Preparation and characterization of silica-coated Fe3O4 nanoparticles used as precursor of ferrofluids

Fe₃O₄ magnetic nanoparticles (MNPs) were synthesized by the co-precipitation of Fe³⁺ and Fe²⁺ with ammonium hydroxide. The sodium citrate-modified Fe₃O₄ MNPs were prepared under Ar protection and were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). To improve the oxidation resistance of Fe₃O₄ MNPs, a silica layer was coated onto the modified and unmodified MNPs by the hydrolysis of tetraethoxysilane (TEOS) at 50 °C and pH 9. Afterwards, the silica-coated Fe₃O₄ core/shell MNPs were modified by oleic acid (OA) and were tested by IR and VSM. IR results revealed that the OA was successfully grafted onto the silica shell. The Fe₃O₄/SiO₂ core/shell MNPs modified by OA were used to prepare water-based ferrofluids (FFs) using PEG as the second layer of surfactants. The properties of FFs were characterized using a UV-vis spectrophotometer, a Gouy magnetic balance, a laser particle size analyzer and a Brookfield LVDV-III+ rheometer.     

Li0.5Fe2.5−xMnxO4 ferrite sintered from microwave-induced combustion

Li_0.5Fe_2.5−xMn_xO₄ (0≦x≦1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, ferric nitrate, manganese nitrate and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Mn-substituted lithium ferrite powders. The resultant powders annealed at 650 ^°C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the Mn content were strongly influenced the magnetic properties and Curie temperature of Mn-substituted lithium ferrite powder. As for sintered Li_0.5Fe_2.5−xMn_xO₄ specimens, substituting an appropriate amount of Mn for Fe in the Li_0.5Fe_2.5−xMn_xO₄ specimens markedly improved the complex permeability and loss tangent.     

Structural, AC, and DC magnetic properties of Zn1−xCoxFe2O4

Structural, AC and DC magnetic properties of polycrystalline Zn_1−xCo_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures (1100-1300 °C), and various dwell times (0.2-15 h) have been investigated thoroughly. The bulk density of the Zn_0.60Co_0.40Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1100 to 1250 °C, and above 1250 °C the bulk density decreases slightly. The Zn_0.80Co_0.20Fe₂O₄ samples show similar behavior of changes to that of Zn_0.60Co_0.40Fe₂O₄ samples except that the bulk density is found to be highest at 1200 °C. The DC magnetization as a function of temperature curves show that the Zn_0.60Co_0.40Fe₂O₄ sample is ferrimagnetic at room temperature while the Zn_0.80Co_0.20Fe₂O₄ sample is paramagnetic at room temperature. The T_c of Zn_0.80Co_0.20Fe₂O₄ sample is found to be 170 K from DC magnetization measurement. Separate measurement (AC magnetization), initial permeability as a function of temperature shows that the T_c of the Zn_0.60Co_0.40Fe₂O₄ sample is 353 K. Slight variation of T_c is observed depending on sintering condition. The initial permeability for the Zn_0.60Co_0.40Fe₂O₄ composition sintered at 1250 °C is found to be maximum.     

Magnetic and bioactivity evaluation of ferrimagnetic ZnFe2O4 containing glass ceramics for the hyperthermia treatment of cancer

Glass ceramics of the composition xZnO·25Fe₂O₃·(40−x)SiO₂·25CaO·7P₂O₅·3Na₂O were prepared by the melt-quench method using oxy-acetylene flame. Glass-powder compacts were sintered at 1100 °C for 3 h and then rapidly cooled at −10 °C. X-ray diffraction (XRD) revealed 3 prominent crystalline phases: ZnFe₂O₄, CaSiO₃ and Ca₁₀(PO₄)₆(OH)₂. Vibrating sample magnetometer (VSM) data at 10 KOe and 500 Oe showed that saturation magnetization, coercivity and hence hysteresis area increased with the increase in ZnO content. Nano-sized ZnFe₂O₄ crystallites were of pseudo-single domain structure and thus coercivity increased with the increase in crystallite size. ZnFe₂O₄ exhibited ferrimagnetism due to the random distribution of Zn²⁺ and Fe³⁺ cations at tetrahedral A sites and octahedral B sites. This inversion/random distribution of cations was probably due to the surface effects of nano-ZnFe₂O₄ and rapid cooling of the material from 1100 °C (thus preserving the high temperature state of the random distribution of cations). Calorimetric measurements were carried out using magnetic induction furnace at 500 Oe magnetic field and 400 KHz frequency. The data showed that maximum specific power loss and temperature increase after 2 min were 26 W/g and 37 °C, respectively for the sample containing 10% ZnO. The samples were immersed in simulated body fluid (SBF) for 3 weeks. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDX) and XRD results confirmed the growth of precipitated hydroxyapatite phase after immersion in SBF, suggesting that the ferrimagnetic glass ceramics were bioactive and could bond to the living tissues in physiological environment.     

Synthesis and characterization of a novel magnetic carrier with its composition of Fe3O4/carbon using hydrothermal reaction

In this study, a simple method to prepare a novel magnetic carrier based on carbon matrix has been built by heating the aqueous solution of glucose and oleic acid-stabilized Fe₃O₄ nanoparticle at 170 °C for 3 h. The results show that the surface hydrophobic modification of Fe₃O₄ nanoparticle is necessary for the successful synthesis of Fe₃O₄/C nanocomposition, and a possible formation mechanism of Fe₃O₄/C nanocomposition was presented. The influence of the reaction parameters such as the concentration of oleic acid-stabilized Fe₃O₄ nanoparticle, the reaction time, etc. on the product was also investigated. In the typical reaction (2.5 g/L of oleic acid-stabilized Fe₃O₄ nanoparticle, 0.5 M of glucose), Fe₃O₄/C nanocompositions with the average diameter in the range 100–200 nm were obtained and its saturation is 12.4 emu/g. In order to characterize Fe₃O₄/C nanocompositions, XPS, XRD, FT–IR, and Mössbauer spectra were employed.     

Synthesis of iron oxide nanoparticles under oxidizing environment and their stabilization in aqueous and non-aqueous media

Synthesis of magnetite (Fe₃O₄) nanoparticles under oxidizing environment by precipitation from aqueous media is not straightforward because Fe²⁺ gets oxidized to Fe³⁺ and thus the ratio of Fe³⁺:Fe²⁺=2:1 is not maintained during the precipitation. A molar ratio of Fe³⁺:Fe²⁺ smaller than 2:1 has been used by many to compensate for the oxidation of Fe²⁺ during the preparation. In this work, we have prepared iron oxide nanoparticles in air environment by the precipitation technique using initial molar ratios Fe³⁺:Fe²⁺?2:1. The phases of the resulting powders have been determined by several techniques. It is found that the particles consist mainly of maghemite with little or no magnetite phase. The particles have been suspended in non-aqueous and aqueous media by coating the particles with a single layer and a bilayer of oleic acid, respectively. The particle sizes, morphology and the magnetic properties of the particles and the ferrofulids prepared from these particles are reported. The average particle sizes obtained from the TEM micrographs are 14, 10 and 9 nm for the water, kerosene and dodecane-based ferrofluids, respectively, indicating a better dispersion in the non-aqueous media. The specific saturation magnetization (σ_s) value of the oleic-acid-coated particles (∼53 emu/g) is found to be lower than that for the uncoated particles (∼63 emu/g). Magnetization σ_s of the dodecane-based ferrofluid is found to be 10.1 emu/g for a volume fraction of particles ?=0.019. Zero coercivity and zero remanance on the magnetization curves indicate that the particles are superparamagnetic (SPM) in nature.     

Magnetic and Mössbauer studies of Ni substituted Li-Zn ferrite

Li-Zn ferrites substituted with Ni having the compositional formula Li_0.4−0.5xZn_0.2Ni_xFe_2.4−0.5xO₄ where x=0.02?x?0.1 in steps of 0.02 were fabricated by the citrate precursor method. This method has been employed to get nanosized particles and good magnetic properties. The spinel phase structure of the prepared ferrites was confirmed by XRD analysis. The effect of Ni concentration on magnetic properties such as saturation magnetization and Curie temperature were investigated. A good knowledge of these magnetic properties is desirable from application point of view. The values observed are large and both quantities were found to decrease with substitution. The saturation magnetizations were found to vary from 78 to 94 emu/gm while the Curie temperature which limits the operating temperature of the system ranges between 563 and 584 °C. Mössbauer data were also recorded at room temperature and the hyperfine parameters like isomer shift, quadrupole splitting and internal magnetic field estimated. The results obtained and mechanisms involved are discussed.     

Preparation of the SrFe12O19-based magnetic composites via boron oxide glass devitrification

Magnetic composites were obtained in the system SrO–Fe₂O₃–B₂O₃ by oxide glass heat treatment at 600–950 °C. Samples of the composites were investigated using XRD analysis, magnetic measurements, electron microcopy, and thermal analysis. It was shown that chemical composition of the precursor oxide glass and thermal treatment conditions influenced on the SrFe₁₂O₁₉ particles morphology and magnetic properties. The composites and powders were obtained containing hexaferrite as single domain platelet crystals or polycrystalline aggregates with a coercive force up to 6300 Oe in the former case and 4200 Oe in the latter case.     

Enhanced magnetoresistance effects in bulk polycrystalline Ag-added magnetite

A series of bulk polycrystalline Ag-added Fe₃O₄ with a nominal composition, (Fe₃O₄)_1−xAg_x (x is molar fraction) with x=0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by conventional solid-state reaction. X-ray diffraction patterns show that the pure Fe₃O₄ sample (x=0) has a single-phase inverse spinel structure, while the Ag-added samples are two-phase composites consisting of a ferrimagnetic Fe₃O₄ phase and a non-magnetic metal Ag phase. The bright-field transmission electron microscopy images exhibit that the samples are typical granular solids with a porosity of about 22%. The addition of Ag slightly increases the average grain size of the Fe₃O₄ phase and significantly enhances the MR effect of bulk polycrystalline Fe₃O₄ samples. Of all the samples the x=0.3 sample has a maximal MR of −5.1% at 300 K in a magnetic field of 1 T, and −6.8% in 5 T, which are approximately three times greater than the corresponding MR values (−1.8% at 1 T and −2.4% at 5 T) of the Fe₃O₄ sample. This enhancement of the MR can be attributed to the combination effect from the spin-dependent scattering at the interfaces between the Fe₃O₄ grains and the Ag granules and the spin-polarized tunneling at grain boundaries of Fe₃O₄ phase of the spin-polarized electrons.     

Simple recipe to synthesize single-domain BaFe12O19 with high saturation magnetization

We report a new synthesis route for preparation of single-domain barium hexaferrite (BaFe₁₂O₁₉) particles with high saturation magnetization. Nitric acid, known as a good oxidizer, is used as a mixing medium during the synthesis. It is shown that formation of BaFe₁₂O₁₉ phase starts at 800 °C, which is considerably lower than the typical ceramic process and develops with increasing temperature. Both magnetization measurements and scanning electron microscope micrographs reveal that the particles are single domain up to 1000 °C at which the highest coercive field of 3.6 kOe was obtained. The best saturation magnetization of ≈60 emu/g at 1.5 T was achieved by sintering for 2 h at 1200 °C. Annealing at temperatures higher than 1000 °C increased the saturation magnetization, on the other hand, decreased the coercive field which was due to the formation of multi-domain particles with larger grain sizes. It is shown that the best sintering to obtain fine particles of BaFe₁₂O₁₉ occurs at temperatures 900-1000 °C. Finally, magnetic interactions between the hard BaFe₁₂O₁₉ phase and impurity phases were investigated using the Stoner-Wohlfarth model.     

Magnetic field-induced cluster formation and variation of magneto-optical signals in zinc-substituted ferrofluids

Fine magnetic particles (size≅100 Å) belonging to the series Zn_xFe_1−xFe₂O₄ were synthesized by cold co-precipitation methods and their structural properties were evaluated using X-ray diffraction. Magnetization studies have been carried out using vibrating sample magnetometry (VSM) showing near-zero loss loop characteristics. Ferrofluids were then prepared employing these fine magnetic powders using oleic acid as surfactant and kerosene as carrier liquid by modifying the usually reported synthesis technique in order to induce anisotropy and enhance the magneto-optical signals. Liquid thin films of these fluids were prepared and field-induced laser transmission through these films was studied. The transmitted light intensity decreases at the centre with applied magnetic field in a linear fashion when subjected to low magnetic fields and saturate at higher fields. This is in accordance with the saturation in cluster formation. The pattern exhibited by these films in the presence of different magnetic fields was observed with the help of a CCD camera and was recorded photographically.     

Study on electro-magnetic properties of La substituted Ni–Cu–Zn ferrite synthesized by auto-combustion method

(Ni_0.25Cu_0.20Zn_0.55)La_xFe_2−xO₄ ferrite with x=0.00, 0.025, 0.050 and 0.075 compositions were synthesized through nitrate–citrate auto-combustion method. Crystalline spinel ferrite phase with about 16–19 nm crystallite size was present in the as-burnt ferrite powder. These powders were calcined, compacted and sintered at 950 °C for 4 h. Initial permeability, magnetic loss and AC resistivity of different compositions were measured in the frequency range from 10 Hz to 10 MHz. Saturation magnetization and hysteresis parameters were measured at room temperature with a maximum magnetic field of 10 kOe. Permeability and AC resistivity were found to increase and magnetic loss decreased with La substitution for Fe, up to x=0.025. Saturation magnetization and coercive field also increases up to that limit. The electromagnetic properties were found best in the ferrite composition of x=0.025, which would be better for more miniaturized multi layer chip inductor.     

Magnetoresistive properties of Zn1−xCoxFe2O4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn_1−xCo_xFe₂O₄ (x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn_1−xCo_xFe₂O₄ ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR(H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe₂O₄. The MR for ZnFe₂O₄ at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR(H) are linear for x=0.2 and 0.4 ferrimagnetic Zn_1−xCo_xFe₂O₄ samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn_1−xCo_xFe₂O₄ mixed ferrites.     

Enhancement of initial permeability due to Mn substitution in polycrystalline Ni0.50−xMnxZn0.50Fe2O4

The structural and magnetic properties of Mn substituted Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.00, 0.10 and 0.20) sintered at various temperatures have been investigated thoroughly. The lattice parameter, average grain size and initial permeability increase with Mn substitution. Both bulk density and initial permeability increase with increasing sintering temperature from 1250 to 1300 °C and above 1300 °C they decrease. The Ni_0.30Mn_0.20Zn_0.50Fe₂O₄ sintered at 1300 °C shows the highest relative quality factor and highest initial permeability among the studied samples. The initial permeability strongly depends on average grain size and intragranular porosity. From the magnetization as a function of applied magnetic field, M(H), it is clear that at room temperature all samples are in ferrimagnetic state. The number of Bohr magneton, n(μ_B), and Neel temperature, T_N, decrease with increasing Mn substitution. It is found that Mn substitution in Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.20) decreases the Neel temperature by 25% but increases the initial permeability by 76%. Possible explanation for the observed characteristics of microstructure, initial permeability, DC magnetization and Neel temperature of the studied samples are discussed.     

The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

Investigating the formation and magnetic properties of the Dy0.75Fe1.25O3 orthoferrite prepared by sol-gel method

In this paper, the Dy_0.75Fe_1.25O₃ orthoferrite nanoparticles were synthesized successfully by sol-gel method. Dy_0.75Fe_1.25O₃ orthoferrite nanoparticles are obtained by calcining the flakes at 600 and 700 °C. The magnetic properties of the different samples are investigated using Quantum Design MPMS SQUID magnetometer and MS-500 Mössbauer spectrometer. Magnetic phase γ-Fe₂O₃ coexists in the samples calcined at 600 °C and orthoferrite phase is completely recovered in the samples calcined at 700 °C. Although excessive Fe³⁺ ions were introduced, none of these iron spins couple magnetically with Dy³⁺ ions.     

Electromagnetic properties of samarium-substituted NiCuZn ferrite prepared by auto-combustion method

Ni_0.25Cu_0.2Zn_0.55Sm_xFe_2−xO₄ ferrite with x=0.00, 0.025, 0.05 and 0.075 compositions were synthesized through the nitrate-citrate auto-combustion method. These powders were calcined, compacted and sintered at 900 °C for 4 h. Effect of Sm substitution on phase composition, microstructure and relative density were studied. Permeability, magnetic loss and AC resistivity were measured in the frequency range of 1 kHz-10 MHz. Permeability and AC resistivity were found to increase and loss decreased with Sm substitution up to x=0.05. Saturation magnetization also increased up to that substitution limit. Observed variations in electromagnetic properties have been explained.     

Mn–Zn ferrite nanoparticles for ferrofluid preparation: Study on thermal–magnetic properties

Mn_1−xZn_xFe₂O₄ (with x   varying from 0.1 to 0.5) ferrite nanoparticles used for ferrofluid preparation have been prepared by chemical co-precipitation method and characterized. Characterization techniques like elemental analysis by atomic absorption spectroscopy and spectrophotometry, thermal analysis using simultaneous TG-DTA, XRD, TEM, VSM and Mossbauer spectroscopy have been utilized. The final cation contents estimated agree with the initial degree of substitution. The Curie temperature (_Tc$T_{\mathrm{c}}$) and particle size decrease with the increase in zinc substitution. In the case of particles with higher zinc concentration, both ferrimagnetic nanoparticles and particles exhibiting superparamagnetic behavior at room temperature are present. In addition, some of the results obtained by slightly altering the preparation condition are also discussed. The precipitated particles were used for ferrofluid preparation. The fine particles were suitably dispersed in heptane using oleic acid as the surfactant. The volatile nature of the carrier chosen helps in altering the number concentration of the magnetic particles in a ferrofluid. Magnetic properties of the fine particles and ferrofluids are discussed. Ferrofluids having Mn_0.5Zn_0.5Fe₂O₄ particles can be used for the energy conversion application utilizing the magnetically induced convection for thermal dissipation.     

Structural,electrical transport,and magnetic properties of Ni1−xZnxFe2O4

Structural, electrical, and magnetic properties of Ni_1−xZn_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures have been investigated thoroughly. The bulk density of the Ni_0.8Zn_0.2Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1200 to 1300 °C and above 1300 °C the bulk density decreases slightly. The Ni_0.6Zn_0.4Fe₂O₄ samples show similar behavior of changes to that of Ni_0.8Zn_0.2Fe₂O₄ samples, except that the bulk density is found to be the highest at 1350 °C. The DC electrical resistivity, ρ(T)$\rho (T)$, decreases as the temperature increases indicating that the samples have semiconductor-like behavior. As the Zn content increases, the Curie temperature (T_c), resistivity, and the activation energy decrease while the magnetization, initial permeability, and the relative quality factor (Q) increases. A Hopkinson peak is obtained near T_c in the real part of the initial permeability vs. temperature curves. The ferrite with higher permeability has a relatively lower resonance frequency. The initial permeability and magnetization of the samples has been found to correlate with density, average grain sizes. Possible explanation for the observed structural, magnetic, and changes of resistivity behavior with various Zn content are discussed.     

A facile method to synthesize magnetic polymer nanospheres with multifunctional groups

Magnetic poly(styrene methyl methacrylate)/Fe₃O₄ nanospheres with ester groups were prepared by a modified one-step mini-emulsion polymerization in the presence of Fe₃O₄ ferrofluids. The effects of monomer dose, surfactant content, ferrofluid concentration and initiator content on the particle characteristics such as the size, morphology and magnetic properties were investigated by Fourier-transform infrared spectroscopy, transmission electron microscopy, thermogravimetric analysis and vibrating sample magnetometer. The results indicated that magnetic nanospheres were superparamagnetic with high saturation magnetization of 51.0 emu/g and corresponding magnetite content of 61.5 wt%. Subsequently, magnetic nanospheres with carboxyl and amino groups were also obtained by hydrolysis and ammonolysis reaction. These magnetic nanospheres with multifunctional groups have biomedical applications.