Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3−xO4) nanoparticles

Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3−xO4) nanoparticles

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 314 (2007) 60–67 www.elsevier.com/locate/jmmm Synthesis and magnetic characterization o...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 314 (2007) 60–67 www.elsevier.com/locate/jmmm

Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3xO4) nanoparticles Victoria L. Calero-DdelC, Carlos Rinaldi Department of Chemical Engineering, University of Puerto Rico, Mayagu¨ez Campus, P.O. Box 9046, Mayagu¨ez, PR 00680, Puerto Rico Received 13 May 2006; received in revised form 8 December 2006 Available online 16 January 2007

Abstract Cobalt-substituted ferrite nanoparticles were synthesized with a narrow size distribution using reverse micelles formed in the system water/AOT/isooctane. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (CoxFe3xO4) and some indication of g-Fe3O4 when 4:1 and 5:1 Fe:Co ratios were used. Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) showed bands at 560 and 400 cm1, characteristic of the metal–oxygen bond in ferrites. Transmission electron microscopy showed that the number median diameter of the particles was 3 nm with a geometric deviation of 0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a ¼ 8.388 A˚ for Co0.61Fe0.39O4, which is near that of CoFe2O4 (a ¼ 8.394 A˚). Magnetic properties were determined using a superconducting quantum interference device (SQUID). Coercivities higher than 8 kOe were observed at 5 K, whereas at 300 K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field and an expression relating w0 and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of 106 erg/cm3 were determined using the Debye model, whereas anisotropy constants in the order of 107 erg/cm3 were calculated assuming Ot ¼ 1 at the temperature peak of the in-phase component of the susceptibility curve as commonly done in the literature. Our analysis demonstrates that the assumption Ot ¼ 1 at the temperature peak of w0 is rigorously incorrect. r 2007 Published by Elsevier B.V. PACS: 74.25.Ha; 75.30.Gw; 75.50.Tt; 75.90.+w Keywords: Reverse micelle; Cobalt ferrite; AC susceptibility; Anisotropy constant

1. Introduction Research in suspensions of magnetic nanoparticles has increased in recent years due to their potential in a variety of applications [1] such as magneto-responsive colloidal extractants [2], targeted drug delivery vectors for gene therapy, genetic screening, biochemical, and toxicity cleansing [3–6], in vivo imaging and contrast agents [7], magnetocytolysis agents for treatment of localized cancerous tumors [7–14], magnetic cell sorting schemes [12], and binding of magnetic nanoparticles to antibodies to label

Corresponding author. Tel.: +1 787 832 4040x3585; fax: +1 787 834 3655. E-mail address: [email protected] (C. Rinaldi).

0304-8853/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.12.030

molecules, cell populations, structures, or microorganisms or for capturing cells and other biological targets from blood or other fluid and tissue samples [3]. A great variety of techniques has been used to obtain magnetic nanoparticles such as milling, co-precipitation, and synthesis in reverse micelles. Various authors [15–21] have used the latter technique to obtain particles with diameters ranging from 2 to 20 nm, with narrow size distribution and control of the nominal size by varying the water to surfactant ratio. Because of its high coercivity and anisotropy constant, cobalt ferrite nanoparticles have been widely studied [15,22–30]. High coercivity gives cobalt ferrite potential in high-capacity magnetic storage, whereas high magnetic anisotropy forces the particles to relax through the Brownian mechanism, giving them potential applications as sensors [1,31–37].

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Typically, the anisotropy constant value is determined by assuming that Ot ¼ 1 (O is the radian field frequency and t the Ne´el relaxation time) at the peak of the in-phase component of the alternating current (AC) susceptibility curve as a function of temperature. Values in the order of 107 erg/cm3 have been reported [23,30,38], which are higher than those for the bulk material (1.8–3.0  106 erg/cm3). We have used the Debye model for a magnetic dipole in an oscillating field to demonstrate that the assumption Ot ¼ 1 is incorrect at the peak of the in-phase component of the AC susceptibility curve as a function of temperature. Based on our analysis, an expression relating the peak in-phase susceptibility and temperature was obtained and used to calculate values of the magnetic anisotropy constant which are of the same order as those found in the bulk material (106 erg/cm3). 2. Experimental details 2.1. Materials The precursor salts ferrous sulfate heptahydrate (FeSO4  7H2O) and cobalt chloride (II) pentahydrate (CoCl2  5H2O) and the surfactant, bis(2-ethylhexyl) sulfosuccinate, (AOT), were obtained from Fisher and used without modification. Aqueous ammonia (28% NH4OH) and isooctane were obtained from Sigma-Aldrich. 2.1.1. Reverse micelle preparation and nanoparticle synthesis Reverse micelles were prepared with ferrous sulfate heptahydrate (FeSO4  7H2O), cobalt chloride (II) pentahydrate (CoCl2  5H2O), and aqueous ammonia 10% (NH4OH) and combined to form nanoparticles in the water pool of the micelles. For each Fe:Co ratio (3:1, 4:1, and 5:1) two reverse micelle precursor solutions were used. One contained both metal salts in the specified molar ratio and the other the ammonia solution. Reverse micelles were prepared by adding a specific amount of aqueous solution to 0.5 M AOT in isooctane. In the case of the metal salt solution, the total concentration of metal ions was kept at 1 M. The metal ion solution was added to the AOT/ isooctane solution in a 1:11 volume ratio in order to obtain a water to surfactant molar ratio w ¼ 10. The ammonia reverse micelle solution was prepared by adding 10% ammonia solution to the AOT/isooctane solution in a volume ratio of 1:8. Magnetic nanoparticles were obtained by mixing under vigorous stirring equal volumes of the metal salt and

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ammonia reverse micelle solutions. The reaction was allowed to take place for 2 h. Afterwards, the isooctane and water were removed by drying in a vacuum oven at 40 1C for 12 h, leaving a dry dark residue of particles and surfactant. To remove the surfactant and excess ions, the residue was washed several times using methanol, ethanol, and DI water successively, with each wash step followed by centrifugation. The final nanoparticle powder was obtained by drying the washed precipitate in a vacuum oven at 100 1C for 14 h. 2.2. Nanoparticle characterization X-ray diffractograms were obtained using a Siemens D5000 diffractometer with a scan rate of 0.0008 1/s. Magnetic properties of the powder sample were measured using a quantum design MPMS XL-7 SQUID magnetometer equipped with an AC susceptometer. The samples were diluted and immobilized in wax in order to minimize particle–particle interaction and to keep the particles from rotating rigid-body like as the applied field changes direction. Fourier transform infrared (FTIR) spectra were obtained with a Remspec mid-IR GAP interfaced to a Bruker Vector-22 spectrometer equipped with DTGS detector and a potassium bromide (KBr) beamsplitter. Elemental analysis was performed using ICP–MS at an independent laboratory (Galbraith Laboratories Inc.). 3. Results and discussion As shown in Table 1, elemental analysis indicates an Fe:Co ratio higher than expected for CoFe2O4 and higher than that used in the synthesis. This indicates that a cobalt substituted ferrite CoxFe3xO4 was produced. The calculated x values are shown in Table 1. FTIR spectra of the washed and dried samples indicated presence of AOT even after washing with ethanol and methanol multiple times. FTIR spectra (Fig. 1) showed bands in 1725, 1459, 1218, and 1039 cm1, which were attributed to shift bands of AOT. The small shift (10 cm1) is an indication of chemical bonding between AOT and the nanoparticles. Bands at 590 and 400 cm1, characteristic of the metal–oxygen bonds of ferrites [39–42], were also observed. X-ray diffraction of the powder, shown in Fig. 2, confirms the formation of the face-centered cubic (fcc) crystalline structure. The diffractogram for the sample synthesized with Fe:Co 3:1 presents higher background

Table 1 Nanoparticle composition, lattice parameters, interplanar spacing, and diameter determined using X-ray diffraction and TEM Preparation Fe:Co ratio

Measured Fe:Co ratio

x

A (A˚)

d311

DXray (nm)

DTEM (nm)

ln(sg)

3:1 4:1 5:1

3.9:1 5.8:1 6.4:1

0.61 0.44 0.41

8.388 8.311 8.356

2.529 2.500 2.520

3.56 4.77 4.76

3.20 3.44 3.04

0.21 0.12 0.18

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Bragg’s law (Eq. (2)) [43]. The values obtained are shown in Table 1: l , 2 sin y

(2)

a d hkl ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi . 2 h þ k2 þ l 2

(3)

d hkl ¼

Fig. 1. FTIR spectra for samples 3:1, 4:1, and 5:1 of cobalt-substituted ferrites and for AOT.

The lattice parameter and interplanar spacing for the Co0.61Fe2.39O4 (3:1) sample are similar to values reported for bulk cobalt ferrite (a0 ¼ 8.394 and d311 ¼ 2.531) [44,45], while those for Co0.44Fe2.56O4 (4:1) and Co0.41Fe2.59O4 (5:1) are lower. The values obtained for sample 4:1 and 5:1 can be attributed to simultaneous formation of another phase such as g-Fe2O3 (a0 ¼ 8.347 and d311 ¼ 2.517) [46]. A TEM image of the Co0.61Fe2.39O4 (3:1) sample is shown in Fig. 3. The particles are spherical as expected. The particle size distribution was determined from diameter measurements of 350 nanoparticles, fitted using a log normal size distribution function. The number median diameter Dpg and geometric deviation ln sg are shown in Table 1. Because the diameter obtained by X-ray diffraction is similar to the diameter obtained by TEM, we can consider the particles as single crystals. For the samples synthesized with an Fe:Co ratio of 4:1 and 5:1, the diameters determined by X-ray diffraction are a little higher than those obtained by TEM. This can be attributed to the high background noise of the diffractograms, which can introduce an error when determining the crystal diameter. Also, the crystallite size determined by Scherrer’s equation is a volume median diameter, which

Fig. 2. X-ray diffractograms of cobalt-substituted ferrites prepared with Fe:Co ratios of 3:1, 4:1, and 5:1. The 3:1 sample was measured with a scan rate of 0.01 1/s while samples 4:1 and 5:1 were measured with a scan rate of 0.0008 1/s.

noise, because a faster scan rate (0.01 1/s) was used. A scan rate of 0.0008 1/s was used for the other two samples. One can observe the peaks for the 3 1 1 and 5 1 1 planes typical of the inverse spinel of a ferrite. The background noise and presence of broad peaks are characteristic of particles with nanometer dimensions. The mean crystallite diameter was estimated using Scherrer’s Equation (1) and the 3 1 1 plane peak D

0:9l , b cos Y

(1)

where l is the wavelength of the incident X-ray (1.54056 A˚), Y is the diffraction angle, and b is the fullwidth at half maximum. The lattice parameter a and interplanar spacing dhkl were determined by Eq. (3) and

Fig. 3. TEM of cobalt-substituted ferrite nanoparticles prepared with an Fe:Co ratio of 3:1.

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Fig. 5. Magnetization curve for cobalt-substituted ferrite powder at 5 K.

Fig. 4. Magnetization curve for cobalt-substituted ferrite dispersed in wax at (a) 5 K and (b) 300 K.

Table 2 Magnetic properties of cobalt substituted ferrites prepared with various Fe:Co ratios Fe:Co ratio

TB (K) 100 Oe

Hc (5 K) (kOe)

Mr (5 K) (emu/g)

Ms (5 K) (emu/g)

Mr/ Ms (300 K) Ms (emu/g)

3:1 4:1 5:1

60.8 83.4 79.0

9.2 8.0 10.0

4.2 7.6 4.6

19.4 26.6 15.2

0.22 8.8 0.29 16.7 0.30 9.0

should be larger than the number median diameter determined from TEM. Magnetization curves were obtained at 5 and 300 K with applied fields of up to 50 kOe (5 T). Fig. 4 shows results for the three samples. In all cases the magnetization curve at 5 K shows hysteresis with coercivity higher than 8 kOe (0.8 T, Table 2), which is characteristic of single-domain cobalt ferrite nanoparticles. Similar results were observed for cobalt ferrite nanoparticles by Ahn et al. [47], Song and Zhang [48], Ammar et al. [49], and Tung et al. [23].

For the sample with 4:1 Fe:Co ratio, the magnetization curve at 5 K shows a step change in the magnetization at low fields. A similar observation was made by Ammar et al. [49], who attributed this step change to particle–particle interactions when the powder samples are not well dispersed. Fig. 5 shows the magnetization curves for the cobalt-substituted particles without dispersion in wax, in agreement with the observations of Ammar et al. [49]. We suggest that the observed step change is due to rigid body rotation of the particles with body-locked magnetic moment. As the applied field switches direction during measurement, the particle dipole moment becomes antiparallel with respect to the applied field, and hence unstable. A small disturbance causes the particles to rotate in order to align their magnetic dipoles parallel with the field. The saturation magnetization was determined by plotting M vs. 1/H at high fields and determining the infinite field value through extrapolation. The saturation magnetization value calculated for samples synthesized using reverse micelles is less than reported in the literature (Table 2). Tung et al. [23] obtained a saturation magnetization value of 67.95 emu/g at 300 K and 97.51 emu/g at 5 K for particles of 3.3 nm in magnetic diameter. Ammar et al. [49] observed values of 85.1 emu/g at 5 K and 65 emu/g at 300 K for saturation magnetization of cobalt ferrite particles with 5.5 nm in diameter. Hanh et al. [30] observed for 4 nm particles a Ms ¼ 75 emu/g at 5 K for cobalt-substituted ferrite with a Fe/Co ratio of 1.85. Our low values of saturation magnetization can be attributed to AOT binding to the particle surface which could result in increased non-collinearity structure in such small particles of cobalt-substituted ferrite [24]. The sample synthesized with a 4:1 ratio presents Ms (300 K) higher than sample 3:1, which is due to its larger particle size (0.2 nm greater) and lower cobalt content. Reduced remanence obtained for the three samples is less than 0.3, which is an indication of uniaxial anisotropy [24,49].

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Table 3 Calculated values of interaction parameter T0, anisotropy constant determined using the Debye model, Kw0 , and the assumption Ot ¼ 1 for the temperature peak of the in-phase component of the susceptibility, Kt, and the calculated value of t0 using the assumption Ot ¼ 1 Fe:Co ratio 3:1 4:1 5:1

T0 (K) 53.14 71.41 83.34

Kw0 (erg/cm3) 6

1.1  10 1.6  106 4.3  106

Kt (erg/cm3) 7

2.2  10 2.6  107 3.1  107

t0 (s) 9.9  1027 1.6  1022 5.5  1023

Fig. 7. Field cooled and zero field cooled magnetization as a function of temperature for cobalt-substituted ferrites dispersed in wax under an applied field of 100 Oe.

Fig. 6. (a) Magnetization curves at low field as a function of temperature and (b) relation between w0 and temperature for the cobalt-substituted ferrite synthesized using an Fe:Co ratio of 3:1.

The dependence of w0 with temperature was determined from magnetization measurements at low fields. Fig. 6 shows the linear behavior between magnetization and applied field, with slope w0. As shown in Fig. 6, the observed dependence of w0 with temperature agrees with the relation of Bradbury et al. (Eq. (4)) [50] for collections of interacting magnetic dipoles wi ¼

pfm0 M 2d d 3 , 18k ðT  T 0 Þ

(4)

here Md is the domain magnetization, d is the magnetic diameter of the particle, m0 is the free space permeability, f is the volume fraction, T0 is the interaction parameter, and k is Boltzman’s constant. The values for T0 are shown in Table 3. High values of T0 (T0450 K) were obtained despite the fact that the samples were dispersed in paraffin, indicating that interparticle interaction is non-negligible and must be considered during the analysis.

Zero-field cooled (ZFC) and field cooled (FC) magnetization curves were obtained for each sample at 100 Oe (Fig. 7). ZFC curves were obtained by cooling the sample under zero applied field to 5 K, then applying the desired field and slowly warming the sample to 300 K. FC curves were obtained by cooling the sample from 300 to 5 K under the applied field. Phenomenologically, the peak of the ZFC curve corresponds to a state where the particles cross from superparamagnetic behavior to ferromagnetic behavior with decreasing temperature. The temperature at which this peak occurs is commonly referred to as the blocking temperature. Our results are summarized in Table 2. Hanh et al. [30] observed at 100 Oe, a blocking temperature of 180 K for cobalt ferrite nanoparticles with diameter of 4 nm, which is higher than our observed temperatures of 60.8, 83.4 and 79 K for samples synthesized with 3:1, 4:1 and, 5:1 Fe:Co ratio, respectively. Comparing the values obtained of blocking temperature and particle diameter, we observe that the highest blocking temperature corresponds to the largest particle size whereas the lowest blocking temperature corresponds to the smallest particle size. The in-phase component of the magnetic susceptibility w0 was measured as a function of temperature for various frequencies. As seen in Fig. 8, the in-phase component of

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Fig. 8. In-phase component of the AC susceptibility w0 as a function of temperature for various frequencies in the cobalt-substituted ferrite prepared with Fe:Co ratio of 3:1 and dispersed in wax. The data for 0 Hz correspond to w0 values from Fig. 6.

the AC susceptibility displays a peak with respect to temperature. The shift in this peak with applied frequency can be used to estimate the magnetic anisotropy. According to the Debye model for dipolar systems [51], the in-phase component of the AC susceptibility w0 is given by w0 w0 ¼ , (5) 1 þ t2 O 2 where w0 and t are functions of temperature, t is the magnetic relaxation time, and O is the applied field frequency. Arguably, the Debye model may seem simplistic, however, we note that the results of a rigorous analysis by Raikher and Stepanov [52] reduce to this equation for the limits OtD 51; O2 t2 5KV =kT; KV =kT b1,

Fig. 9. Relation between the in-phase component of the AC susceptibility w0 and temperature corresponding to the maximum Tmax for the cobaltsubstituted ferrite prepared with Fe:Co ratio of 3:1 considering (a) Debye model and (b) Ot ¼ 1 at the peak of the in-phase susceptibility.

(6)

where tD ¼ ðKV =kTÞt0 . The first condition is easily met for the frequencies considered here, whereas the other conditions may be easily checked after obtaining values for K from analyzing the experimental measurements. Because the small-amplitude AC field represents a small departure from equilibrium, we assume the low-field limit of the Langevin equation applies and w0 is given by Eq. (4). We further assume magnetic relaxation occurs by the Ne´el process, hence the characteristic relaxation time is given by   KV t ¼ t0 exp , (7) kT where t0 is regarded as an attempt frequency (i.e., the natural frequency of gyromagnetic precession [53]), K is the magnetic anisotropy constant, and V ¼ pd3/6 is the volume of the magnetic cores. According to Raikher and Stepanov [52], t0 is of the order 1010–109 s. Combining Eqs. (4), (5), and (7) we obtained expressions for Ot and for the temperature corresponding to the

maximum wm0 of the in-phase component of the AC susceptibility: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 Ot ¼ 2KV ðTT Þ (8) 0  1 2 kT T 2max 2KV 36KV  ¼ ðT max  T 0 Þw0max . k T max  T 0 pfm0 M 2d D3

(9)

Fig. 9a shows the curve described by Eq. (12) using measurements for the 3:1 sample. Similar results were obtained for the other two samples. The intercept of the curve of T 2max =ðT max  T 0 Þ vs. ðT max  T 0 Þw0max was used to obtain the values of the magnetic anisotropy Kw0 in Table 3. Our values for the magnetic anisotropy constant differ considerably from values reported for similarly synthesized cobalt ferrite particles. For example, Hyeon et al. [54] report values with equivalent orders of magnitude, whereas Tung et al. [23], Ammar et al. [49], Moumen et al. [24] and Hanh et al. [30] report values that are one order of larger

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magnitude. We attribute the difference to a discrepancy in the way in which the AC susceptibility measurements have been analyzed. Commonly, it is assumed that the peak of the w0 vs. T curve corresponds to the condition Ot ¼ 1, hence plotting ln(1/O) vs. 1/T would yield a linear dependence in which the slope provides an estimate for K. Eq. (8) clearly shows that this assumption is incorrect. Fig. 9b shows data plotted taking this assumption. The corresponding values of Kt (Table 3) are in agreement with those reported in the literature. From the intercept of this curve, we obtained values of t0 in the range 1022–1027 s, which are clearly not physical. 4. Conclusions Cobalt-substituted ferrite nanoparticles with FCC structure have been synthesized using reverse micelles. Because the reverse micelles act as templates, we obtained nanoparticles with approximately 3 nm diameter and narrow size distribution. Despite the advantages posed by the reverse micelles, the method produces particles that are chemically bound to AOT, which is difficult to eliminate with a wash process. The presence of AOT in the nanoparticles can be inconvenient when the particles must be funtionalized with biomolecules, such as for use in a biosensor. Magnetic measurements show interparticle interaction despite dispersing the particles in paraffin. Superparamagnetic behavior was observed at 300 K whereas ferrimagnetic behavior was observed at 5 K. The ferrimagnetic behavior of the cobalt-substituted ferrite is characterized by coercivity values higher than 8 kOe, characteristic of cobalt ferrite nanoparticles. A Debye model provides a new alternative to determine the anisotropy constant using AC susceptibility measurements. Anisotropy values in the order of 106 erg/cm3 (in agreement with the bulk value) were obtained whereas values of 107 erg/cm3 were obtained assuming Ot ¼ 1, as is typically done in the literature. A rigorous analysis based on the Debye model for a magnetic dipole in an oscillating field was used to determine the temperature dependence of the peak of the in-phase component of the AC susceptibility curve when the relaxation time corresponds to a Ne´el process. This analysis demonstrates the assumption that Ot ¼ 1 at the peak of the in-phase AC susceptibility with respect to temperature is not rigorously valid. Acknowledgments The authors are grateful to Prof. Vijay John and Prof. Oscar Perales–Perez for helpful discussions, to the UPRRio Piedras Center for Nanoscale Materials, financed by NASA-URC grants number NASA-NCC3-1034, for the use of the high resolution transmission electron microscope, to Mr. Oscar Resto for assisting in taking TEM images, and to Prof. Samuel Hernandez and Mr. Leonardo Pacheco for assisting in making FTIR measurements.

This work was supported by the US National Science Foundation (CTS-0320534, DMR-0351449, and NSF0223152). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jmmm.2006.12.030 References [1] V.L. Calero-DdelC, C. Rinaldi, M. Zahn, Magnetic fluid and magnetic nanoparticle based sensors, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 5, American Scientific Publishers, 2006, pp. 223–234. [2] G.D. Moeser, K.A. Roach, W.H. Green, P.E. Laibinis, T.A. Hatton, Ind. Eng. Chem. Res. 41 (2002) 4739. [3] S.K. Sahoo, V. Labhasetwar, Drug Discov. Today 8 (2003) 1112. [4] P. Gould, Mater. Today (2004) 36. [5] B.M. Berkovsky, V.F. Medvedev, M.S. Krakov, Magnetic Fluids, Engineering Applications, Oxford, UK, 1993. [6] B. Berkovsky, V. Bashtovoi, Magnetic Fluids Applications Handbook, 1996. [7] E. Blums, A. Cebers, M.M. Maiorov, Magnetic Fluids, Berlin, 1996. [8] R.E. Rosensweig, Ferrohydrodynamics, Mineola, NY, 1997. [9] M. Chastellain, A. Petri, A. Gupta, K.V. Rao, H. Hofmann, Adv. Eng. Mater. 6 (2004) 235. [10] K.G. Hofer, Eur. Cells. Mater. 3 (2002) 67. [11] A. Halbreich, et al., Biochimie 80 (May–Jun) (1998) 379. [12] C.N. Ramchand, P. Pande, P. Kopcansky, R.V. Mehta, Indian J. Pure Appl. Phys. 39 (2001) 683. [13] M. Mabincova´, D. Leszczynska, P. Sourivong, P. Cicmanec, P. Babinec, J. Magn. Magn. Mater. 225 (2001) 109. [14] A. Jordan, et al., J. Magn. Magn. Mater. 194 (1999) 185. [15] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243. [16] V. Turco, M. Rossi, G. D’Arrigo, D. Manno, G. Micocci, Appl. Phys. A 69 (1999) 369. [17] M.P. Pileni, J. Phys. Chem. 97 (1993) 6961. [18] M.P. Pileni, Langmuir 13 (1997) 3266. [19] I. Lisiecki, M.P. Pileni, J. Am. Chem. Soc. 115 (1993) 3887. [20] C.T. Seip, E.E. Carpenter, C.J. O’Connor, IEEE Trans. Magn. 34 (1998) 1111. [21] C.J. O’Connor, C.T. Seip, E.E. Carpenter, S. Li, V.T. John, Nanostruct. Mater. 12 (1999) 65. [22] S. Li, L. Liu, V.T. John, C.J. O’Connor, V.G. Harris, in: Proceedings of the IEEE Transactions Magnetron, San Antonio, TX, 2001. [23] L.D. Tung, et al., J. Appl. Phys. 93 (2003) 7486. [24] N. Moumen, M.P. Pileni, J. Phys. Chem. 100 (1996) 1867. [25] E.J. Choi, et al., J. Magn. Magn. Mater. 262 (2003) L198. [26] A.J. Rondinone, A.C.S. Samia, Z.J. Zhang, Appl. Phys. Lett. 76 (2000) 3624. [27] S. Li, C.J. O’Connor, V.G. Harris, E. Carpenter, J. Appl. Phys. 87 (2000) 6223. [28] C.N. Chinnasamy, et al., J. Colloid Interface Sci. 263 (2003) 80. [29] V. Blaskov, et al., J. Magn. Magn. Mater. 162 (1996) 331. [30] N. Hanh, O.K. Quy, N.P. Thuy, L.D. Tung, L. Spinu, Phys. B: Condens. Mater. 327 (2003) 382. [31] R. Kotitz, T. Bunte, W. Weitschies, L. Trahms, J. Appl. Phys. 81 (1997) 4317. [32] R. Kotitz, W. Weitschies, L. Trahms, W. Semmler, J. Magn. Magn. Mater. 201 (1999) 102. [33] Y.R. Chemla, et al., Proc. Natl. Acad. Sci. 97 (2000) 14268. [34] C. Wilhelm, et al., Phys. Rev. E 65 (2002) 031404. [35] P. Daveze, H. Sahsah, J. Monin, Meas. Sci. Technol. 7 (1996) 157. [36] J.M. Perez, L. Josephson, T. O’Loughlin, D. Hogemann, R. Weissleder, Nat. Biotechnol. 20 (2002) 816.

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