Surface anisotropy change of CoFe2O4 nanoparticles depending on thickness of coated SiO2 shell
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- Coşkun, M., Can, M.M., Coşkun, Ö.D. et al. J Nanopart Res (2012) 14: 1197. doi:10.1007/s11051-012-1197-6
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We systematically investigated the effective surface anisotropy of CoFe2O4 nanoparticles dependant on the thickness of SiO2 shell. XRD (X-ray powder diffraction) patterns and TEM (transmission electron microscopy) micrographs were used to investigate the structure of particles and thickness of SiO2 shell, respectively. The thicknesses of SiO2 shell with 5.41 nm on CoFe2O4 nanoparticles were increased up to 14.04 ± 0.05 nm by changing the amount of added TEOS by, 0.10, 0.25, 0.50, 1.00, 1.50, and 2.50 mL. The increase of the SiO2 thickness shell decreased the effective anisotropy due to decline the effectiveness of the dipolar magnetostatic interactions, determined from Vogel–Fulcher equation, between the particles. The declines in the Keff values stabled at around 3.76 ± 0.11 × 105 J/m3 for TEOS amount higher than 1.5 mL.
KeywordsDipolar interactionMagnetic anisotropyFerrite nanoparticlesSiO2 shell
The ferrite nanoparticles have been widely used in technological areas such as biomedical (Giri et al. 2008; Latorre-Esteves et al. 2009), magnetic disk media (Harasawa et al. 2010; Matsumoto et al. 2001), microwave devices (Fujiwara et al. 2008; Pardavi-Horvath 2000), and waste treatment (Demirel et al. 1999; Hencl et al. 1995) due to their unique magnetic, optic, and electrical properties. CoFe2O4 is one of the promising material in ferrites for future technology due to its good chemical stability for biological applications, (Giri et al. 2008; Latorre-Esteves et al. 2009) high mechanical hardness for magnetic disk media application (Matsumoto et al. 2001), and high magnetic storage capacity for tape storage systems (Harasawa et al. 2010).
The high anisotropy is the one of the main attractive property of CoFe2O4 nanoparticles for many technological areas such as magnetic fluids (Davies et al. 1995) in magnetostrictive torque sensor applications (Chen et al. 1999) and microwave devices (Pardavi-Horvath 2000). The effective surface anisotropy of nanosized ferrites is approximately fifteen times bigger than their bulk anisotropy values The observed effective anisotropy of CoFe2O4 nanoparticles, 3.3 nm in diameter, was 3.1 × 106 J/m3, while its bulk value was 1.8 × 105 J/m3 (Lizuka and Lida 1996; Tung et al. 2003). The effective surface anisotropy becomes dominant on the magnetic properties of ferrites (Bakuzis et al. 1999), while the magnetocrystalline anisotropy decreasing with decreasing particle size. The surface effects on anisotropy lead the magnetic properties of nanoparticles due to its higher surface/volume ratio. While the surface atoms are 0.3 % of total atoms for 1 mm particles in diameter, this ratio goes up to 30.0 % for 10 nm particles in diameter. The rise of the ratio for magnetic nanoparticles increases the amount of surface spins. The broken surface bonds and surface spins create new magnetic contributions (Kodama et al. 1997; Trohidou 2005). Furthermore, the enhanced surface interactions such as magnetic dipolar interaction and exchange interaction between the sequential nanoparticles are another outcome of increased number of surface spins (Bansmann et al. 2005).
However, the surface reactivity of the magnetic nanoparticles restricts to their technological applications. That is why; magnetic nanoparticles with nonmagnetic shell, such as silver, gold, carbon, TiO2, and SiO2, etc., have been adapted as a usual way to decline the surface reactivity and interparticle interactions (Bansmann et al. 2005; Chen et al. 2011; Vogt et al. 2010; Zhoua et al. 2001). SiO2 is the one of the common material to coat magnetic nanoparticles due to its stability with many chemicals (Vogt et al. 2010; Yi et al. 2006) and temperature variations (Tang et al. 2007). Moreover, SiO2 does not take part in any reduction or oxidation reactions with the core material (Yang et al. 2009).
The purpose of this study is to understand the effects of dipolar interactions in effective anisotropy of CoFe2O4 nanoparticles with varying SiO2 shell thickness (Cannas et al. 2010; Chen and Tang 2007; Limaye et al. 2009; Tago et al. 2002; Tang et al. 2007). TEM micrographs were used to obtain the thicknesses of the SiO2 shell. The magnetization variation by temperature and magnetic filed were studied using DC and AC magnetization measurement techniques in the temperature range of 5–300 K. The AC susceptibility was measured at the frequencies of 10, 30, 100, 300, 1000, 3000, and 10000 Hz under the field of 10 Oe, were used to reveal the anisotropy change by increasing thickness of SiO2.
The synthesis of nanoparticles coated with SiO2 was done by the combinations of the study of Caruntu et al. (2002, 2004, 2007) and Lee et al. (2006). The CoFe2O4 nanoparticles with oleic acid were produced by the following three steps: (1) forming metal compounds by combining CoCl2·6H2O (98 %) with FeCl3·6H2O (97 %), DEG (99 %), and sodium hydroxide (NaOH) (97 %), (2) hydrolyzing/compensating, and (3) coating with oleic acid (OA) (95 %). During the reaction diethyl glycol (DEG) was used as a catalysis. The resulting solid product obtained with centrifugation at 8000 rpm for 20 min and washed with methanol (99 %) once, ethanol (99.5 %) twice, and dried in a flow of air. The CoFe2O4 nanoparticles were coated with SiO2 using water in oil microemulsion technique with base catalysis of TEOS as following the previous procedure in detail (Lee et al. 2006; Coskun et al. 2010). SiO2-coated nanoparticles with a different shell thickness were prepared by adding different amount of TEOS during the process.
The structures of synthesized particles were analyzed using EQUINOX 1000 model X-ray diffractometer (XRD). The patterns were recorded in 2θ range from 5o to 100o with 0.03o resolution for 20 min using Co Kα (λ = 1.7902 Å) radiation. The particle sizes were obtained using a JEOL 2010-F model transmission electron microscopy (TEM) with 200-kV field emission gun.
Magnetization and AC susceptibility measurements were carried out by using a Quantum Design Physical Property Measurement System (PPMS) magnetometer. The magnetization versus temperature σ(T) variations were obtained using the standard zero field cooled (ZFC) and field cooled (FC) procedures in the temperature range of 5–300 K with applied field of 500 Oe. The magnetization versus field σ(H) variations measured at 5, 50, and 300 K temperatures in the field of ±3 T. The AC susceptibility measurements were carried out in the frequency range of 10 Hz–10 kHz as a function of temperature in the range of 5–300 K operating at AC amplitudes of 10 Oe to get AC susceptibility χ(T) variations, for both real (χ′), and imaginary (χ′′) parts.
Results and discussion
The obtained effective anisotropy, T0, τ0, and blocking temperature values changes depending on thicknesses of SiO2 shell
Thickness of SiO2 shell (nm) over 5.41 nm CoFe2O4
TB (K) ± 1 (K)
Keff (J/m3) (×105)
1.36 ± 0.06 × 10−19
9.0 ± 0.1 × 105
89 ± 2
0.20 ± 0.08
71 ± 1
4.09 ± 0.04
7.94 ± 0.08
69 ± 2
3.89 ± 0.05
8.97 ± 0.09
66 ± 3
3.83 ± 0.08
10.87 ± 0.05
67 ± 3
3.81 ± 0.11
13.63 ± 0.04
65 ± 3
3.76 ± 0.11
14.04 ± 0.05
63 ± 3
3.76 ± 0.09
On the other hand, coating with SiO2 under 0.10 mL TEOS media cause a decrease at T0 values from 89.4 K (for the uncoated sample) to 70.5 K after (inset of Fig. 6b). As shown in Table 1 and Fig. 6b, a sharp decrease at T0 continued by adding TEOS due to separation between the nanoparticles, and also decreased the dipolar interactions (Coskun et al. 2010).
SiO2-coated CoFe2O4 nanoparticles were successfully synthesized using chemical route and by water-in-oil microemulsion technique. The effective anisotropy was found using Vogel–Fulcher equation due to the dominant role of magnetic dipolar interactions. The increases in the effective anisotropy and blocking temperature were associated with new orientation of the surface spins caused by the initial SiO2 coating on the surface of CoFe2O4 nanoparticles. Further increase in the SiO2 thickness produced decreases in the effective anisotropy and blocking temperature attributed to the following factor: the decrease in the strength of the dipole–dipole interactions between the CoFe2O4 nanoparticles due to their separations from each other.
The authors would like to thank Dr. S. Ismat Shah for giving an opportunity to take TEM micrographs.