Synthesis and characterization of CoxZn1−xFe2O4 magnetic nanoparticles via a PEG-assisted route

We present an investigation of properties of Co_xZn_1−xFe₂O₄ (x=0.0-1.0) nanoparticles synthesized by a polyethylene glycol (PEG)-assisted hydrothermal route. X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and vibrating scanning magnetometry (VSM) were used to characterize the structural, morphological and magnetic properties. The particle size obtained from TEM and XRD are consistent with each other. It was observed that the lattice constant for each composition decreases with increasing Co substitution and follows Vegard's law. Magnetization measurements show that while the materials with high Zn substitution are superparamagnetic at room temperature, they are ferromagnetic at temperatures lower than the blocking temperature. The materials with less Zn substitution are ferromagnetic below room temperature. Magnetizations and the coercivities of the samples decrease with the Zn substitution. The resultant overall magnetic behavior of the superparamagnetic samples are found to be considerably different than that of conventional superparamagnetic systems due to the antiferromagnetic interactions both in intra- and inter-cluster spins, and size (effective moment) distribution of the particles.     

Synthesis and magnetic characterization of Zn0.7Ni0.3Fe2O4 nanoparticles via microwave-assisted combustion route

We report on the synthesis of Zn_0.7Ni_0.3Fe₂O₄ nanoparticles via microwave assisted combustion route by using urea as fuel. XRD and FT-IR analyses confirm the composition and structure as spinel ferrite. The crystallite size estimated from XRD (16.4 nm) and the magnetic core size (15.04 nm) estimated from VSM agree well, while a slightly smaller magnetic diameter reflects a very thin magnetically dead layer on the surface of the nanoparticles. Morphological investigation of the products was done by TEM which revealed the existence of irregular shapes such spherical, spherodial and polygon. Magnetization measurements performed on Zn_0.7Ni_0.3Fe₂O₄ nanoparticles showed that saturation was not attained at even in the high magnetic field. The sample shows superparamagnetic behavior at around the room temperature and ferromagnetic behavior below the blocking temperature which is measured as 284 K.     

Microwave anneal effect on magnetic properties of Ni0.6Zn0.4Fe2O4 nano-particles prepared by conventional hydrothermal method

Ni_0.6Zn_0.4Fe₂O₄ ferrite nano-particles with a crystallite size of about 20 nm were prepared by the conventional hydrothermal method, followed by annealing in a microwave oven for 7.5-15 min. The microstructure and magnetic properties of the samples were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and vibrating sample magnetometry. The microwave annealing process has slight effect on the morphology and size of Ni_0.6Zn_0.4Fe₂O₄ ferrite nano-particles. However it reduces the lattice parameter and enhances the densification of the particles, and then greatly increases the saturation magnetization (50-56 emu/g) and coercive force of the samples as compared to the non-annealing condition. The microwave annealing process is an effective way to rapidly synthesize high performance ferrite nano-particle.     

Electro-precipitation of Fe3O4 nanoparticles in ethanol

The preparation of superparamagnetic magnetite (Fe₃O₄) nanoparticles by electro-precipitation in ethanol is proposed. Particle average size can be set from 4.4 to 9 nm with a standard deviation around 20%. Combination of wide-angle X-ray scattering (WAXS), Electron energy loss spectroscopy (EELS) and Mössbauer spectroscopy characterizations clearly identifies the particles as magnetite single-crystals (Fe₃O₄).     

Mössbauer study of Zn0.4Cu0.6Fe1.2Cr0.8O4

Zn_0.4Cu_0.6Fe_1.2Cr_0.8O₄ has been studied by Mössbauer spectroscopy, SQUID magnetometry, and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant The iron ions are in ferric states and occupy both the tetrahedral (A) and octahedral (B) sites; the fractions of the iron ions at the A-sites and B-sites are 0.52 and 0.34, respectively. While spin orderings are collinear at higher temperatures, spin canting begins to appear around 25 K and increases with decreasing temperature; the canting angle at 4.7 K reaches up to 27°. Debye temperatures of the tetrahedral and octahedral sites are determined to be 339 and 335 K, respectively.     

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.     

Synthesis and characterization of CoFe2O4 octahedrons via an EDTA-assisted route

Octahedral-like CoFe₂O₄ ferrite was fabricated using an ethylenediaminetetraacetic acid (EDTA)-assisted route under mild conditions. EDTA plays important roles in the formation of the products in the process. Also, the magnetic properties of the samples were characterized on a vibrating sample magnetometer (VSM).     

Synthesis and characterization of single-crystalline MnFe2O4 nanorods via a surfactant-free hydrothermal route

Uniform, high-quality, single-crystalline MnFe₂O₄ nanorods with diameter around 25 nm and length up to 500 nm, have been reproducibly synthesized via a surfactant-free hydrothermal route. The growth direction of the obtained nanowires was determined to be its [1 1 1] direction, resulting in the increase of saturation magnetization. Mn²⁺ is responsible for one-dimensional growth of the nanorods, and the effects of reaction time and solution concentration on the morphology and crystallization of the MnFe₂O₄ nanorods were investigated. Saturation magnetization of the nanorods is 74.0 emu/g, which is among the best value reported so far.     

The effect of grinding on magnetic properties of agglomereted MnFe2O4 nanoparticles

The effects of grinding on interparticle magnetic interactions for an ensemble of agglomerated MnFe₂O₄ nanoparticles have been studied. Structural analyses showed that by grinding the samples, a small variation in size of crystallites and lattice strain will occur. ac Magnetic susceptibility measurements under different conditions and spin dynamics analysis suggest that freezing temperature is frequency dependent and it is in good agreement with critical slowing down model. This is an indication that these nanoparticles have superspin glass behavior. The estimated zυ and τ₀ parameters using critical slowing down model show that by increasing the grinding time the interaction between nanoparticles decreases. ac Susceptibility measurements in cooling and heating process show a thermal hysteresis. The thermal hysteresis decreased by increasing the grinding time. Also, the thermal hysteresis is frequency dependent and it increased as frequency decreased. These results showed that interparticle interactions such as dipole-dipole and exchange interactions between nanoparticles become weaker by grinding.     

Superspin glass state in MnFe2O4 nanoparticles

Nanosized MnFe₂O₄ ferrites were synthesized by a simple method, which is based on the solid state ball-milling and calcinations of nitrate precursors and citric acid. The samples were characterized by using different methods. The results indicate that the products mainly consist of MnFe₂O₄ nanoparticles. The effect of different annealing temperatures on particle sizes and crystallinity of the samples was also studied. By increasing the particle size, the coercivity and magnetization of the samples increase. The increase of magnetization by increasing the crystallite size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. Our analysis of ac susceptibility measurements shows that the interparticle magnetic interaction leads to the superspin glass-like behavior in these nanoparticle samples.     

Preparation and characterizations of SiO2-coated nanoparticles of Mn0.4Zn0.6Fe2O4

Core/shell nanoparticles consisting of a magnetic core of zinc-substituted manganese ferrite (Mn_0.4Zn_0.6Fe₂O₄) and a shell of silica (SiO₂) are prepared by a sol-gel method using tetraethyl orthosilicate (TEOS) as a precursor material for silica and salts of iron, manganese and zinc as the precursor of the ferrite. Three weight percentages of the shell materials of SiO₂ are used to prepare the coated nanoparticles. The X-ray diffractograms (XRD) of the coated and uncoated magnetic nanoparticles confirmed that the magnetic nanoparticles are in their mixed spinel phase in an amorphous matrix of silica. Particles sizes of the samples annealed at different temperatures are estimated from the width of the (3 1 1) line of the XRD pattern using the Debye-Sherrer equation. The information regarding the crystallographic structure together with the particles sizes extracted from the high-resolution transmission electron microscopy (HRTEM) of a few selected samples are in agreement with those obtained from the XRD. HRTEM observations revealed that particles are coated with silica. The calculated thickness is in agreement with that obtained from the HRTEM pictures. Hysteresis loops observed in the temperature range 300 down to 5 K and Mössbauer spectra at room temperature indicate superparamagnetic relaxation of the nanoparticles.     

Helicoidal structure of the mixed ferrite-spinel Zn0.75Ni0.25Fe2O4

The mixed Zn_0.75Ni_0.25Fe₂O₄ ferrite was studied by the neutronographic method within the temperature region of 1.8–77 K. It is shown that besides the usual ferrimagnetic order at 1.8 K there exist superstructure reflections. The experimental results are treated in the framework of the helicoidal magnetic structure. The transition temperatures and some other parameters of the magnetic structure are given. The results of calculation are in good agreement with the experimental data.     

Studies on the preparation and ethanol gas sensing properties of spinel Zn0.6Mn0.4Fe2O4 nanomaterials

Zn_1−xMn_xFe₂O₄ (x = 0, 0.2 and 0.4) nanomaterials were synthesized by sol–gel citrate method and studied structural and gas sensing properties. The structural characteristics of synthesized nanomaterials were studied by X-ray diffraction measurement (XRD) and transmission electron microscope (TEM). The results revealed that the particle size is in the range of 30–35 nm for Mn–Zn ferrite with good crystallinity. The gas sensing properties were studied towards reducing gases like LPG, CH_4, CO and ethanol and it is observed that Mn–Zn ferrite shows high response to ethanol at relatively lower operating temperature. The Zn_0.6Mn_0.4Fe₂O₄ nanomaterial shows better sensitivity towards ethanol at an operating temperature 300 °C. Incorporation of 1.5 wt.% Pd improved the sensitivity, selectivity, response time and reduced the operating temperature from 300 °C to 230 °C for ethanol sensor. The response time of 200 ppm ethanol in air is about 10s.     

Hollow glass microspheres coated with CoFe2O4 and its microwave absorption property

Spinel CoFe₂O₄ coating on the surface of hollow glass microspheres of low density was synthesized by co-precipitation method. The phase structures, morphologies, particle size, shell thickness, chemical compositions of the composites have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDS). The results show that CoFe₂O₄ coating on hollow glass microspheres can be achieved, and the coating layers are constituted by CoFe₂O₄ nanoparticles of mean size ca. 10 nm. The as-synthesized powder materials were uniformly dispersed into the phenolic cement, then the mixture was pasted on metal plate with the area of 200 mm×200 mm as the test plate. The test of microwave absorption was carried out by the radar-absorbing materials (RAM) reflectivity far field radar cross-section (RCS) method. The results indicate that the coated CoFe₂O₄/hollow glass microspheres composites can be applied in lightweight and strong absorption microwave absorbers.     

Effects of synthesis variables on the magnetic properties of CoFe2O4 nanoparticles

Cobalt ferrite nanoparticles (CoFe₂O₄) have been synthesized using precipitation in water solution with polyethylene glycol as surfactant. Influence of various synthesis variables included pH, reaction time and annealing temperature on the magnetic properties and particle sizes has also been studied. Structural identification of the samples was carried out using Thermogravimetric and Differential thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy, Scanning electron microscopy, High resolution transmission electron microscopy. Vibrating sample magnetometer was used for the magnetic investigation of the samples. Magnetic properties of nanoparticles show strong dependence on the particle size. The magnetic properties increase with pH of the precipitating medium and annealing temperature while the coercivity goes through a maximum, peaking at around 25 nm.     

Synthesis and characterization of NiMn2O4 nanoparticles using gelatin as organic precursor

Nanoparticles of NiMn₂O₄ were successfully obtained by mixing gelatin and inorganic salts NiCl₂·6H₂O and MnCl₂·4H₂O in aqueous solution. The mixture has been synthesized at different temperatures and resulted in NiMn₂O₄ nanoparticles with crystallites size in the range of 14–44 nm, as inferred from X-ray powder diffraction (XRPD) data. We have also observed that both the average crystallite size and the unit cell parameters increase with increasing synthesis temperature. Magnetic measurements confirmed the presence of a magnetic transition near 110 K.     

The effect of cation distribution on magnetization of ZnFe2O4 nanoparticles

In this work zinc ferrite (ZnFe₂O₄) nanoparticles have been prepared by sol-gel method in two different media, one acidic and another one basic and then annealed at different temperatures from 350 to 800 °C. XRD investigations show that both samples have a single phase spinel structure. Mean crystallite sizes of the samples were calculated, using Scherrer’s formula, which are 13 and 16 nm for the samples prepared in acidic and basic media, respectively. The variation of cation distribution in the samples was estimated by the ratio of (2 2 0) and (2 2 2) intensity diffraction peaks and the results show that as-prepared nanoparticles have different ionic distributions in comparison with that of bulk zinc ferrite. Also the results show that by increasing annealing temperature the ionic distribution of the zinc ferrite nanoparticles tends to that of bulk sample. The magnetic properties of the samples were studied by VSM and the results show that zinc ferrite nanoparticles have a ferrimagnetic behavior. Also the morphology of the powders was examined by TEM.     

Magnetic properties of Co2.4Al0.6O4/SiO2 nanocomposite obtained via sol-gel method

Nanocomposite made of 10 wt% of Co_2.4Al_0.6O₄ particles dispersed in an amorphous SiO₂ matrix has been synthesized by a sol-gel method. X-ray diffraction, transmission electron microscopy and magnetic measurements have been used to characterize the properties of nanocomposite. Most of the particles are well crystallized and have an average diameter below 100 nm. Smaller particles with size below 10 nm have also been observed. A large value of the effective magnetic moment per Co²⁺ ion of 5.08 μ_B and negative and the low Curie-Weiss paramagnetic temperature Θ∼−6 K, obtained from the high-temperature susceptibility data, indicate a possible mixing of Co²⁺ and Co³⁺ ions between tetrahedral and octahedral sites of the spinel crystal lattice. The measurements of static and dynamic magnetic susceptibilities have shown that Co_2.4Al_0.6O₄ particles in SiO₂ matrix display a spin glass behavior at low temperatures.     

Low-temperature synthesis and growth of superparamagnetic Zn0.5Ni0.5Fe2O4 nanosized particles

Mono-disperse spinel Ni_0.5Zn_0.5Fe₂O₄ nanosized particles have been synthesized via a hydrothermal method at low temperature. X-ray diffraction (XRD), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) analysis indicated that the synthesized nanocrystals were of pure cubic spinel structure with the size about 6-20 nm. The activation energy of grain growth is 35.06 kJ/mol experimented by the Arrhenius equation. A primary experimental model was put forward to shed light on the growth mechanism of crystallined spinel Ni-Zn ferrite nanosized particles under hydrothermal conditions. The magnetic measurements shows that the prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticle possess good superparamagnetic behavior.     

Study of dielectric and magnetic properties of PbZr0.52Ti0.48O3-Mn0.3Co0.6Zn0.4Fe1.7O4 composite

Results of detailed structural, dielectric, magnetic and magnetoelectric studies of (x)PbZr_0.52Ti_0.48O₃-(1−x)Mn_0.3Co_0.6Zn_0.4Fe_1.7O₄ composites where x=65, 70, 75 and 80 are shown in this work. Manganese substituted cobalt ferrites are known to exhibit large strain derivative (dx/dH) and on the other hand substitution of Zn in pure cobalt ferrite is known to enhance its permeability μ and permittivity ε. The choice of ferrite as Mn, Zn simultaneously substituted cobalt ferrite (MCZFO) is made keeping in view that for good magnetoelectric (ME) voltage coefficient the magnetostrictive constituent phase of the composite should have large strain derivative (dx/dH) along with large permittivity and permeability. It is shown here that although the dielectric transition temperature changes significantly with change in the mole ratio of the two component phases, magnetic transition temperature (much less compared to the bulk cobalt ferrite) is relatively non-responsive to the changing molar ratio of the two component phases. In the vicinity of the magnetic transition temperature we observed an anomaly in tan δ vs. T plots, which indicates a possible magnetoelectric coupling in the samples. Magnetoelectric voltage coefficient (α_E) has been measured using static magnetoelectric method. Highest magnetoelectric voltage coefficient (α_E=0.312 mV/cmOe) is obtained for sample 80:20 at H_DC=1000 Oe.