Synthesis and characterization of CoFe₂O₄ nanoparticles with high coercivity

Magnetic nanoparticles have been widely investigated because of their potential application in exchange-coupled nanocomposite magnets, magnetic data storage, magnetic fluids, and biomedicine.^1–5 Nanoparticle size, shape, crystal structure, and composition are often related to nanoparticle magnetism such as anisotropy, magnetization, coercivity, and ordering temperature.^6–11 CoFe₂O₄ has been studied widely due to its interesting magnetic properties such as high anisotropy and coercivity, excellent chemical stability, and good mechanical hardness. Many advances to synthesize magnetic nanostructures have been made using a variety of chemical and physical approaches.^12–16 A number of efforts have been made to increase coercivity in cobalt ferrite. The maximum coercivity of 9.3 kOe at room temperature reported to date in thin films deposited on SiO₂ single crystal substrate that was due to the small grain size (50 nm) and lattice strain in the films.¹⁷ The large coercivity of 4.6 kOe was reported in CoFe₂O₄ nanoparticles synthesized using seed-mediated growth method.¹⁸ High coercivity of 9.4 kOe was reported in oleic acid capped CoFe₂O₄ nanoparticles with low magnetization of 7.1 emu/g.¹⁹ 

In this work, we report the synthesis of single crystalline ferromagnetic CoFe₂O₄ nanoparticles through a one-step hydrothermal method, which was employed along with the surfactants to control morphology, size, and composition of the nanoparticles. This method is suitable for mass production of magnetic CoFe₂O₄ nanoparticles.

In a typical synthesis, FeCl₃.6H₂O (1.08 g, 4 mmol), CoCl₂ (0.26 g, 2 mmol), NaOH (0.8 g, 20 mmol), and cetyltrimethylammonium bromide (CTAB) (1.0 g, 2.75 mmol) were dissolved in 7 ml deionized (DI) water by intensive stirring till a homogeneous solution was obtained. Subsequently, a 14 ml of ethylenediamine (EDA) was added dropwise into the above mentioned aqueous solution. After stirring for 30 min, the slurry mixture obtained was transferred into a Teflon-lined stainless steel autoclave. This autoclave was kept in a furnace and heated at 200 °C for 10 h. After cooling to room temperature, the black powder consisting of cobalt ferrite nanocrystals was separated from reaction fluid by centrifugation at 6000 rpm for 15 min. The powder was collected then redispersed in 30 ml ethanol using an ultrasonic bath. The sample was centrifuged once again at 6000 rpm for 15 min and the ethanol was discarded. This purification step was repeated two more times.

The transmission electron microscopy (TEM) images were recorded on a JEOL 1200 EX electron microscope at an accelerating voltage of 120 kV. HRTEM images were obtained with Hitachi H-9500 with an accelerated voltage of 300 kV. Powder X-ray diffraction (XRD) spectra were recorded on a Rigaku Ultima IV diffractometer with a Cu Kα X-ray source. The Raman spectra measurements were carried out using DXR Raman microscope (Thermo Scientific) with a laser (532 nm and 5 mW) excitation source. Magnetic measurements of the metallic samples were performed using a Quantum Design MPMS magnetometer. Samples for magnetic characterization were prepared by depositing a drop of the final ethanol dispersion on a silicon substrate evaporating the solvent at room temperature and by hardening randomly oriented CoFe₂O₄ powders in epoxy.

CoFe₂O₄ nanocrystals with different size and shape were synthesized via the hydrothermal route. Various combinations of NaOH and CTAB were studied at fixed precursor concentration and fixed reaction temperature. We first studied the effect of surfactant addition on size and shape of the nanocrystals. The surfactant CTAB is cationic and is responsible for the rod shape assembly of nanoparticles. Upon the ionization, CTAB constitutes the CTA⁺ ion that forms the pair with negatively charged Fe–OH–Co, thus constituting the crystal growth initializers.²⁰ 

Fig. 1 shows the TEM images for the samples prepared using different concentration of NaOH and CTAB by the hydrothermal method. Figs. 1(a)–1(b) show typical CoFe₂O₄ nanoparticles synthesized using 12 mmol and 20 mmol NaOH without adding CTAB into the solution mixture. The particles shown in Fig. 1(a) have different shapes with a broad particle size distribution. Fig. 1(b) shows a narrow size distribution of the nanoparticles with diameters of 20–40 nm. Fig. 1(c) shows the typical TEM image of ∼40 nm CoFe₂O₄ nanoparticles prepared with 2.75 mmol of CTAB and 20 mmol of NaOH. When 5.5 mmol of CTAB was used, we observed a mixture of nanoparticles with diameter 10–30 nm and shuttle like nanorods with diameter of 40–50 nm and length of 60–100 nm as shown in Fig. 1(d). Fig. 1(e) shows the HRTEM image of representative nanoparticles from Fig. 1(c). The nanoparticle is single crystalline and has a lattice fringes of 0.48 nm spacing of the (111) planes in inverse spinel structured face centered cubic (fcc) CoFe₂O₄.

The crystal structure of nanoparticles was further characterized using XRD. Fig. 2 shows the XRD patterns of the CoFe₂O₄ nanocrystals prepared by varying the concentration of NaOH and CTAB. It shows the characteristic (220), (311), (222), (400), (422), (511), (440), and (533) diffraction peaks of CoFe₂O₄ phase with fcc crystal structure (Figs. 2(a)–2(c)). The average crystallite size of CoFe₂O₄ nanoparticles was estimated from x-ray diffraction patterns using Scherrer formula (d = 0.9 λ/β cos θ_B).The average crystallite size values determined from XRD patterns for the samples shown in Figs. 2(a)–2(c) were ∼28 nm, ∼38 nm, and ∼42 nm, respectively, which are in good agreement with the particles size observed by TEM (Figs. 1(a), 1(c), and 1(d)) that further confirm CoFe₂O₄ nanoparticles are single crystalline. We observed the presence of α-Fe₂O₃ phase along with CoFe₂O₄ phase (Fig. 2(a)) in the XRD pattern when 12 mmol of NaOH (Fig. 1(a)) was used in the reaction. When NaOH amount was increased to 20 mmol, the final product obtained was pure CoFe₂O₄ phase without the α-Fe₂O₃ impurity as seen in Figs. 2(b) and 2(c).

The information on the coordination of the metal ions in the nanocrystals was obtained from the Raman spectroscopy. CoFe₂O₄ also has the inverse spinel structure like Fe₃O₄ with cubic $Oh7(Fd3¯m)$ symmetry.^21–23 According to previous studies, the low-frequency vibrations (below 600 cm⁻¹) are attributed to the motion of oxygen around the octahedral lattice site whereas the higher frequencies are attributed to oxygen around tetrahedral sites.²⁴ In this work, the intense band in the Raman spectra at 682 cm⁻¹ is characteristic of the tetrahedral site and the band at 470 cm⁻¹ is due to Co²⁺ at octahedral sites (Fig. 3).

Fig. 4(a) shows hysteresis loop of CoFe₂O₄ nanoparticles measured at 300 and 5 K, using an applied field of 5 T. The maximum coercivity of 5.0 kOe and a saturation magnetization of 73 emu/g were obtained at 300 K for the 40 nm particles (Fig. 1(c)). At 5 K, the coercivity of 17.7 kOe and saturation magnetization of 79.7 emu/g were obtained, which can be attributed to the increase in magnetocrystalline anisotropy at low temperatures. The value of squareness ratio (M_r/M_s) of 0.66 indicates that the system consists of randomly oriented equiaxial particles with cubic magnetocrystalline anisotropy. The presence of Co²⁺ ions on the octahedral sites of the spinel structure is the primary reason for the strong anisotropy of cobalt ferrite.²⁵ Furthermore, the larger anisotropy in nanosized materials compared to bulk materials is expected due to the additional contributions from surface anisotropy and size-related effects.²⁶ It has been reported that the critical size of single-domain CoFe₂O₄ particles is about 40 nm from both the high switching field obtained by the magnetic torque analysis and the relatively high coercive field obtained for CoFe₂O₄ particles.¹⁸ It is therefore expected that CoFe₂O₄ particles about 40 nm can have high coercivity. The method of synthesis applied in the present work favors good crystallinity and magnetic ordering, resulting in a high saturation magnetization. Fig. 4(b) shows the variation of coercivity and magnetization as a function of temperature. Both the coercivity and the magnetization for the 40 nm particle size (Fig. 1(c)) decreases with temperature.

In summary, single crystalline CoFe₂O₄ nanoparticles were prepared using a simple hydrothermal method. The size and shape of the CoFe₂O₄ nanocrystals were found to be dependent on the reaction conditions such as NaOH concentration and CTAB surfactant concentration. A maximum saturation magnetization of 73 emu/g and coercive force of 5.0 kOe were obtained at 300 K, which is the highest reported via one step hydrothermal method to our best knowledge. An in-depth evaluation of the influence of the bases on the morphology, structure, chemical composition, and magnetic properties of the nanocrystals is provided. This work sheds new light on the design of magnetic nanoparticles with improved properties through the hydrothermal method. The obtained results also suggest the possibility to optimize magnetic property of CoFe₂O₄ nanoparticles with controlled magnetic alignment for magnetic energy and data storage applications.

This work has been supported by the U.S. DoD/ARO under grant W911NF-11–1–0507, and the Center for Nanostructured Materials and Characterization Center for Materials and Biology at the University of Texas at Arlington.