SAED pattern of CoFe2O4 ferrite particles are shown in Figs. 2(c) and (d), respectively. TEM observation reveals that the as-prepared CoFe2O4 ferrite particles are monodisperse and the particle size is about 25 nm. It is found this size is approximately three times larger than that estimated by the Scherrer formula. This implies that our CoFe2O4 ferrite particles are polycrystalline. The SAED pattern is also used to identify the crystallite structure of the CoFe2O4 particles. The rings in the electron diffraction pattern are indexed with their respective hkl from the PDF database. The lattice parameter calculated was 8.38770.002 A˚, which is consistent with the result estimated from XRD. Fig. 3 is the diameter distribution of the CoFe2O4 ferrite particles according to Fig. 2(a). The obtained CoFe2O4 particles show a Gaussian distribution with a maximum at grain size of 25 nm. This implies that the sample has a narrow grain size distribution and the diameters of most CoFe2O4 ferrite particles are estimated to be between 20 and 30 nm. The magnetic behavior of the CoFe2O4 ferrite particles will vary with different Co2+ site occupations since the Co2+ ion is highly anisotropic [23]. To investigate the magnetic properties of the synthesized CoFe2O4 ferrite particles, the temperature dependence of magnetization M(T) and the applied magnetic field dependence of magnetization M(H) are measured. The asprepared CoFe2O4 ferrite sample was heated from room temperature to 800 K and then cooled to room temperature, while magnetic data were collected in a magnetic field of 100 Oe. The measured M(T) curve of the CoFe2O4 ferrite particles is shown in Fig. 4. With increasing temperature, a large drop in magnetization occurred. This indicates a sharp ferrimagnetic to paramagnetic transition, and the Curie temperature TC derived by the peak of dM(T)/dT as shown in the inset of Fig. 4 is 677.0 K, which is slightly lower than that of the bulk. This can be explained by the finitesize-scaling effect [24]. When the sample was subsequently cooled to room temperature, it showed a different transition at 668.6 K. This phenomenon may be attributed to the fact that the structure of the as-prepared CoFe2O4 ferrite sample is unstable and can be altered in the process of increasing-temperature measurement. Heating may change the distribution of metal ions between tetrahedral and octahedral sites of ferrite, which has been confirmed by neutron diffraction and experiments [25,26]. In ideal inverted CoFe2O4 ferrite, all the cobalt is on the octahedral sites, hence none on the tetrahedral sites of the spinel lattice. Heating changed the distribution of metal ions between tetrahedral and octahedral sites, which means the different magnetic transition observed in our sample may be the result of site redistribution. In order to verify this hypothesis, the as-prepared CoFe2O4 ferrite sample was annealed at 800 K by simulating experimental measuring process. XRD result of the sample post-heat treated at 800 K is shown in Fig. 1. The lattice parameter for annealed CoFe2O4 ferrite sample was estimated as 8.37470.002 A˚, which is a little lesser than the value of the as-prepared CoFe2O4 ferrite sample (8.38570.002 A˚). In general, there existed enhanced
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Fig. 2. (a) FE-SEM image of as-prepared CoFe2O4 ferrite particles, (b) EDS image analysis of the CoFe2O4 particles, (c) typical TEM image, and (d) SAED pattern of the CoFe2O4 particles.
CoFe2O4
16
H = 100 Oe
0.0
8
4
dM/dT (emu/g)
M (emu/g)
12
-0.2 -0.4 677.0 K 668.6 K
-0.6 300
0 300
400
400
500 600 T (K)
700
500
800
600
700
800
T (K) Fig. 3. Particle diameter distribution of CoFe2O4 ferrite particles synthesized by the modified chemical coprecipitation method.
lattice parameters in the case of nanoparticles due to interface structure with a large volume fraction [27,28]. This reduced lattice parameter in our sample could be attributed to an increased degree of inversion [29]. Nanosized as-prepared CoFe2O4 ferrite particles, generally have some antisite defects. In the spinel lattice, only one eighth of the tetrahedral sites are occupied by Fe3+ ions and half of the octahedral sites are occupied by Co2+ and Fe3+ ions. This large fraction of empty interstitial sites makes the spinel structure a very open structure conducive to cation migration [30]. After the sample was annealed, the relatively larger Co2+ ions would remigrate to their conventionally preferred octahedral sites from the larger tetrahedral sites. In such a case,
Fig. 4. Magnetization versus temperature curve for the CoFe2O4 particles. Arrows indicate direction of temperature change. The inset is the temperature dependence of the first derivative of M(T), dM(T)/dT.
the Co2+ ions need not expand the lattice, which results in the reduced lattice parameter in annealed CoFe2O4 nanoparticles. There have been some reports on extracting the information related to cation distribution in spinel ferrites using the integrated intensity ratios of X-ray diffraction peaks. The integrated intensity of (2 2 0) reflection depends exclusively on the cations occupying the tetrahedral sites and the intensity of (2 2 2) reflection depends on the cations occupying the octahedral sites [31]. Therefore, the decrease in the intensity ratio of I(2 2 0)/I(2 2 2) in Fig. 1 means a reduction in the concentration of Co2+ ions that fill the tetrahedral sites, and as a result an increase of an equal number of Co2+ ions in
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octahedral sites, which could otherwise have filled the octahedral sites. This further supports that heating changed the distribution of metal ions between tetrahedral and octahedral sites in our sample. This result can be used to qualitatively explain the change of TC in increasing- and decreasing-temperature measuring processes. Since the superexchange interaction between Fe3+ ions in tetrahedral and octahedral sites is stronger than that between Co2+ ions in tetrahedral and Fe3+ ions in octahedral sites [32], the increase of the degree of inversion causes the average exchange interaction to decrease, which in turn results in a decrease in TC. This explains why the as-prepared CoFe2O4 nanoparticles with a low inversion degree has a higher TC, while the sample measured in the process of increasing temperature with a high inversion degree has a lower TC. The M(H) loop for the as-prepared CoFe2O4 ferrite particles at 300 K is shown in Fig. 5. It is clear that the value of saturation magnetization, Ms, comes out to be 61.77 emu/g at room temperature, which is smaller than the bulk value (74.08 emu/g) [33]. The remanence magnitude, Mr, can be extracted from the hysteresis loop at the intersections of the loop with the vertical magnetization axis. The Mr value of 14.39 emu/g is obtained from the inset in Fig. 5. For nanosized ferrite particles, the surface areas are larger and thus the surface energy and surface tension are high. This results in changes in cationic preferences and leads to an increased degree of antisite defects and thus lesser magnetizations [34,35]. The coercivity for a ferromagnet or ferrimagnet can be reflected by the coercivity field, Hc. This value refers to the intensity of the magnetic field required to reduce the magnetization of the magnetic sample to zero, after the magnetization of the sample has reached saturation. The obtained Hc value (0.519 kOe) for our sample is much lower than that in the literature [17]. Generally there are two reasons for reduced coercivity. On the one hand, the CoFe2O4 ferrite particles may have multi-domain structure. The formation of multi-domains and the easy movement of the domain walls can result in a decrease of coercivity. The critical size of a mono-domain particle is estimated using the formula [36] Dm ¼ 9sw =2pM2s , where sw ¼ ð2kB T C jK 1 j=aÞ1=2 is the wall density energy, |K1| the magnetocrystalline anisotropy constant, TC the Curie temperature, Ms the saturation magnetization, kB the Boltzmann constant, and a the lattice constant. For D4Dm, the particles exist in a multi-domain structure, while for DoDm, the particles exist in a mono-domain structure. For
80 CoFe2O4 300 K
0 20 M (emu/g)
M (emu/g)
40
-40
Mr
0 Hc
-20 -1000 -500
-80 -40
-20
0
0 H (Oe)
20
500
1000
40
H (kOe) Fig. 5. Room-temperature M–H hysteresis loop of the CoFe2O4 particles. The inset contains the minor loop.
CoFe2O4 ferrite, TC ¼ 677 K, a ¼ 8.385 108 cm, |K1| ¼ 3.8 106 erg/cm3, and Ms ¼ 310 Gs, the estimated value of Dm is about 430 nm, which is far larger the average diameter of the obtained CoFe2O4 ferrite particles. So the grains exhibit a mono-domain behavior. On the other hand, the decrease in coercivity for our sample may be attributed to a typical particle-size-dependent behavior [37]. For DoDm, the particles exist in a mono-domain structure. As the particle size decreases in succession, the coercivity decreases to some extent, which could be characteristically described as a transformation from ferrimagnetic to superparamagnetic state.
4. Conclusions A modified chemical coprecipitation method by digestion was used to prepare CoFe2O4 ferrite nanoparticles. The XRD results showed that the derived sample was in pure phase with space group of Fd3m O7h . FE-SEM and TEM results showed that the grains were regular sphere-shaped nanoparticles with sizes from 20 to 30 nm. The magnetic properties, including TC, roomtemperature Ms, Mr, and Hc, were investigated. The shift of TC can be explained as due to changed Co2+ ion redistribution in tetrahedral and octahedral sites on heating. The reduction of coercivity can be understood by the transformation from ferrimagnetic to superparamagnetic state.
Acknowledgements This work was financially supported by the National Key Basic Research Program of China under Contract no. 2007CB925002, National Nature Science Foundation of China under Contract nos. 10774146, 10774147, 50672099, and 50701042, and Director’s Fund of Hefei Institutes of Physical Science, Chinese Academy of Sciences. References [1] Y. Yin, A.P. Alivisatos, Nature (London) 437 (2005) 664. [2] M.A. El-Sayed, Acc. Chem. Res. 37 (2004) 326. [3] B.Y. Geng, J.Z. Ma, X.W. Liu, Q.B. Du, M.G. Kong, L.D. Zhang, Appl. Phys. Lett. 90 (2007) 043120. [4] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [5] A.H. Lu, E.L. Salabas, F. Schu¨th, Angew. Chem. 46 (2007) 1222. [6] M. Grigorova, H.J. Blythe, V. Blaskov, V. Rusanov, V. Petkov, V. Masheva, D. ˜ oz, M. Mikhov, J. Magn. Magn. Mater. 183 Nihtianova, L.M. Martinez, J.S. Mun (1998) 163. [7] H. Shenker, Phys. Rev. 107 (1957) 1246. [8] L. Piraux, J.M. George, J.F. Despres, C. Leroy, E. Ferain, R. Legras, K. Ounadjela, A. Fert, Appl. Phys. Lett. 65 (1994) 2484. [9] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, V.G. Harris, Appl. Phys. Lett. 80 (2002) 2341. [10] K.E. Mooney, J.A. Nelson, M.J. Wagner, Chem. Mater. 16 (2004) 3155. [11] R. Arulmurugan, G. Vaidyanathan, S. Sendhilnathan, B. Jeyadevan, J. Magn. Magn. Mater. 298 (2006) 83. [12] A.R. West, in: Basic Solid State Chemistry, 1998, p. 356. [13] M. Rajendran, R.C. Pullar, A.K. Bhattacharya, D. Das, S.N. Chintalapudi, C.K. Majumdar, J. Magn. Magn. Mater. 232 (2001) 71. [14] F.X. Cheng, Z.Y. Peng, C.S. Liao, Z.G. Xu, S. Gao, C.H. Yan, D.J. Wang, J. Wang, Solid State Commun. 107 (1996) 471. [15] N. Moumen, P. Veillet, M.P. Pileni, J. Magn. Magn. Mater. 149 (1995) 67. [16] A.T. Ngo, P. Bonville, M.P. Pileni, J. Eur. Phys. B 9 (1999) 583. [17] Y.C. Mattei, O.P. Perez, M.S. Tomar, F. Roman, P.M. Voyles, W.G. Stratton, J. Appl. Phys. 103 (2008) 07E512. [18] A.F. Ju´nior, V. Zapf, P. Egan, J. Appl. Phys. 101 (2007) 09M506. [19] C.H. Yan, Z.G. Xu, F.X. Cheng, Solid State Commun. 111 (1999) 287. [20] S. Ammar, A. Helfen, N. Jouini, J. Mater. Chem. 11 (2001) 186. [21] M.I. Mendelson, J. Am. Ceram. Soc. 52 (1969) 443. [22] T. Meron, Y. Rosenberg, Y. Lereah, J. Magn. Magn. Mater. 292 (2005) 11. [23] S.C. Li, L.M. Liu, V.T. John, C.J. O’Connor, V.G. Harris, IEEE Trans. Magn. 37 (2001) 2350. [24] Z.X. Tang, C.M. Sorensen, K.J. Klabbunde, Phys. Rev. Lett. 67 (1991) 3602. [25] J.M. Hastings, L.M. Corliss, Phys. Rev. 104 (1956) 128.
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