Influence of Zn–Zr ions on physical and magnetic properties of co-precipitated cobalt ferrite nanoparticles

Cobalt ferrite nanoparticles having the chemical formula CoFe_2−2xZr_xZn_xO₄ with x ranging from 0.0 to 0.4 were prepared by chemical co-precipitation method. The powder X-ray diffraction pattern confirms the spinel structure for the prepared compound. The particle size was calculated from the most intense peak (3 1 1) using Scherrer formula. The particle size of the samples was found within the range of 12–23 nm for all the compositions. The magnetic and electrical properties of these materials have been studied as a function of temperature. Activation energy and drift mobility have been calculated from the DC electrical resistivity measurements. Dielectric properties such as dielectric constant and dielectric loss tangent were measured at room temperature in the frequency range 100 Hz–1 MHz.     

Correlation between structural and magnetic properties of substituted (Cd,Zr) cobalt ferrite nanoparticles

This work correlates the magnetic properties to the microstructure of the calcined nanocrystalline Cd_xCo_1-xZr_0.05Fe_1.95O₄ (0.0 ≤ x ≤ 0.3 in a step of 0.05) powders produced by Pechini sol–gel method. The dry gel was grinded and calcined at 700 °C in a static air atmosphere for 1 h. The thermal decomposition process of dried gel was studied by thermo gravimetric analysis (TGA) combined with differential analysis (DTA). Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometer (VSM) were carried out to investigate the structural bonds identification, crystallographic properties, morphology and magnetic properties of the obtained powders. The XRD pattern of the samples showed that the synthesized materials were of a single cubic phase with the nanocrystalline Co–Zr–Cd ferrite which had an average crystallite size of 32–40 nm and particle size of 55 nm resulted from FE-SEM. The magnetic properties were measured from the hysteresis loops. The magnetic measurements had indicated that the coercivity and the magnetization decreased by increasing the Cd content.     

Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles

Cobalt ferrite, CoFe₂O₄, nanoparticles in the size range 2–15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D _TEM. Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M _S, are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below ~160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T ² law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D _TEM, and a thickness of 3.7 to 5.1 Å was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T ≤ 160 K), observing a large hysteresis at magnetic fields H > ~1000 Oe compared to smaller ones (H ≤ ~1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T ₀, as deduced by assuming a Vogel–Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T _b, observed in the FC magnetization curves.     

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.     

Study of magnetic and structural properties of ferrofluids based on cobalt-zinc ferrite nanoparticles

Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 30 nm, dispersed in a liquid carrier. Magnetic Co_(1−x)Zn_xFe₂O₄ (x=0.25, 0.50, 0.75) ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through Energy Dispersive X-ray Spectroscopy and X-ray diffraction, respectively. Transmission Electron Microscopy (TEM) studies permitted determining nanoparticle size; grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer (VSM); and finally, a magnetic force microscope was used to visualize the magnetic domains of Co_(1−x)Zn_xFe₂O₄ nanoparticles. X-ray diffraction patterns of Co_(1−x)Zn_xFe₂O₄ show the presence of the most intense peak corresponding to the (311) crystallographic orientation of the spinel phase of CoFe₂O₄. Fourier Transform Infrared Spectroscopy confirmed the presence of the bonds associated to the spinel structures; particularly for ferrites. The mean size of the crystallite of nanoparticles determined from the full-width at half maximum of the strongest reflection of the (311) peak by using the Scherrer approximation diminished from (9.5±0.3) nm to (5.4±0.2) nm when the Zn concentration increases from 0.21 to 0.75. The size of the Co-Zn ferrite nanoparticles obtained by TEM is in good agreement with the crystallite size calculated from X-ray diffraction patterns, using Scherer's formula. The magnetic properties investigated with the aid of a VSM at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. In this study, we established that the coercive field of Co_(1−x)Zn_xFe₂O₄ magnetic nanoparticles, the crystal and nanoparticle sizes determined by X-ray Diffraction and TEM, respectively, decrease with the increase of the Zn at%. Finally, our magnetic nanoparticles are not very hard magnetic materials given that the hysteresis loop is small and for this reason Co_(1−x)Zn_xFe₂O₄ nanoparticles are considered as soft magnetic material.     

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.     

Influence of synthesis conditions on structural,optical and magnetic properties of iron oxide nanoparticles prepared by hydrothermal method

The iron oxide nanoparticles were synthesized by a simple hydrothermal method at different heating temperatures and pH conditions. The synthesized materials were characterized by X-ray diffractometer, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, UV–visible spectrometer and vibrating sample magnetometer. With increment in pH of the synthesized materials were resulted in orthorhombic (goethite) and cubic (magnetite) structures at pH 6 and 12, respectively. The banding nature of synthesized materials was analyzed by infrared spectra. The synthesized powders at 130?°C showed higher percent of nanorods (length = 90–120 nm) in addition to lower percentage of nanoparticles. The material at pH 12 consisted of maximum nanoparticles with size = 10–60 nm with small agglomerations. Band gap energy of synthesized materials was 2.2–2.8 eV. Herein, the reaction conditions tuned the saturation magnetization (M_S). The maximum M_S (59.38 emu/g) was obtained at pH 12 and lower M_S (0.65 emu/g) was observed at pH 6 due to intrinsic property of goethite phase.     

Effect of calcinations on the structural and magnetic properties of magnesium ferrite nanoparticles prepared by sol gel method

Magnesium ferrite nanoparticles calcined at 300 °C, 350 °C, 400 °C, 450 °C were synthesized by sol-gel method. The effects of calcinations on the cation distribution, structural and magnetic properties have been investigated. X-ray diffraction (XRD) and vibrating scanning magnetometer (VSM) were used to characterize the structural and magnetic properties. X-ray diffraction analysis revealed the formation of single phase MgFe₂O₄ in all the samples. Lattice constant and crystallite size increased with calcination. X-ray diffraction data were used to estimate the average cationic distribution among A site and B site. Cationic distribution shows that there is migration of cation between tetrahedral A site and octahedral B site. Saturation magnetization increased with particle size. Coercivity decreased with calcination temperature as a result of decrease in pinning effect at the grain boundary. Curie temperature (T_C) decreased slightly due to weakening of A-B exchange interaction. Low temperature magnetic measurement revealed that blocking temperature (T_B) increased due to strong magnetic interaction.     

Influence of copper cations on the magnetic properties of NiCuZn ferrite nanoparticles

Ni_0.6−xCu_xZn_0.4Fe₂O₄ (x=0-0.5) ferrite nanoparticles were prepared, employing a reverse micelle process. X-ray diffraction and transmission electron microscopy evaluations demonstrated that single phase spinel ferrites with narrow size distribution were obtained. Vibrating sample magnetometer was employed to probe the magnetic properties of the samples. It was found that with an increase in copper content, the saturation magnetization decreases. Magnetic dynamics of the samples was studied by measuring a.c. magnetic susceptibility versus temperature at different frequencies. The phenomenological Néel-Brown and Vogel-Fulcher models were employed to distinguish between the interacting or non-interacting systems. The system exhibits that there is strong interaction among fine particles.     

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.     

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.     

Ionic magnetic fluid based on cobalt ferrite nanoparticles: Influence of hydrothermal treatment on the nanoparticle size

Magnetic fluid based on cobalt ferrite nanoparticles was obtained using a hydrothermal treatment added to the Massart procedure. This treatment increases the average size of the nanoparticles from 11.9 to 18.7 nm and also improves the dispersity and crystallinity of the cobalt ferrite particles. The nanoparticles obtained after the hydrothermal treatment were dispersed in aqueous solvent by the classical procedure for ionic magnetic fluids. The ferrofluid thus obtained is stable at pH 7 and may be useful for hyperthermia applications.     

Influence of stabilizing agents on structural and optical properties of cobalt oxide nanoparticles synthesized by an ultrasound-assisted method

Abstract

Cobalt oxide nanoparticles were synthesized using a facile and convenient sonochemical method, followed by a calcination process. For the synthesis, cobalt nitrate, and sodium borohydride were used as precursors. Polyvinyl alcohol, 3-mercaptopropionic acid, and styrene were used as a stabilizing agent. To identify the structure, optical properties and morphology, the synthesized nanoparticles were characterized using X-ray diffraction, Ultraviolet-Visible Spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy. To explain the formation mechanism of cobalt oxide nanoparticles, some investigations were carried out before thermal annealing. In addition, the particle size was tuned by variation of the concentration of 3-mercaptopropionic acid.     

Structural and magnetic properties of zinc- and aluminum-substituted cobalt ferrite prepared by co-precipitation method

Spinal ferrites having the general formula Co_{1 − x} Zn _x Fe_{2 − x} Al _x O₄ (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were prepared using the wet chemical co-operation technique. The samples were annealed at 800°C for 12 h and were studied by means of X-ray diffraction, magnetization and low field AC susceptibility measurements. The X-ray analysis showed that all the samples had single-phase cubic spinel structure. The variation of lattice constant with Zn and Al concentration deviates from Vegard’s law. The saturation magnetization σ _s and magneton number n _B measured at 300 K using high field hysteresis loop technique decreases with increasing x, suggesting decrease in ferrimagnetic behaviour. Curie temperature T _C deduced from AC susceptibility data decreases with x, suggesting a decrease in ferrimagnetic behaviour.      

Synthesis and magnetic properties of the size-controlled Mn-Zn ferrite nanoparticles by oxidation method

Size-controlled Mn_0.67Zn_0.33Fe₂O₄ nanoparticles in the wide range from 80 to 20 nm have been synthesized, for the first time, using the oxidation method. It has been demonstrated that the particle size can be tailor-made by varying the concentration of the oxidant. The magnetization of the 80 nm particles was 49 A m² kg⁻¹ compared to 34 A m² kg⁻¹ for the 20 nm particles. The Curie temperatures for all the samples are found to be within 630±5 K suggesting that there is no size-dependent cation distribution. The critical particle size for the superparamagnetic limit is found to be about 25 nm. The effective magnetic anisotropy constant is experimentally determined to be 7.78 kJ m⁻³ for the 25 nm particles, which is about an order of magnitude higher than that of the bulk ferrite.     

Structural and magnetic properties of nanocrystalline stannic substituted cobalt ferrite

The structural and magnetic properties of the spinel ferrite system Co_1+xFe_2−2xSn_xO₄ (x=0.0–1.0) have been studied. Samples in the series were prepared by the ceramic technique. The structural and microstructural evolutions of the nanophase have been studied using X-ray powder diffraction and the Rietveld method. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is partially an inverse spinel. Far infrared absorption spectra show two significant absorption bands, around 600 cm⁻¹ and 425 cm⁻¹, which are respectively attributed to tetrahedral (A) and octahedral [B] vibrations of the spinel. Scanning Electron Microscopy (SEM) was used to study surface morphology. SEM images reveal particles in the nanosize range. The transmission electronic microscope (TEM) reveals that the grains are spherical in shape. TEM analysis confirmed the X-ray results. The magnetic properties of the prepared samples were characterized by using a vibrating sample magnetometer.     

钴纳米粒子自组装有序阵列与磁性

采用高温液相分解法制备出平均粒径不同的单分散的钴纳米粒子.用自组装的方法得到二维和三维的钴纳米粒子有序阵列，用透射电子显微镜研究了粒径、温度、有机溶剂以及浓度对钴纳米粒子的自组装的影响.用超导量子干涉仪研究了钴纳米粒子的超顺磁性.这些研究结果为深入研究磁性纳米粒子的物性和在纳米器件中的应用奠定了良好的基础. 关键词： 钴纳米粒子 自组装 超顺磁性     

Carbon-encapsulated cobalt nanoparticles: synthesis,properties, and magnetic particle hyperthermia efficiency

A facile and low-cost method for structuring carbon-encapsulated cobalt nanoparticles (Co@C) is presented. Three samples were solvothermally prepared in one step at 220 °C and one in two steps at 200 °C. Three different polyols such as propylene glycol, triethylene glycol, and tetraethylene glycol were used as carbon sources, solvents, and reducing agents. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. Concerning the crystal structure of the particles, a mixture of hcp/fcc Co phases was obtained in three of the samples, independently of the polyol used. The coexistence of cubic and hexagonal phases was revealed both from XRD and high-resolution TEM (HRTEM). The formation of the cubic fcc structure, despite the relatively low reaction temperature, is attributed to the role of the interface between carbon coating and metallic core. The presence of carbon coating was demonstrated by Raman spectrometry, exhibiting the characteristic D and G graphitic bands, and by HRTEM observations. All samples showed ferromagnetic behavior with saturation magnetization up to 158 emu/g and coercivity up to 206 Oe. From the magnetic particle hyperthermia measurements recorded at a frequency of 765 kHz, a maximum SLP value of 241 W/g was obtained.