Magnetic properties of superparamagnetic γ-Fe2O3 nanoparticles prepared by coprecipitation technique

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Journal of Magnetism and Magnetic Materials 286 (2005) 5–9 www.elsevier.com/locate/jmmm

Magnetic properties of superparamagnetic g-Fe2O3 nanoparticles prepared by coprecipitation technique Jong-Ryul Jeonga,, Sung-Chul Shina, Seung-Jun Leeb, Jong-Duk Kimb a

Department of Physics and Center for Nanospinics of Spintronic Materials, Korea Advanced Institute of Science and Technology,Yuseong-gu, Daejeon 305-701, Republic of Korea b Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-701, Republic of Korea Available online 26 October 2004

Abstract g-Fe2O3 nanoparticles have been synthesized by a chemical coprecipitation technique using the typical pipette drop method and the novel piezoelectric nozzle method. Nanoparticles made by the piezoelectric nozzle method show smaller size and very narrow size distribution compared to the nanoparticles produced by the pipette drop method. The superconducting quantum interference device (SQUID) measurements show superparamagnetism of nanoparticles and reveal that the anisotropy constants of the nanoparticles made by the pipette drop method and the piezoelectric nozzle method are 2.2
106 and 9.0
106 erg/cm3, respectively. By measuring the magnetic relaxation of the magnetization at 5 K, we also obtained magnetic viscosity of g-Fe2O3 nanoparticles. r 2004 Elsevier B.V. All rights reserved. PACS: 87.61.c; 61.46.+w; 75.90.+w; 75.50.Mm Keywords: Maghemite; Superparamagnetism; Piezoelectric nozzle method

1. Introduction Magnetic nanoparticles are widely investigated because of their excellent electromagnetic and chemical properties [1–3]. The maghemite (gFe2O3) nanoparticles especially attract great deal Corresponding

author. Tel.: +82 42 869 8163; +82 42 869 8162 E-mail address: [email protected] (J.-R. Jeong).

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of interest due to their technological and fundamental importance, such as information storage, magnetic resonance imaging contrast agent, and macroscopic quantum tunnelling associated with size quantization and electronic quantum confinement effects [1,4–7]. Various methods have been reported for the synthesis of g-Fe2O3 nanoparticles, such as sonochemical synthesis, sol–gel reactions, and chemical solution. One of the most suitable techniques for preparing ultrafine

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.09.129

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nanoparticles in a controlled way is the chemical coprecipitation method [1,7]. In this study, we have synthesized g-Fe2O3 nanoparticles by a chemical coprecipitation technique through the typical pipette drop and the novel piezoelectric nozzle method, and magnetic properties of gFe2O3 nanoparticles were studied using SQUID magnetometer from 5 to 300 K.

2. Experimental The g-Fe2O3 nanoparticles were prepared by the chemical coprecipitation technique of ferric and ferrous ions in an alkali solution. The reaction steps in our process are as follows: FeCl2 ð1 molÞ þ FeCl3 ð2 molÞ ! Fe3 O4 ! g  Fe2 O3 A molar ratio of Fe(II)/Fe(III)=0.5 was dissolved in water with sonication. The resulting solution was poured into an alkali solution in two different ways: (a) the pipette drop-wise (pipette diameter: 2000 mm, 0.04 ml/drop) method and (b) the piezoelectric nozzle (nozzle size: 50 mm, 0.01 ml/drop) method. The schematic diagram of the typical pipette drop method and novel ink-jet method is shown in Fig. 1. In the typical pipette drop method for synthesizing the maghemite nanoparticle, it is difficult to control the size of droplets and dropping times. Also, big droplets are formed due to a big orifice diameter of the pipette’s nozzle. Therefore, we used the piezoelectric nozzle method to form small droplets and control them. The piezoelectric nozzle method is generally used in an ink-jet process. Details of preparation procedure will be published elsewhere. Briefly, in the piezoelectric nozzle process a solution is sprayed though a small orifice which is vibrated by a piezoelectric transducer. This method helps in achieving precise control of the sphere size and uniform size distribution. By using this method, we could reduce the size of droplets in an uniform manner and also control the dropping times. The shape and size of maghemite nanoparticles were determined by a transmission electron

Fig. 1. The schematic diagram of (a) the typical pipette drop method and (b) the piezoelectric nozzle method.

microscope (TEM). Magnetization and relaxation measurements were performed using a SQUID magnetometer from 5 to 300 K to investigate the magnetic properties of nanoparticles.

3. Results and discussion The size and shape of maghemite nanoparticles were investigated using TEM. Fig. 2 shows the TEM images of maghemite nanoparticles for (a)

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the pipette and (b) the piezoelectric nozzle method. Fig. 2 clearly shows that the particle size obtained by the piezoelectric method is smaller than that obtained by the pipette method. In Fig. 3, we demonstrate g-Fe2O3 nanoparticle size distribution measured from the TEM images. It is worthwhile to note that the size distribution of the maghemite nanoparticles prepared by the typical pipette drop method is from 5 to 8 nm, as shown in Fig. 3(a). However, the nanoparticles made by the piezoelectric nozzle method show smaller size and very narrow size distribution from 3 to 5 nm, as shown in Fig. 3(b). In order to analyze the size distribution quantitatively, it was fitted using a log-normal function [8,9]:

 ! x 1 1 x 2 pffiffiffiffiffiffi exp  2 ln P ln ¼ ; (1) x0 x0 2sd Asd 2p where sd is the standard deviation of the diameter and x0 is the mean diameter. A mean diameter of x0, as determined from Eq. (1), is about 3.8 nm with a standard deviation sd ¼ 0:15 for the nanoparticles prepared by the piezoelectric nozzle method, and x0 ¼ 7:0 nm; sd ¼ 0:17 for the nanoparticles prepared by the pipette method. The mean diameter of the nanoparticles is also verified by fitting the magnetization curve using the theoretical Langevin function [10]. 

M mH kB T ¼ coth ; (2)  M0 kB T mH where m is the true magnetic moment of each particle, kB is the Boltzmann constant, T is the absolute temperature, and M0 is the saturation magnetization. Figs. 4(a) and (b) show the fitting result for the magnetization curve of the nanoparticles by the pipette method and the piezo-

30

20

10

0

4

6

8

10

12

Size of particle ( nm )

(a)

50 40

Frequency ( % )

Fig. 2. TEM images of g-Fe2O3 nanoparticles for (a) the typical pipette drop method and (b) the piezoelectric nozzle method.

Frequency ( % )

40

30 20 10 0

(b)

2

4

6

Size of particle ( nm )

Fig. 3. Particle size distributions measured from TEM images for (a) the typical pipette drop method and (b) the piezoelectric nozzle method.

electric nozzle method at 300 K, respectively. All magnetization curves fit well with the Langevin function as shown in Fig. 4. It is well known that the diameter of the magnetic cluster at a particular temperature can be estimated using the Langevin function [10]. From analysis of these curves, the average size of magnetic nanoparticles is found to be 4.070.3 and 7.170.3 nm for the piezoelectric method and the pipette drop method, respectively. These values show good agreement with those obtained from TEM measurements. Since the magnetic moments follow the Langevin function as shown in Figs. 4(a) and (b), we could estimate the anisotropy constant of the nanoparticles using a Ne´el–Arrhenius relation, if

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nanoparticles made by the pipette drop method and the piezoelectric nozzle method, respectively [11]. Therefore, we could estimate the anisotropy constant of nanoparticles using the value of particle diameter x0 ¼ 7:1 and 4.0 nm, and t0 1010 s [1]. The estimated anisotropy constants are 2.2
106 and 9.0
106 erg/cm3 for the nanoparticles made by the pipette drop method and the piezoelectric nozzle method, respectively. It is

0.5

0.0

-0.5

-1.0 -0.005 (a)

0.000 0.005 Field/Temperature ( T/K )

Normalized magnetization

1.0

0.5

Magnetization (emu/g - arb. value)

Normalized Magntization

1.0

1000 Oe

0.333

200 Oe

0.330

0.327

100 Oe

0.0 101

-0.5

(a)

-1.0

102 103 Time ( sec. )

104

-0.05

(b)

0.000

0.005

Field/Temperature (T/K)

Fig. 4. The Langevin function fit for the magnetization curve of nanoparticles made by (a) the typical pipette drop method and (b) the piezoelectric nozzle method.

we assume the saturation magnetic moment of a particle is proportional to its volume [1,10]: KV ¼ T B kB ln

t ; t0

Magnetic viscosity

-0.005

(3)

where TB is the blocking temperature, K is the anisotropy constant, kB is the Boltzmann’s constant, V is the volume of the nanoparticle, and t0 is a microscopic attempt time. In the previous study, we have reported that the blocking temperature of the nanoparticles is T B ¼ 119 and 94 K for the

-0.10

-0.15

-0.20 0 (b)

1000

2000

Field (Oe)

Fig. 5. (a) Field dependence of the magnetic relaxation for the nanoparticles made by the piezoelectric nozzle method at 5 K. (b) Magnetic viscosity estimated from the slope of magnetic relaxation curve.

ARTICLE IN PRESS J.-R. Jeong et al. / Journal of Magnetism and Magnetic Materials 286 (2005) 5–9

noteworthy that the anisotropy constant of nanoparticles made by the piezoelectric nozzle method is three times larger than that of the nanoparticles made by the typical pipette method. These results suggest that the piezoelectric method enhance not only the particle size distribution but also the crystallinity of g-Fe2O3 nanoparticles. We have also carried out magnetic relaxation experiments for the nanoparticles made by the piezoelectric nozzle method. In Fig. 5(a), we plot the magnetization measured at 5 K as a function of time on a logarithmic scale. For most superparamagnetic fine particles the time-dependent magnetization (M) at a given temperature (T) and applied field (H) can be described by logarithmic time dependence as follows [12–14]: MðtÞ ¼ const: þ S ln t;

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distribution but also the crystallinity of g-Fe2O3 nanoparticles. The magnetic viscosity of the nanoparticles made by the piezoelectric nozzle method was obtained by measuring the magnetic relaxation of the magnetization at 5 K.

Acknowledgements This work was supported by Ministry of Science and Technology through the Creative Research Initiatives Project and Center for Ultramicrochemical Process Systems Project. The authors would like to express sincere thanks to Dr. Sang-Jun Oh in KBSI for helping in SQUID measurements.

(4)

where the parameter S ¼ dM=dðln tÞ is the magnetic viscosity caused by the thermal activation of magnetization reversal over the activation energy. The viscosity of the nanoparticles made by the piezoelectric nozzle method is estimated from Fig. 5(b) under several magnetic field strengths. It is found that the viscosity increases from 0.2 to 0.05 by varying an applied magnetic field from 200 to 2000 Oe.

4. Conclusions The g-Fe2O3 nanoparticles have been produced by a chemical coprecipitation technique using the pipette drop method and the piezoelectric nozzle method. The size distribution of the g-Fe2O3 nanoparticles prepared by the typical pipette drop method is from 5 to 8 nm. However, the nanoparticles made by the piezoelectric nozzle method show smaller size and very narrow size distribution from 3 to 5 nm. TEM measurement and magnetic property measurement reveal that the piezoelectric method enhances not only the particle size

References [1] J.L. Dormann, D. Fiorani (Eds.), Magnetic Properties of Fine Particles, North-Holland, Amsterdam, 1992. [2] G.C. Hadjipanayis, R.W. Siegel (Eds.), Nanophase Materials: Synthesis, Properties, Application, Kluwer Academic Publishers, Dordrecht, 1994. [3] R.C. Ashoori, Nature 379 (1996) 413. [4] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, J. Am. Chem. Soc. 123 (2001) 12798. [5] A.O. Caldeira, A.J. Leggett, Phys. Rev. Lett. 46 (1981) 211. [6] J. Tejada, X.X. Zhang, Li Balcells, J. Appl. Phys. 73 (1993) 6709. [7] L. Zhang, G.C. Papaefthymiou, J.Y. Ying, J. Phys. Chem. B 105 (2001) 7414. [8] D.K. Kim, Y. Zhang, W. Voit, K.V. Rao, M. Muhammed, J. Magn. Magn. Mater. 225 (2001) 30. [9] C.G. Granqvist, R.H. Buhrman, J. Appl. Phys. 47 (1976) 2200. [10] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, MA, 1972, pp. 410–418. [11] J.-R. Jeong, S.-J. Lee, J.-D. Kim, S.-C. Shin, Phys. Stat. Sol. 241 (7) (2004) 1593. [12] R. Street, J.C. Woolley, Proc. Phys. Soc. A 62 (1987) 562. [13] L.F. Kiss, t. Keme´ny, I. Vincze, J. Phys.: Condens. Matter 9 (1997) 10501. [14] J.R. Friedman, U. Voskoboynik, M.P. Sarachik, Phys. Rev. B 56 (1997) 10793.