Cobalt Ferrite Nanocrystals Research Articles

Self-assembly and magnetic properties of shape-controlled monodisperse CoFe2O4 nanocrystals

Ordered arrays of monodisperse cobalt ferrite (CoFe2O4) nanocrystals with highly controllable spherical or cubic shapes have been synthesized via solution-based thermolysis of an intimately mixed Co2+Fe23+-oleate precursor. The evolution from spherical to cubic morphology is achieved by simply changing the precursor concentration, thereby controlling the nanocrystal growth rate. Magnetic studies indicate that the saturation magnetization is independent of the shape and is solely determined by the size of the nanocrystal. However, the coercivity does exhibit a small shape dependence, primarily resulting from the influence of surface anisotropy.

Tuning of magnetic properties in cobalt ferrite nanocrystals

Cobalt ferrite (CoFe2O4) possesses excellent chemical stability, good mechanical hardness, and a large positive first order crystalline anisotropy constant, making it a promising candidate for magneto-optical recording media. In addition to precise control of the composition and structure of CoFe2O4, its practical application will require the capability to control particle size at the nanoscale. The results of a synthesis approach in which size control is achieved by modifying the oversaturation conditions during ferrite formation in water through a modified coprecipitation approach are reported. X-ray diffraction, transmission electron microscopy (TEM) diffraction, and TEM energy-dispersive x-ray spectroscopy analyses confirmed the formation of the nanoscale cobalt ferrite. M-H measurements verified the strong influence of synthesis conditions on crystal size and hence, on the magnetic properties of ferrite nanocrystals. The room-temperature coercivity values increased from 460 up to 4626Oe under optimum synthesis conditions determined from a 23 factorial design.

Studies on the magnetism of cobalt ferrite nanocrystals synthesized by hydrothermal method

Fe–Co/CoFe 2O 4 nanocomposite and CoFe 2O 4 nanopowders were prepared by the hydrothermal method. The structure of magnetic powders were characterized by X-ray diffraction diffractometer (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermal gravity analysis (TGA) and differential thermal analysis (DTA) analysis, X-ray photoelectron spectrometry (XPS), and Fourier transform infrared spectra (FTIR) techniques, while magnetic properties were determined by using a vibrating sample magnetometer (VSM) at room temperature. The effects of hydrothermal reaction conditions on magnetic properties were also discussed in details. The values of saturation magnetization ( M s) and coercive fore ( H c) for Fe–Co/CoFe 2O 4 nanocomposite are 113 emu/g and 1.4 kOe, respectively. Furthermore, CoFe 2O 4 ferrite with a single-domain critical size of 70 nm was fabricated by controlling the hydrothermal reaction conditions carefully, which presents high coercive force (ca. 4.6 kOe) and high squareness ratio (ca. 0.65). One interesting thing is M s value of CoFe 2O 4 ferrite with a diameter of 40 nm is 86 emu/g which is comparable to that of the bulk counterpart.

Correlation between Spin−Orbital Coupling and the Superparamagnetic Properties in Magnetite and Cobalt Ferrite Spinel Nanocrystals

The superparamagnetic properties of CoFe2O4 and Fe3O4 nanocrystals have been systematically investigated. The observed blocking temperature of CoFe2O4 nanocrystals is at least 100 deg higher than that of the same sized Fe3O4 nanocrystals. The coercivity of CoFe2O4 nanocrystals at 5 K is over 50 times higher than the same sized Fe3O4 nanocrystals. The drastic difference in superparamagnetic properties between the similar sized spherical CoFe2O4 and Fe3O4 (or FeFe2O4) spinel ferrite nanocrystals was correlated to the coupling strength between electron spin and orbital angular momentum (L-S) in magnetic cations. Compared to the Fe2+ ion, the effect of much stronger spin-orbital coupling at Co2+ lattice sites leads to a higher magnetic anisotropy and results in the dramatic discrepancy of superparamagnetic properties between CoFe2O4 and Fe3O4 nanocrystals. These results provide some insight to the fundamental understanding of the quantum origin of superparamagnetic properties. Furthermore, they suggest that it is possible to control the superparamagnetic properties through magnetic coupling at the atomic level in spinel ferrite nanocrystals for various applications.

Magnetic properties of CoFe2O4 ferrite doped with rare earth ion

A series of CoFe2−xNdxO4 ferrite nanocrystals have been synthesized by the emulsion method. Powder X-ray diffraction (XRD), thermal gravity analysis (TGA) and differential thermal analysis (DTA), TEM, Mössbauer spectroscopy and vibrating sample magnetometer (VSM) were employed to characterize the crystallite sizes, structure and magnetic properties of these ferrite nanocrystals. The effect of substitution Fe3+ ions by Nd3+ ions on structure, magnetic properties of cobalt ferrite nanocrystals was investigated. The value of saturation magnetization for Nd3+-doped samples is less than that of pure cobalt ferrite. The value of coercivity is increased with the Nd3+-doped content. The pure cobalt ferrite powders with crystallite sizes of 10.3 nm achieved values of Ms=53.4 emu/g and Hc=172.0 Oe. However, nanocrystalline CoFe1.8Nd0.2O4 powders with crystallite sizes of 10.0 nm achieved desirable values of Ms=28.0 emu/g and Hc=488.6 Oe.

One step synthesis of highly crystalline and high coercive cobalt-ferrite nanocrystals

Highly crystalline and almost monodisperse spinel cobalt-ferrite nanocrystals are synthesized in a one step process, which has very high coercivity at 10 K and exhibits superparamagnetic behaviour at 300 K.

Spindly cobalt ferrite nanocrystals: preparation, characterization and magneticproperties

In this paper we describe the preparation of homogeneously needle-shaped cobalt ferrite(CoFe2O4)nanocrystals on a large scale through the smooth decomposition of urea and the resulting co-precipitationof Co2+ and Fe3+ in oleic acid micelles. Furthermore, we found that other ferritenanocrystals with a needle-like shape, such as zinc ferrite(ZnFe2O4) and nickelferrite (NiFe2O4), can be prepared by the same process. Needle-shapedCoFe2O4 nanocrystals dispersed in an aqueous solution containing oleic acid exhibit excellentstability and the formed colloid does not produce any precipitations after twomonths, which is of prime importance if these materials are applied in magneticfluids. X-ray diffraction (XRD) measurements were used to characterize the phaseand component of the co-precipitation products, and demonstrate that they arespinel ferrite with a cubic symmetry. Transmission electron microscopy (TEM)observation showed that all the nanocrystals present a needle-like shape with a 22 nmshort axis and an aspect ratio of around 6. Varying the concentration of oleic aciddid not bring about any obvious influence on the size distribution and shapes ofCoFe2O4. The magnetic properties of the needle-shapedCoFe2O4 nanocrystals were evaluated by using a vibrating sample magnetometer (VSM), electronparamagnetic resonance (EPR), and a Mössbauer spectrometer, and the results all demonstrated thatCoFe2O4 nanocrystals were superparamagnetic at room temperature.

Synthesis and assembly of high-quality cobalt ferrite nanocrystals prepared by a modified sol–gel technique

Colloidal cobalt ferrite nanocrystals were produced using a new sol–gel-like synthesis based on the procedure developed by O’Brien et al. (J. Am. Chem. Soc. 123 (2001) 12085) for the synthesis of BaTiO3 nanocrystals. This synthesis involves the single-stage high-temperature hydrolysis of the metal alkoxide precursors to obtain crystalline, uniform, organically coated nanoparticles which are well-dispersed in an organic solvent. The particles could be further manipulated to form Langmuir–Blodgett films consisting of close-packed nanocrystal monolayers. The structural and magnetic properties of these nanocrystals indicate similarity to bulk CoFe 2O 4 with a very high coercivity at low temperatures.

Superparamagnetic Cobalt Ferrite Nanocrystals Synthesized by Alkalide Reduction.

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Superparamagnetic Cobalt Ferrite Nanocrystals Synthesized by Alkalide Reduction

CoFe2O4 nanocrystallites have been synthesized by alkalide reduction of Co2+ and Fe3+ to form nanoscale CoFe2, followed by oxidation with aerated water at room-temperature resulting in the nanocrystalline ferrite. As produced, the material consists of 2−4 nm nanocrystals that are superparamagnetic with an average blocking temperature of ∼350 K. Annealing at 100 °C in air results in a decrease in the blocking temperature to ∼250 K, with no detectable nanocrystallite growth. The dramatic change in the magnetic properties upon annealing is probably due to removal of crystal defects, namely oxygen vacancies. Further annealing to temperatures as high as 400 °C results in little change in the nanocrystallite size or the magnetic properties. Annealing at 500 °C results in the onset of significant growth in the nanocrystallite size, reaching 30 nm for material annealed at 1000 °C. The saturation magnetization, remanence, and squareness ratio, measured at 300 K, increase smoothly with increasing annealing temperature above 500 °C reaching 30 nm, 75 emu/g (94% of bulk value), 28 emu/g, and 0.37 respectively, for material that had been annealed at 1000 °C. The unannealed material has the largest coercivity observed in this study, 5.13 kOe at 5 K falling to 116 Oe at 300 K. The coercivity at 300 K declines dramatically to 0.9 Oe upon annealing at 100 °C, rising sharply to 67 Oe for material annealed at 500 °C, falling to 44 Oe for material annealed at 600 °C, and then steadily growing with increasing annealing temperature to 59 Oe for material annealed at 1000 °C. The anomalous increase in coercivity observed for samples annealed at 500 °C appears to be due to an increase in the average crystallite aspect ratio, which declines upon annealing at higher temperature.

Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals.

By combining nonhydrolytic reaction with seed-mediated growth, high-quality and monodisperse spinel cobalt ferrite, CoFe(2)O(4), nanocrystals can be synthesized with a highly controllable shape of nearly spherical or almost perfectly cubic. The shape of the nanocrystals can also be reversibly interchanged between spherical and cubic morphology through controlling nanocrystal growth rate. Furthermore, the magnetic studies show that the blocking temperature, saturation, and remanent magnetization of nanocrystals are solely determined by the size regardless the spherical or cubic shape. However, the shape of the nanocrystals is a dominating factor for the coercivity of nanocrystals due to the effect of surface anisotropy. Such magnetic nanocrystals with distinct shapes possess tremendous potentials in fundamental understanding of magnetism and in technological applications of magnetic nanocrystals for high-density information storage.

Synthesis of Highly Crystalline and Monodisperse Cobalt Ferrite Nanocrystals

Highly crystalline and monodisperse cobalt ferrite nanocrystals were fabricated by the high-temperature aging of a metal−surfactant complex followed by mild oxidation. Particle sizes were varied from 4 to 9 nm by changing the experimental conditions. Transmission electron microscopic (TEM) images of the particles showed two- and three-dimensional assembly of the particles, demonstrating the uniformity of the nanocrystals. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic images of the nanocrystals confirmed the highly crystalline nature of the cobalt ferrite structure. The elemental analysis confirmed the stoichiometry of cobalt ferrite, despite some variations in the relative atomic composition of nanocrystals. The nanocrystals were found to have typical behaviors of magnetic nanocrystals and the narrow energy barrier distributions of magnetic anisotropy, implying that the nanocrystals obtained are very uniform.

Spin canting and size effects in nanoparticles of nonstoichiometric cobalt ferrite

The magnetic properties of nonstoichiometric cobalt ferrite nanocrystals differing by their sizes were investigated by magnetic measurements and Fe57 Mössbauer spectroscopy. We found that the spin-wave Bloch exponent increases with increasing particle size, in agreement with previous studies. Under a magnetic field of 7 T, we observed that the partial reorientation of the two ferrimagnetic Fe3+ sublattice magnetizations towards the field direction increases with increasing the particle size; we discuss the applicability of an intrinsic spin canting model as a function of the particle size. An evaluation of the relative amounts of cobalt and iron in each spinel site is given.