Synthesis of cobalt ferrite nano-particles and their magnetic characterization

Cobalt ferrite nano-particles (CoFe₂O₄) were synthesized by the co-precipitation method with ammonium hydroxide as an alkaline solution. The reactions were carried out at different temperatures between 20 and 80 °C. The nano-particles have been investigated by magnetic measurements, X-ray powder diffraction and transmission electron microscopy. The average crystallite size of the synthesized samples was between 11 and 45 nm, which was found to be dependent on both pH value of the reaction and annealing temperatures. However, lattice parameters, interplane spacing and grain size were controlled by varying the annealing temperature. Magnetic characterization of the nano-samples were carried out using a vibrating sample magnetometer at room temperature. The saturation magnetization was computed and found to lie between 5 and 67 emu/g depending on the particle size of the studied sample. The coercivity was found to exhibit non-monotonic behavior with the particle size. Such behavior can be accounted for by the combination between surface anisotropy and thermal energies. The ratio of remanence magnetization to saturation magnetization was found to exhibit almost linear dependence on the particle size.     

Temperature dependence of magnetic anisotropy constant in cobalt ferrite nanoparticles

The temperature dependence of the effective magnetic anisotropy constant K(T) of CoFe₂O₄ nanoparticles is obtained based on the SQUID magnetometry measurements and Mössbauer spectroscopy. The variation of the blocking temperature T_B as a function of particle radius r is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and the magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and T_B. The resultant effective magnetic anisotropy constant K(T) decreases markedly with increasing temperature from 1.1×10⁷ J/m³ at 5 K to 0.6×10⁵ J/m³ at 280 K. The attempt time τ₀ is also determined to be 6.1×10⁻¹² s which together with the K(T) best explains the temperature dependence of superparamagnetic fraction in Mössbauer spectra.     

Hydrothermal synthesis and magnetic properties of gadolinium-doped CoFe2O4 nanoparticles

CoFe_2−xGd_xO₄ (x=0-0.25) nanoparticles were synthesized via a simple hydrothermal process at 200 °C for 16 h without the assistance of surfactant. The as-synthesized powders were characterized by X-ray diffraction, transmission electron microscopy, and a vibrating sample magnetometer. The X-ray diffraction results showed that the as-synthesized powders were in the pure phase with a doping amount of ≤0.25, and the peaks could be readily indexed to the cubic spinel cobalt ferrite. Transmission electron microscopy and high resolution transmission electron microscopy observations revealed that the gadolinium-doped cobalt ferrite nanoparticles were single crystal, roughly spherical, uniformly distributed, and not highly agglomerated. The room temperature magnetic field versus magnetization measurements confirmed a strong influence of gadolinium doping on the saturation magnetization and coercivity due to large lattice distortion and grain growth of small particles.     

Structure and magnetic properties of nanocrystalline cobalt ferrite powders synthesized using organic acid precursor method

Nanocrystalline octahedra of cobalt ferrite CoFe₂O₄ powders were synthesized using the organic acid precursor route. The effect of the calcination temperature, Fe³⁺/Co²⁺ molar ratio, calcination time and type of organic acid (oxalic, benzoic and tartaric acids) on the formation, crystallite size, microstructure and magnetic properties was studied systematically. The Fe³⁺/Co²⁺ molar ratio was varied from 2 to 1.739 while the annealing temperature was controlled from 400 to 1000 °C for various periods from 0.5 to 2 h. The resulting powders were investigated using X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). XRD results indicate that a well crystallized, single spinel cobalt ferrite phase was formed for the precursors annealed at 600-800 °C for 2 h, using oxalic and tartaric acids as precursors for Fe³⁺/Co²⁺ molar ratio 1.818. The crystallite size of as-formed powders was in the range of 38.0-92.6 nm at different operating conditions. The calcination temperature and Fe³⁺/Co²⁺ molar ratio have a significant effect on the microstructure of the produced cobalt ferrite. The microstructure of the produced powders was found to be octahedra-shaped. The crystalline, pure cobalt ferrite powders with magnetic properties having a maximum saturation magnetization (76.1 emu/g) was achieved for the single phase at Fe³⁺/Co²⁺ molar ratio 1.818 and annealing temperature of 600 °C for 2 h using tartaric acid precursor.     

Cobalt ferrite nanoparticles: The control of the particle size and surface state and their effects on magnetic properties

In order to improve the efficacy of magnetic fluid hyperthermia (MFH) mediators, we synthesised cobalt ferrite nanoparticles with different sizes (between 5 and 7 nm) via successive polyol synthesis. The static and dynamic magnetic properties of the prepared particles, dispersed in a solid matrix, were investigated in order to evaluate the possibility of applying cobalt ferrite as magnetic susceptors in MFH. The effect on magnetic properties coming from the surface anchoring of the model molecule cetyl phosphate, was also investigated.     

Composition and magnetic properties of cobalt ferrite nano-particles prepared by the co-precipitation method

Cobalt ferrite nano-particles were prepared using the co-precipitation method followed by annealing treatment. The formation of nano-particles with different composition, microstructure and sizes were confirmed by X-ray diffraction, Raman, thermogravimetric-differential thermal analysis and transmission electron microscope. The magnetic hysteresis loops measured at room temperature revealed smaller effective magnetic anisotropy constant, coercivity and remanence ratio for the samples prepared by adding the NaOH solutions into the mixed solutions of Co²⁺ and Fe³⁺ ions due to the formation of Co³⁺ ions. A small saturation magnetization and an enhanced coercivity were observed for the nano-particles prepared by adding the mixed solutions of Co²⁺ and Fe³⁺ ions into the NaOH solutions, which was related to the formation of outer layers with poor crystallization on the surfaces of the cobalt ferrite nano-crystals. Furthermore, the existence of these outer layers induced the oxidation of Co²⁺ ions in cobalt ferrite nano-crystals at 200 and 300 °C, and led to a large change on the composition and magnetic properties.     

Time-dependent magnetization in co-precipitated cobalt ferrite

A study of the magnetic aftereffect in co-precipitated cobalt ferrite is presented. Measurements of the magnetic viscosity S were performed at room temperature along the demagnetization curve for different applied fields H_ap over a wide range of fields (0 kOe<H_ap<−7 kOe). The interrelation function η=(∂M_rev/∂M_irr)_Hi between the DCD reversible M_rev and irreversible M_irr magnetization components was determined as well. The experimental results for S_η(H_i), where H_i is the internal field, showed a broad distribution with a maximum at H_i=2.7 kOe. However, the irreversible susceptibility χ_irr displays a maximum at H_c=0.75 kOe, the coercivity of the material. The experimental behavior of η and the non-proportionality between S_η and χⁱ_irr suggest that the magnetic viscosity in this material is principally supplied by events of nucleation of inverse domains and the depinning of domain walls. When the main mechanism of reversal magnetization changes to rotation of magnetic moments for all the grains, the magnetic viscosity decreases.     

Synthesis and characterization of manganese ferrite nanoparticles by thermal treatment method

Cubic structured manganese ferrite nanoparticles were synthesized by a thermal treatment method followed by calcination at various temperatures from 723 to 873 K. In this investigation, we used polyvinyl pyrrolidon (PVP) as a capping agent to control the agglomeration of the nanoparticles. The characterization studies were conducted by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average particle sizes of manganese ferrite nanoparticles were determined by TEM, which increased with the calcination temperature from 12 to 22 nm and they had good agreement with XRD results. Fourier transform infrared spectroscopy confirmed the presence of metal oxide bands at all temperatures and the absence of organic bands at 873 K. Magnetic properties were demonstrated by a vibrating sample magnetometer, which showed a super-paramagnetic behavior for all samples and also saturation magnetization (M_s) increases from 3.06 to 15.78 emu/g by increasing the calcination temperature. The magnetic properties were also confirmed by the use of electron paramagnetic resonance spectroscopy, which revealed the existence of unpaired electrons and also measured peak-to-peak line width, resonant magnetic field and the g-factor.     

Synthesis and magnetic properties of Mn–Zn ferrite nanoparticles

Mn–Zn ferrite nanoparticles (Mn_1−xZn_xFe₂O₄) are synthesized by a hydrothermal precipitation approach using metal sulfate solution and aqueous ammonia. The analysis methods of XRPD, TEM, TGA, and VSM are used to characterize the magnetic nanoparticles. Through the characterization of the precipitated nanoparticles, the effects of the reacting component proportions and preparation techniques on the Curie temperature, the magnetization, and the size distribution of Mn–Zn ferrite nanoparticles are discussed. Furthermore, the Mn–Zn ferrite nanoparticles are used to prepare ferrofluid. Variation of the magnetic properties of the ferrite nanoparticles with the composition content x of Zn and the magnetic moment of the nanoparticles are discussed.     

Effect of aluminum substitution on the structural and magnetic properties of cobalt ferrite synthesized by sol-gel auto combustion process

Aluminum substituted cobalt ferrite powders (CoFe_2−xAl_xO₄) with varying composition from 0.0 to 1.0 in the step of 0.2 have been obtained by sol-gel auto combustion technique using citric acid as a fuel. The metal nitrate to fuel ratio was maintained 1:4 throughout the synthesis of CoFe_2−xAl_xO₄. The thermal analysis of as prepared samples is done by TGA technique. The compositional stoichiometry of the prepared samples is confirmed by Energy dispersive X-ray analysis technique. Single phase cubic spinel structure and nano phase structure of the synthesized powders were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size of 16-26 nm was obtained using Scherrer formula. SEM analysis shows the formation of uniform grain growth. The grain size obtained from SEM results is of the order of 30 nm. Maximum specific surface area was observed to be of the order of 52 m²/gm. The highest value of saturation magnetization and coercivity was observed for pure cobalt ferrite sample and it decreases as the aluminum content x increases. A strong co-relation between the saturation magnetization and aluminum content was observed. The decrease in magnetic properties is due to the substitution of aluminum ions in place of Fe³⁺.     

Investigations on structural and magnetic properties of cobalt ferrite/silica nanocomposites prepared by the coprecipitation method

Magnetic nanocomposites consisting of cobalt ferrite nanoparticles embedded in silica matrix were prepared by the coprecipitation method using metallic chlorides as precursors for ferrite. Subsequently composites were annealed at 100, 200 and 300 °C for 2 h. The samples were structurally characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The magnetic properties were measured in the temperature range of 10-300 K using vibrating sample magnetometer (VSM). The effects of thermal treatment on structural and magnetic properties of nanocomposites were investigated. When the samples were annealed, CoFe₂O₄ nanocrystallites were observed in the SiO₂ matrix, whose size increases with increase in annealing temperature. The coercivity and saturation magnetization of nanocomposite (annealed at 300 °C for 2 h) are much higher than that of bulk cobalt ferrite. The realization of adjustable particle sizes and controllable magnetic properties makes the applicability of the CoFe₂O₄ nanocomposite more versatile.     

Influence of vacuum sintering on microstructure and magnetic properties of magnetostrictive cobalt ferrite

Differences in the microstructure and magnetic properties of highly magnetostrictive cobalt ferrite resulting from the effects of different vacuum sintering temperatures and times have been investigated. A vacuum environment was chosen to allow direct comparison of results with air-sintered samples which are more often reported in the literature. It was found that vacuum sintering resulted in the development of a solid solution second phase with composition Co_1−xFe_xO₄ (x∼0.33). There was a decrease in magnetostriction as a result of the formation of the second phase. Furthermore, differences in sintering temperatures were found to have a greater effect on the magnetostriction than differences in sintering times. It was found that the first order cubic anisotropy coefficient initially increased with both sintering temperature and time, before peaking and decreasing to its lowest measured value. The lowest anisotropy was therefore achieved with samples sintered at higher temperatures and longer times.     

Effect of heat treatment on the magnetic and magnetoelastic properties of cobalt ferrite

The influence of different heat treatments on the magnetic and magnetoelastic properties of highly magnetostrictive CoFe₂O₄ has been investigated. The first order cubic anisotropy coefficient, coercive field, magnetostriction and high strain sensitivity were observed to decrease as the heat treatment temperature increased. The saturation magnetization of the samples on the other hand increased with increase in heat treatment temperature. These changes were not accompanied by any observable changes in crystal structure or composition and are indicative of migration of Co²⁺ from the octahedral sites (B-sites) to the tetrahedral sites (A-sites) and Fe³⁺ from the A-sites to the B-sites of the spinel structure. Different distributions of the cations at the two distinct lattice sites can strongly affect the magnetic and magnetoelastic properties of these materials.     

Size-dependant heating rates of iron oxide nanoparticles for magnetic fluid hyperthermia

Using the thermal decomposition of organometallics method we have synthesized high-quality, iron oxide nanoparticles of tailorable size up to ∼15 nm and transferred them to a water phase by coating with a biocompatible polymer. The magnetic behavior of these particles was measured and fit to a log-normal distribution using the Chantrell method and their polydispersity was confirmed to be very narrow. By performing calorimetry measurements with these monodisperse particles we have unambiguously demonstrated, for the first time, that at a given frequency, heating rates of superparamagnetic particles are dependent on particle size, in agreement with earlier theoretical predictions.     

High magnetostriction coefficient of Mn substituted cobalt ferrite sintered from nanocrystalline powders and after magnetic field annealing

Magnetostriction characteristics of Mn substituted cobalt ferrite, CoFe_2?xMn_xO₄ (0 ≤ x ≤ 0.3), sintered from nanocrystalline powders of average particle size of ～4 nm have been studied. Larger value of magnetostriction at lower magnetic field is achieved after substitution of Mn for Fe. The maximum value of magnetostriction coefficient is not much affected and the slope of the magnetostriction is increased with increasing Mn content. Higher maximum value of magnetostriction coefficient (λ) of 234 ppm comparable to that of the unsubstituted composition with larger strain derivative (dλ/dH) is obtained for x = 0.2 in CoFe_2?xMn_xO₄. The magnetostriction coefficient is increased to 262 ppm with further enhancement in the strain derivative after annealing the sintered compact at 300 °C in a magnetic field of 400 kA/m for 30 min.     

Dielectric,electrical transport and magnetic properties of Er3+substituted nanocrystalline cobalt ferrite

Erbium substituted cobalt ferrite (CoFe_2−xEr_xO₄; x=0.0–0.2, referred to CFEO) materials were synthesized by sol-gel auto-combustion method. The effect of erbium (Er³⁺) substitution on the crystal structure, dielectric, electrical transport and magnetic properties of cobalt ferrite is evaluated. CoFe_2−xEr_xO₄ ceramics exhibit the spinel cubic structure without any impurity phase for x≤0.10 whereas formation of the ErFeO₃ orthoferrite secondary phase was observed for x≥0.15. All the CFEO samples demonstrate the typical hysteresis (M–H) behavior with a decrease in magnetization as a function of Er content due to weak superexchange interaction. The frequency (f) dependent dielectric constant (ε′) revealed the usual dielectric dispersion. The ε′–f dispersion (f=20 Hz to 1 MHz) fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived are ∼10⁻⁴ s and ∼0.61(±0.04), respectively. Electrical and dielectric studies indicate that ε′ increases and the dc electrical resistivity decreases as a function of Er content (x≤0.15). Complex impedance analyses confirm only the grain interior contribution to the conduction process. Temperature dependent electrical transport and room temperature ac conductivity (σ_ac) analyses indicate the semiconducting nature and small polaron hopping.     

Reduced conductivity and enhancement of Debye orientational polarization in lanthanum doped cobalt ferrite nanoparticles

In this work we have investigated the conductivity and dielectric properties of CoLa_xFe_2−xO₄ (x=0.0, 0.03, 0.05, and 0.07) nanoparticles synthesized by chemical co-precipitation route. X-ray diffraction analysis confirms the inverse spinal structure of nanoparticles with slight increase in the lattice constant as La concentration increases. Transmission electron microscopy shows spherical nanoparticles with sizes of ∼20 nm. Impedance spectroscopy of the samples was performed in the frequency range 20 Hz-2 MHz at room temperature. The resistance of the grains and grain boundaries was found to increase with lanthanum concentration while the AC conductivity of the samples was observed to decrease with increasing La concentration. Dipolar orientational polarization was found to play an important role in determining dielectric properties of the samples.     

Mössbauer and magnetic studies in nickel ferrite nanoparticles: Effect of size distribution

The magnetic properties of nickel ferrite nanoparticles in the form of powders, prepared by the sol-gel process and subjected to different annealing temperatures, were investigated using both static and dynamic measurements namely hysteresis, zero field cooled-field cooled magnetization (ZFC-FC) measurements and Mössbauer spectroscopy. The Transmission Electron Microscopy (TEM) studies reveal particle sizes for the as-prepared particles which increases upto 52 nm with annealing. A bimodal distribution, upto an annealing temperature of was observed. ZFC-FC measurements for the as-prepared samples reveal twin peaks, indicative of the bimodal size distribution. ZFC-FC measurements performed for fields varying from 100 Oe to 3 kOe show a superparamagnetic phase with blocking temperatures between 320 and . Numerical simulations for the ZFC-FC studies indicate that the signature of the bimodal size distribution can be seen only at very low fields. The variation of coercivity with particle size, as determined from the hysteresis measurements, shows a transition from a single domain to a multi domain state for particle sizes larger than 35 nm. Mössbauer measurements performed at room temperature for the as-prepared sample shows a six finger pattern for the samples with higher particle size and a doublet pattern for the samples with smaller particle size, which is indicative of their superparamagnetic nature.     

Enhancement of the crystal size and magnetic properties of Mg-substituted Co ferrite

Standard ceramic technique was used to prepare the ferrite Co_1−xMg_xFe₂O₄ 0.0?x?1. FTIR and X-ray diffraction were performed to assure the formation of the sample in the proper form. The obtained lattice parameter was interpreted on the basis of cation distribution. The replacement of Co²⁺ instead of Mg²⁺ on B sites expands slightly the size of the lattice. The sample MgFe₂O₄ does not exhibit high thermal stability. The general trend of χ_M with Mg content is the decrease in its values by decreasing x from 1 to ≈0.6. The obtained data was interpreted also on the basis of redistribution of iron ions between the two sublattices.     

Dependence of the magnetic and magnetoelastic properties of cobalt ferrite on processing parameters

The dependence of the magnetic and magnetoelastic properties of highly magnetostrictive cobalt ferrite on processing parameters has been investigated. The cobalt ferrite samples used in this study were prepared via conventional ceramic processing methods. The processing parameters of interest were sintering temperature, holding time at the sintering temperature and powder compaction pressure. It was observed that the crystal structure, composition and saturation magnetization of the samples studied did not vary with changes in processing parameters but coercive field decreased with increasing sintering temperature. The amplitude of peak to peak magnetostriction was dependent on the holding time and powder compaction pressure. The strain derivative on the other hand was found to depend on powder compaction pressure at any given sintering temperature or holding time. The results show how the magnetoelastic properties of cobalt ferrite can be varied by changing the processing parameters.