CoFe₂O₄, as a type of magnetic materials, has long been of intensive importance in the fundamental sciences and technological applications in various fields of electronics 1, photomagnetism 2, catalysis 3, ferrofluids 4, hyperthermia 5, cancer therapy 6, and molecular imaging agents in magnetic resonance imaging (MRI) 7. The applications of CoFe₂O₄ are strongly influenced by its magnetic properties. For biomedical applications, CoFe₂O₄ nanoparticles are required to have a narrow size distribution, high magnetization values, a uniform spherical shape, and superparamagnetic behavior at room temperature. So far, various synthetic routes have been explored for the preparation of CoFe₂O₄ nanoparticles, such as hydrothermal 8, coprecipitation 910, microemulsion 11, forced hydrolysis 12, reduction–oxidation route 13. However, the main difficulty of these traditional methods is that the as-prepared nanoparticles are severely agglomerated with poor control of size and shape in most cases, which greatly restrict their applications 14. In order to solve the above problems, thermal decomposition of organometallic precursors in high-boiling organic solution has been explored 1516 for the preparation of size- and shape-controlled monodisperse CoFe₂O₄ nanoparticles 14171819. However, the major disadvantages of this method are the need of toxic and expensive reagents, high reaction temperature, and complex operations. To address these concerns, Li et al. adopted a general liquid–solid-solution (LSS) phase transfer and separation method 20. This strategy is based on a general phase transfer mechanism occurring at the interfaces of the liquid, solid, and solution phases present during the synthesis. Through this general method, Li et al. successfully synthesized Fe₃O₄ doped with Co, which has a coercivity about 250 Oe at room temperature 21. However, the synthesis of CoFe₂O₄ nanoparticles with a superparamagnetic behavior at room temperature has not been reported. In this letter, we report a significant improvement of the method of Li et al. 20 to synthesize nearly monodispersed CoFe₂O₄ nanoparticles and systematically investigate the magnetic properties of the as-prepared nanomaterials. At room temperature, these as-prepared nanoparticles were found to have high saturation magnetization values of 50 emu/g and superparamagnetic behavior with negligible coercivity, which is expected to have potential application in biomedicine.

The X-ray pattern of the as-synthesized samples is depicted in Fig. 1. The positions and relative intensities of all the peaks indicate that the crystalline structure of the products favors the formation of cubic spinel phase only, which is accordant to JCPDS card NO. 22-1086. No other impurity phases are observed. Additionally, it clearly shows that the as-synthesized CoFe₂O₄ samples reveal broadening diffraction peak, which is due to the reduced particle size. The average grain size of the as-synthesized nanoparticles calculated by Scherer’s formula 10 is 6 nm. Based of the highest intensity peak of (311), the calculated lattice parameter is 0.8456 nm, which is larger than the bulk CoFe₂O₄ value of 0.8391 nm. The enhancement of the calculated lattice parameter probably results from different distribution of metal cations compared with the bulk spinel cobalt ferrite and the surface distortion of particles induced by the size effect of nanoparticles 13.

The chemical composition of the as-synthesized products is further analyzed by the inductively coupled plasma atomic emission spectroscopy (ICP-AES). The result reveals that the molar ratio of Co and Fe is 1:2.05, which is nearly consistent with the expected stoichiometry of CoFe₂O₄.

Figure 2 shows TEM images of the CoFe₂O₄ nanoparticles obtained without any size-sorting process. It reveals that the as-synthesized nanoparticles were nearly monodisperse with spherical shape. The particle size with a narrow distribution is given in the inset of Fig. 2a. The average particles size is 5.5 nm, which is in good agreement with the particle sizes estimated by Scherer’s formula. This suggests that each individual particle is a single crystal 19. Figure 2b performs the high-resolution (HR) TEM characterizations of the particles, and the highly crystalline nature of the samples is revealed in the inset of Fig. 2b.

FTIR spectra of the samples capped with oleic amine were performed in the range of 500–4,000 cm⁻¹(Fig. 3). The wide peak around 3,374 cm⁻¹ is ascribed to the complexation between -NH₂ and -OH on the surface of CoFe₂O₄. The peak at 3,007 cm⁻¹ is assigned to the stretching of the vinyl group. The peaks at 2,921 and 2,850 cm⁻¹ are attributed to the asymmetric and symmetric stretching of the CH₂ groups, respectively. The sharp peaks are due to the long hydrocarbon chain of oleic amine. The peaks at 1,409 and 1,307 cm⁻¹ correspond to C–H bending of CH₂ group. The peak at 965 cm⁻¹ is attributed to the O–H outplane vibration. The peak at 593 cm⁻¹ is owing to the presence of ferrite nanoparticles. The FTIR spectrum confirms that the as-synthesized nanoparticles are coated by oleic amine, which can provide repulsive (electrostatic repulsion and steric repulsion) forces to balance the attractive forces (dipole–dipole interaction, exchange interaction, and van der Waals force.) between the nanoparticles. Thus, on account of the repulsion, the as-prepared CoFe₂O₄ samples are easily dispersed in the nonpolar solvents and stabilized in the suspension without agglomeration.

The field dependence of the magnetization of as-synthesized particles measured at 300 and 10 K is shown in Fig. 4. Magnetic measurements indicate that the as-prepared particles exhibit superparamagnetic behavior with negligible coercivity (about 11 Oe) and remanence at room temperature.

The saturation magnetization value is 50 emu/g at room temperature, which is less than the bulk value of 74 emu/g 10. For nanoscaled nanoparticles, the loss of the saturation magnetization is due to surface spin canting effect 22 and the presence of a magnetic dead or antiferromagnetic layer on the surface 1323, which is caused by finite-size effect of the small magnetic nanoparticles. Additionally, the magnetic performance of the ferrite-structured nanomaterials is also influenced by the distribution of metal cations, which is different from the bulk ferrite. A summary of the magnetic properties between the as-synthesized products and the reported CoFe₂O₄ is given in Table 1. In our best knowledge, CoFe₂O₄ nanoparticles prepared in this work have a higher saturation magnetization value compared with the reported samples with superparamagnetic behaviors in the applied field of 12,000 Oe at room temperature. The saturation magnetization value (73.8 emu/g) measured at 10 K is close to the value of bulk CoFe₂O₄ (74 emu/g).

The saturation magnetizations are compared at an applied magnetic field of 12,000 Oe

The particles exhibit superparamagnetic behavior with negligible coercivity (about 11 Oe) at room temperature, which is much lower compared with the value (250 Oe) reported by Li et al. 21. The magnetic properties of samples are greatly related to many factors, such as shape, size, and structure, which are influenced by the synthetic method and experimental parameters. This greatly reduced coercivity is understood as follows: The as-synthesized CoFe₂O₄ nanoparticles are spherical in shape, well-isolated, and the particle size of the product is found in the range of the critical size of CoFe₂O₄ for superparamagnetic limit reported in literature 24, which is about 4–9 nm. Additionally, the decrease of coercivity in our samples illuminates that the coercivity has a particle-size-dependent character 29. Whereas, the coercivity of the samples as-synthesized reaches 14.55 kOe, much larger than the value of bulk CoFe₂O₄ (about 5 kOe at 5 K). The comparisons of the magnetic properties measured at 300 and 10 K for our samples are summarized in Table 2.

Figure 5 shows the zero-field-cooled and field-cooled (ZFC/FC) curves of the as-prepared CoFe₂O₄ samples measured at temperatures between 10 and 330 K with an applied field of 100 Oe. As the temperature rises from 10 to 330 K, the ZFC magnetization increases first and then decreases after reaching a maximum at 240 K, which is correspond to the blocking temperature (T_B). This result further proves that the CoFe₂O₄ samples as-prepared display a superparamagnetic behavior at room temperature. Whereas the FC magnetization decreased endlessly as the temperature increased. It is imagined that the difference between ZFC magnetization and FC magnetization below T_B is caused by energy barriers of the magnetic anisotropy 30. The magnetic anisotropy constant K of the samples as-prepared can be calculated by the followed formula 3031:

where k_B is the Boltzman constant, T_B is the blocking temperature of the samples, and V is the volume of a single particle. The calculated magnetic anisotropy constant K of our samples is 3.8 × 10⁶ ergs/cm³, which is slightly larger than that of the K values of bulk CoFe₂O₄ [(1.8–3.0) × 10⁶ ergs/cm³].

The distribution function of the magnetic anisotropy energy barriers f(T) can be obtained through the following equation 1330:

where M_FC (FC magnetization) involves the total magnetization from the contribution of all particles, M_ZFC (ZFC magnetization) is determined by the magnetization from only the contribution of the nanoparticles whose energy barriers are overcomed by the thermal energy (k_BT) at the measuring temperature, and f(T) reflects a quantitative characterization for superparamagnetism of the magnetic nanoparticles.

Figure 6 reveals the calculated anisotropy energy distribution for the as-synthesized CoFe₂O₄ nanoparticles. Generally, the volume and shape distribution of the samples determine the magnetic anisotropy energy distribution. Therefore, the result implies that the thermal energies of most particles have exceeded the energy barriers beyond T_B(about 240 K). So the as-synthesized samples display superparamagnetic behavior at room temperature. In addition, the narrow magnetic anisotropy energy distribution reveals that the as-prepared CoFe₂O₄ nanoparticles possess uniform sizes 1330. The superparamagnetic behavior and narrow size distribution imply that the sample prepared in this work is a good candidate for the possible biomedical applications.

In conclusion, nearly monodispersed CoFe₂O₄ nanoparticles were prepared under a simple hydrothermal condition. The as-synthesized samples are considered to have potential applications in biomedicine for its narrow particle size distribution, high saturation magnetizations, and superparamagnetization at room temperature. The simple synthesis route used in this work is expected to be a general approach for the preparation of binary and ternary metal oxide, especially spinel ferrite.

An erratum to this article can be found athttp://dx.doi.org/10.1007/s11671-010-9777-9