Effect of SnO2 coating on the magnetic properties of nanocrystalline CuFe2O4

Nanocrystalline CuFe₂O₄ and CuFe₂O₄/xSnO₂ nanocomposites (x=0, 1, 5 wt%) have been successfully synthesized by one-pot reaction of urea-nitrate combustion method. The transmission electron microscope study reveals that the particle size of the as synthesized CuFe₂O₄ and CuFe₂O₄/5 wt%SnO₂ are 10 and 20 nm, respectively. The SnO₂ coating on the nanocrystalline CuFe₂O₄ was confirmed from HRTEM studies. The resultant products were sintered at 1100 °C and characterized by XRD and SQUID for compound formation and magnetic studies, respectively. The X-ray diffraction pattern shows the well-defined sharp peak that confirms the phase pure compound formation of tetragonal CuFe₂O₄. The zero field cooled (ZFC) and field cooled (FC) magnetization was performed using SQUID magnetometer from 2 to 350 K and the magnetic hysteresis measurement was carried out to study the magnetic properties of nanocomposites.     

Magnetic structure and collective Jahn-Teller distortions in nanostructured particles of CuFe2O4

The aim of the present work is to compare the structural, the composition and chemical state of the surface and magnetic properties of different nanosized CuFe₂O₄ powders exhibiting collective Jahn-Teller effect. The samples under examination consist of edged nanosized particles (needle like) with average length 1300 ± 20 nm and diameter 300 ± 20 nm obtained after high temperature synthesis, and superparamagnetic (at room temperature) spherical particles (d = 6 ± 2 nm), obtained by soft chemistry techniques. The surface composition of the particles was investigated by X-ray photoelectron spectroscopy (XPS). Mössbauer spectroscopy (MöS), including at high magnetic field up to 5 T and 4.2 K, was used for characterization of cation distribution in the samples. The data yielded by the XPS and MöS analyses for spherical nanosized particles led us to the assumption for the existence of a Jahn-Teller effect gradient—from the B-sublattice on the surface to a compensation of the tetragonal distortion in the two sublattices in the core. The analysis of the contribution of the anisotropy energy in edged and spherical nanoparticles shows that it must be considered as an effective value reflecting the influence of the individual factors depending on the particle shape and surface.     

An investigation on the behavior of fine-grained magnetite particles as a function of size and surface modification

By using a KNO₃-aging ferrous hydroxide gel method, Fe₃O₄ particles with sizes ranging from 35 to 1500 nm were synthesized. The particles were covered with a silica coating to form Fe₃O₄-SiO₂ core-shell structures by using the improved conventional Stöber polycondensation method. The thickness of the SiO₂ covering on magnetite particles surface varies from 10 to 20 nm. The morphology, size and composition of the particles were determined by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The particles with and without coating with SiO₂ were pressed into slices with an oil press at 10 MPa. Subsequently, the coercive forces H_C of the particles were measured by VSM at room temperature, and the critical size for a single domain was estimated. The shape of the particles is basically spherical when the size is smaller than 800 nm, while it is hexagonal for larger particles. The H_C of Fe₃O₄-SiO₂ core-shell structure was larger than that of the uncoated Fe₃O₄ particles by 20%, which was explained to be due to the reduction of inter-particle magnetostatic interaction, supported by an agreement with the packing factor. The dependence of H_C on magnetic particle size could be explained and fitted by the Heewell-Knozam stacking density equation and object-oriented micromagnetic computing framework (OOMMF) micromagnetic software. the results agree well with the experimental data.     

Study of structural and ferromagnetic properties of pure and Cd doped copper ferrite

This report presents the synthesis of copper cadmium ferrite (Cu_1−xCd_xFe₂O₄, x=0.3, 0.4, 0.5, 0.6 and 0.7) by the citrate precursor method and its subsequent characterization by X-ray diffraction (XRD), differential scanning calorimetry, infrared spectroscopy and ferromagnetic resonance. XRD results confirm the single cubic spinel phase formation with the particle size of 40 nm, which decreased up to 20 nm with increase in Cd content, while the lattice parameter increased with increase in Cd content. A significant change in the magnetic properties was observed in the CuFe₂O₄ system with Cd doping. The line width and resonance field variation against change in temperature is noted and the data is fitted to the linearlized model (LM) and Smit and Beljers (SB) model to find out the parameters. The results recorded from the SB approach are in good agreement with those observed in the magnetic measurements carried out by vibrating sample magnetometer (VSM) techniques.     

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.     

Investigation of exchange bias in 0.1MFe2O4/0.9BiFeO3 (M=Co, Cu, Ni) nanocomposite

The 0.1MFe₂O₄/0.9BiFeO₃ (M=Co, Cu, Ni) nanocomposite samples were synthesized by the sol-gel method. Phase composition analysis was carried out, which showed that these bulk samples were composed of a ferrimagnetic MFe₂O₄ (M=Co, Cu, Ni) and a ferroelectric antiferromagnet (FEAF) BiFeO₃ phases, respectively. The magnetic properties of all the samples were investigated by measuring their magnetization as a function of temperature and magnetic field. These results indicated that the magnetic hysteresis loops of 0.1CuFe₂O₄/0.9BiFeO₃ sample sintered in air atmosphere at 550 °C for 3 h exhibited a negative shift and an enhanced coercivity at low temperature ascribed to strong exchange coupling between the BiFeO₃ and CuFe₂O₄ grains. However, there were no magnetic hysteresis loops in both the 0.1CoFe₂O₄/0.9BiFeO₃ sample and the 0.1NiFe₂O₄/0.9BiFeO₃ sample. In view of these results, we tend to think the CuFe₂O₄/BiFeO₃ nanocomposite system may be a useful multifunctional material.     

Magnetic and rheological properties of monodisperse Fe3O4 nanoparticle/organic hybrid

Fe₃O₄ nanoparticle/organic hybrids were synthesized via hydrolysis using iron (III) acetylacetonate at ∼80 °C. The synthesis of Fe₃O₄ was confirmed by X-ray diffraction, selected-area diffraction, and X-ray photoelectron spectroscopy. Fe₃O₄ nanoparticles in the organic matrix had diameters ranging from 7 to 13 nm depending on the conditions of hydrolysis. The saturation magnetization of the hybrid increased with an increase in the particle size. When the hybrid contained Fe₃O₄ particles with a size of less than 10 nm, it exhibited superparamagnetic behavior. The blocking temperature of the hybrid containing Fe₃O₄ particles with a size of 7.3 nm was 200 K, and it increased to 310 K as the particle size increased to 9.1 nm. A hybrid containing Fe₃O₄ particles of size greater than 10 nm was ferrimagnetic, and underwent Verwey transition at 130 K. Under a magnetic field, a suspension of the hybrid in silicone oil revealed the magnetorheological effect. The yield stress of the fluid was dependent on the saturation magnetization of Fe₃O₄ nanoparticles in the hybrid, the strength of the magnetic field, and the amount of the hybrid.     

Synthesis of magnetic hollow silica using polystyrene bead as a template

In this paper, we report a new route to synthesize novel magnetic hollow silica nanospheres (MHSNs) using polystyrene particles as sacrificial templates, and TEOS and Fe₃O₄ as precursors. TEM, EDS, XRD, and SQUID were applied to characterize MHSNs. TEM and EDS results show that the MHSNs consist of about 200 nm of hollow cores and ∼35 nm shells with ∼10 nm of Fe₃O₄ nanoparticles embedded. The polystyrene beads were successfully removed by immersing the as-prepared silica nanocomposite in a toluene solution. XRD results demonstrate that the Fe₃O₄ magnetic nanoparticles still keep spinel structure even heated at low temperature. The surface status of the polystyrene beads and Fe₃O₄ nanoparticles has an important effect on the formation of the MHSNs. The MHSNs present a superparamagnetism at room temperature by SQUID measurement. The MHSNs have potential applications in biosystem and nanomedicine.     

The properties of Mn–CuFe2O4 spinel ferrite nanoparticles under various synthesis conditions

The structural, morphological, magnetic, dielectric, and gas analyzing properties are studied in CuFe₂O₄(Mn–CuFe₂O₄) substituted spinel ferrite nanoparticles synthesized via evaporation and automatic combustion. The obtained nanoparticles are established to possess a spherical shape. The smallest size of Mn–CuFe₂O₄ (~9 nm) nanoparticles is achieved at using automatic combustion. X-ray diffraction and Mössbauer spectroscopy reveal that the crystal lattice constant and the Mn–CuFe₂O₄ nanoparticle size are larger at augmenting the annealing temperature from 600 to 900°С. The dielectric permeability and losses of Mn–CuFe₂O₄ nanoparticles are studied at various synthesis conditions and temperatures of annealing. Various aspects of gas sensibility of synthesized Mn–CuFe₂O₄ nanoparticles are tested, as well. The maximum response to the presence of liquefied petroleum gas is 0.28 at the optimum working temperature of 300°C for Mn–CuFe₂O₄ nanoparticles obtained via automatic combustion and it is 0.23 at 250°C for deposited nanoparticles.

     

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.     

Magnetic behavior of core–shell particles

We have prepared composite magnetic core–shell particles using the process of soap-free emulsion polymerization and the co-precipitation method. The shell of the synthesized composite sphere is cobalt ferrite (CoFe₂O₄) nanoparticles and the core consists of poly(styrene-co-methacrylic acid) polymer. The mean crystallite sizes of the coated CoFe₂O₄ nanoparticles were controlled in the range of 2.4–6.7 nm by the concentration of [NH₄⁺] and heated temperature. The magnetic properties of the core–shell spherical particles can go from superparamagnetic to ferromagnetic behavior depending on the crystalline sizes of CoFe₂O₄.     

Synthesis of carbon nanotubes and iron oxide nanoparticles in MW plasma torch with Fe(CO)5 in gas feed

The MW plasma torch (2.45 GHz) in the mixture of CH₄/H₂/Ar (42/430/1540 sccm) with added Fe(CO)₅vapors was used for the synthesis of iron oxide nanoparticles and carbon nanotubes. The particles with well-defined facets consisting of Fe₃O₄ and -Fe₂O₃ and self-assembled into long chains were produced at the power of 360 W. At higher power of 440-460 W the deposit contained significant amount of multi-walled carbon nanotubes covered by iron oxide nanoparticles. The diameter of CNTs was 8-20 nm. The particles had Fe₃O₄ and/or -Fe₂O₃ cores of spherical shape covered by a thin layer of carbon.     

First nucleation steps of vanadium oxide thin films studied by XPS inelastic peak shape analysis

The initial states of deposition of vanadium oxide thin films have been studied by analysis of the peak shape (both inelastic background and elastic contributions) of X-ray photoemission spectra (XPS) after successive deposition experiments. This study has permitted to assess the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of a XPS spectrometer. Thin films of vanadium oxide have been prepared on Al₂O₃ and TiO₂ by means of thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition. The thin films prepared by the first two procedures consisted of V₂O₄, while those prepared by the latter had a V₂O₅ stoichiometry. The analysis of the inelastic background of the photoemission spectra has shown that the films prepared by thermal evaporation on Al₂O₃ are formed by big particles that only cover completely the surface of the substrate when their height reaches 16 nm. By contrast, the thin films prepared with assistance of ions on Al₂O₃ or with plasma on TiO₂ consist of smaller particles that succeed in covering the substrate surface already for a height of approximately 4 nm. Thin films prepared by plasma-assisted deposition on Al₂O₃ depict an intermediate situation where the substrate is completely covered when the particles have a height of approximately 6 nm. The type of substrates, differences in the deposition procedure or the activation of the adatoms by ion bombardment are some of the factors that are accounted for by to explain the different observed behaviours.     

Synthesis and magnetic properties of platelet γ-Fe2O3 particles for medical applications using hysteresis-loss heating

Platelet γ-Fe₂O₃ particles of particle size less than 100 nm were prepared for medical applications that use the hysteresis-loss heating of ferromagnetic particles. The γ-Fe₂O₃ particles were obtained through the dehydration, reduction, and oxidation of platelet α-FeOOH particles, which were synthesized by the precipitation of ferric ions in an alkaline solution containing ethanolamine, and the crystals grown using a hydrothermal treatment. The γ-Fe₂O₃ particles contained dimples formed by the dehydration of α-FeOOH particles. The coercive force and the saturation magnetization of the γ-Fe₂O₃ particles were in the ranges 11.9 to 12.7 kA/m (150 to 160 Oe), and 70 to 72 Am²/kg (70 to 72 emu/g), respectively. The specific loss power of the γ-Fe₂O₃ particles, estimated from their temperature-raising property measured under a peak magnetic field of 50.9 kA/m (640 Oe) and at a frequency of 117 kHz, was 590 W/g. This value is higher than that of spherical cobalt-containing iron oxide particles having equivalent coercive force and saturation magnetization, reflecting the larger area of the minor hysteresis loop measured under a peak magnetic field of 50.9 kA/m (640 Oe).     

In situ synthesis and characterization of LiNi0.5La0.08Fe1.92O4-polyaniline core-shell nanocomposites

Core-shell-structured LiNi_0.5La_0.08Fe_1.92O₄-polyaniline (PANI) nanocomposites with magnetic behavior were synthesized by in situ polymerization of aniline in the presence of LiNi_0.5La_0.08Fe_1.92O₄ nanoparticles. The structure, morphology and magnetic properties of samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), UV-vis absorption, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) technique. The results of spectroanalysis indicated that there was interaction between PANI chains and ferrite particles. TEM study showed that LiNi_0.5La_0.08Fe_1.92O₄-PANI nanocomposites presented a core-shell structure with a magnetic core of 30-50 nm diameter and an amorphous shell of 10-20 nm thickness. The nanocomposites under applied magnetic field exhibited the hysteresis loops of the ferromagnetic nature. The saturation magnetization and coercivity of nanocomposites decreased with decreasing content of LiNi_0.5La_0.08Fe_1.92O₄. The polymerization mechanism and bonding interaction in the nanocomposites have been discussed.     

Particles size effects of single domain CoFe2O4 on suspensions stability

Single domain magnetic CoFe₂O₄ nanoparticles with spinel structure were prepared by the coprecipitation method. Particles with size of 16, 20, 40 and 60 nm were synthesized by sintering the precursor at 500, 600, 800 and 900 °C, respectively. The magnetic hysteresis measurement of CoFe₂O₄ particles showed that particles were single domain particles with similar saturation magnetization (∼300 emu/cm³) at room temperature. The zeta potential study of suspensions (CoFe₂O₄-acetylacetone system) with various particle sizes showed the suspension systems had similar zeta potential values (∼40 mV). The effects of magnetic particle size on the suspension stability characterized by electrophoretic deposition yields and sediment volumes were studied. The suspension stability decreased with an increase in particle size and a flocculation threshold of particle radius a was found at 30 nm. A suspension stability theory approaching to the phenomenon was established. The theory based on the DLVO theory was developed by introducing an extra magnetic interaction force. Dormann model was adopted, in which the magnetic interactions of two spherical nanoparticles were investigated in terms of dipole-dipole interactions. Compared to DLVO, suspension's physical parameters not only zeta potential ζ and the Debye length 1/κ, but also particles' radius a brought about stable to flocculation transition in the theory.     

Order-disorder transformation in FePt nanoparticles studied by ferromagnetic resonance

We have investigated the ferromagnetic resonance (FMR) response of as-made and temperature annealed FePt magnetic nanoparticles. The as-made nanoparticles, which have been fabricated by a chemical route, crystallize in the low magnetic anisotropy fcc phase and have a diameter in the range of 2-4 nm. The annealing of the particles at high temperatures (T_A=550, 650 and C) in an inert Ar atmosphere produces a partial transformation to the high magnetocrystalline anisotropy L1₀ phase, with a significant increase in particle size and size distribution. FMR measurements at X-band (9.5 GHz) and Q-band (34 GHz) show a single relatively narrow line for the as-synthesized particles and a structure of two superimposed lines for the three annealed samples. The origin of this line shape has been attributed to the presence of the disordered fcc phase. Assuming that the system consists of a collection of identical particles with a random distribution of easy axes, we have been able to estimate a mean value for the magnetic anisotropy constant of the particles in the fcc phase, K∼2×10⁶ erg/cm³. The measured line shape in the annealed samples can be explained if we consider that the magnetic anisotropy of the particles has a gaussian distribution with a relatively broad width.     

Effect of size and synthesis route on the magnetic properties of chemically prepared nanosize ZnFe2O4

Magnetization of the ZnFe₂O₄ sample of average size 4 nm measured with SQUID in the temperature range 5–300 K shows anomalous behaviour in field cooled (FC) and zero-field-cooled (ZFC) conditions. The FC and ZFC curves measured in 50 Oe field cross each other a little before the peaks. No such anomaly is observed with samples of 6 nm particle size made with the same procedure. The characteristics of the FC and ZFC curves are very different in ZnFe₂O₄ samples of the same size (6 nm) made via two different chemical routes. The genesis of these differences are suggested to be in cationic configuration and spin disorder. Fe-extended X-ray absorption fine structure (EXAFS) studies show that there is around 80% inversion in case of zinc ferrite (ZnFe₂O₄) with the particle size 4 nm, whereas ZnFe₂O₄ of size 6 nm shows 40% inversion. The samples with an average particle size of 7 nm and more show negligible inversion. Theoretical simulations suggest that the electrostatic energy of the system plays a crucial role in deciding the cationic configuration of spinel ferrites.     

Preparation of spherical and uniform-sized ferrite nanoparticles with diameters between 50 and 150 nm for biomedical applications

Spherical uniform-sized iron ferrite nanoparticles were synthesized by adding a disaccharide and seed ferrite crystals into an aqueous reaction solution. The average size range 50-150 nm was controlled by choosing one out of five disaccharides and by changing the amount of the seed crystals. The particles had a saturation magnetization and a crystalline structure which are similar to those of intermediate Fe₃O₄-γ-Fe₂O₃. When coated with citrate, the particles with nearly 100 nm diameter were stably suspended in water for 2 days. These novel particles will be utilized as magnetic carriers in biomedical applications.     

Study of polydiethylsiloxane-based ferrofluid with excellent frost resistance property

The polydiethylsiloxane-based ferrofluid was prepared by dispersing finely divided magnetic Fe₃O₄ particles which are modified with oleoyl sarcosine and lauroyl sarcosine. The optimized experiment parameters including molar ratio of surfactant to Fe₃O₄ (1:5), temperature (80 °C), stirring rate (300 RPM), the surfactant content of lauroyl sarcosine (0 to 33 mol%) and the modification time (25 min) were obtained by the orthogonal test. The magnetic liquid was characterized by a transmission electron microscope (TEM), infrared (IR) spectrometer, X-ray diffractometer (XRD), thermogravimetry (TG), vibrating sample magnetometer (VSM) and differential scanning calorimetry (DSC). It is indicated that the surfactant is mainly bonded to the surface of Fe₃O₄ nanoparticles through covalent bond between carboxylate (COO⁻) and Fe atom. The modified magnetic particles are equally dispersed into the carrier and remain stable below −12 °C over 4 months. The ferrofluids exhibit excellent frost resistance property and distinctly reduced temperature coefficient of viscosity compared with polydimethylsiloxane-based ferrofluids and hydrocarbon-based ferrofluids, respectively. The saturation magnetization could reach up to 27.7 emu/g.