J Supercond Nov Magn (2014) 27:2111–2115 DOI 10.1007/s10948-014-2561-9
ORIGINAL PAPER
Magnetic Properties of Fe3O4 Nanoparticles Synthesized by Coprecipitation Method P. H. Linh · D. H. Manh · P. T. Phong · L. V. Hong · N. X. Phuc
Received: 17 January 2014 / Accepted: 8 April 2014 / Published online: 29 May 2014 © Springer Science+Business Media New York 2014
Abstract In this paper, we elucidate several specific magnetic properties of Fe3 O4 nanoparticles synthesized by coprecipitation method. The characterizations by Xray diffraction technique (XRD) and scanning electron microscopy (SEM) showed the particles to be of spinel structure and spherical shapes whose diameter could be controlled in the range from 14 to 22 nm simply by adjusting the precursor salts concentration and coprecipitation temperature. Magnetic properties of the Fe3 O4 nanoparticles measured by using vibration sample magnetometer (VSM) indicated the saturation magnetization and blocking temperature to increase with the particles size. Fe3 O4 nanoparticles with crystal size smaller than 22 nm exhibits superparamagnetic behavior at room temperatures. Characteristic magnetic parameters of the particles including saturation magnetization, effective anisotropy constant, and magnetocrystalline anisotropy constant have been deter-
P. H. Linh · D. H. Manh () · L. V. Hong · N. X. Phuc Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam e-mail:
[email protected] P. T. Phong () Nha Trang Pedagogic College, 01 Nguyen Chanh Street, Nha Trang City, Khanh Hoa Province, Vietnam e-mail:
[email protected] P. T. Phong Department of Advanced Materials Chemistry, Dongguk University Gyeongju, 707 Suckjang-dong, Gyeongju-Si, Gyeongbuk, 780-714, South Korea
mined. The observed decrease of saturation magnetization was explained on the base of core-shell model. A simple analysis indicated that the shell thickness decreases with an increase in particle size. Keywords Magnetic properties · Magnetite nanoparticles · Superparamagnetic · Core-shell model
1 Introduction Iron oxide (Fe3 O4 ) nanoparticles have attracted a great attention of scientists because they not only provide the basic understanding of magnetic nanoscience but also have potential application in various biomedical fields such as: diagnostics, therapy, and drug delivery [1]. At nanoscale, magnetic properties of Fe3 O4 particles are influenced by shape, size, and surface effects. When the size of Fe3 O4 nanoparticles is below a critical value, the multidomain structure of the bulk is replaced by single domain structure in each particle and the numbers of surface atoms are comparable with that of volume atoms. Thus, these features make magnetic nanoparticles different from their bulk counterparts. So far, there are many various methods to prepare Fe3 O4 nanoparticles, which have been reported in other papers, such as microemulsion technology, hydrothermal, and thermal decomposition synthesis [2]. These methods may be able to prepare magnetite particles with several controllable particle diameters. Among the liquid phase methods, the chemical coprecipitation method is able to yield Fe3 O4 nanoparticles of good quality with a facile possibility to get various sizes depending on chemical environment and the precursor used [3]. On the other hand, for applying in
2 Experimental Fe3 O4 nanoparticles were prepared by the chemical coprecipitation method presented in our previous report [5]. In order to obtain different particles sizes, we adjusted the concentration of the initial salt solutions (FeCl2 .4H2 O and FeCl3 .6H2 O) and the reaction temperature. The samples have been defined as S1, S2 corresponding to the salt concentrations of 0.02 M0.01 M and reaction temperature of 80 and 100 ◦ C, respectively. The sample S3 was synthesized with the salt concentrations of 2M1M and at room temperature. Crystal structure, phase composition and particle size of the obtained Fe3 O4 nanoparticles were determined by the X-ray powder diffraction (XRD) and field emission scanning electron microcopy (FESEM). Magnetic properties were examined by a vibrating sample magnetometer (VSM).
3 Results and Discussion The XRD patterns of the three samples are shown in Fig. 1. The existence of the peaks located at (111), (220), (311), (222), (400), (422), (440), and (511) in the XRD patterns illustrates the single phase spinel cubic structure with the Fd/3m space group. The lattice parameters of S1, S2, and ˚ respectively. For S3, S3 were 8.381, 8.388, and 8.394 A, the value of lattice parameter is very close to that of the magnetite bulk. The broad (311) peak indicates that the particles of the Fe3 O4 sample are of nanosize. The average
10
20
30
40 50 degree (2 )
60
(511)
(422) (440)
(400)
(311) (222)
(220)
S1 S2 S3
(111)
biomedicine, Fe3 O4 nanoparticles are required to exhibit the superparamagnetic property at room temperature and have high saturation magnetization to avoid aggregation and enhance the ability to respond to applied magnetic field. Thus, most of researches on Fe3 O4 nanoparticles for biomedicine applications concentrated on superparamagnetic behavior and surface effect. Previous studies confirmed that the saturation magnetization of magnetic nanoparticles is lower than that of bulk [1]. The core-shell model was introduced to explain the reduced magnetization in nanoparticles. Recently, the magnetic anisotropies of Fe3 O4 were measured [4]; however, the calculating of magnetocrystalline anisotropy constant of Fe3 O4 nanoparticles by using the law of approach to saturation is not yet well understood. In this study, the magnetic behavior of Fe3 O4 nanoparticles with the average particle diameter of 14, 16, and 20 nm prepared by coprecipitation method has been investigated. The decrease in saturation magnetization and the superparamagnetic behavior at room temperatures are interpreted by focusing on the role of particle size.
J Supercond Nov Magn (2014) 27:2111–2115
Intensity (a.u)
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70
Fig. 1 X-ray diffraction patterns of the Fe3 O4 samples with different average particle sizes
particle sizes of the three samples were calculated by the Hall–Williamson method [6]: β cos θ =
0.89λ + 4ε sin θ D
(1)
˚ β is the full in which, λ is the X-ray wavelength (1.546 A), width at half maximum (FWHM) of the peak, and ε is the internal strain. The average particle sizes were found to be approximately of 10, 12, and 22 nm for the samples S1, S2, and S3, respectively. From the FESEM images of all three samples shown in Fig. 2, we observed that the average particle sizes are 14, 16, and 20 nm for samples S1, S2, and S3, respectively. The average grain sizes calculated from the XRD data are slightly different than those estimated from the FESEM images. Figure 3 shows the thermal variation of field cooled (FC) and zero-field cooled (ZFC) magnetization of differently sized Fe3 O4 nanoparticle samples at 100 Oe. All the samples exhibited a separation between MZFC and MFC curves at temperatures below the magnetic irreversibility temperatures Tirr . Magnetic irreversibility shows some degree of spin disorder and frustration related to the domain freezing/pinning in the ferromagnetic ground state of the bulk sample of magnetite. Spin disorder and frustration are more pronounced in nanoparticle size samples. The magnetic irreversibility temperature Tirr showed up to increase with the particle size. Moreover, the MZFC exhibits a maximum at TB (
