Role of Cr ions on the microstructure development, and magnetic phase evolution of Ni0.7Zn0.3Fe2O4 ferrite nanoparticles

A series of ferrite samples with the chemical formula Ni_0.7Zn_0.3Cr_xFe_2−xO₄ (x = 0.0-0.5) were prepared by a sol-gel auto-combustion method and annealed at 600 °C for 4 h. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and permeability studies. The prepared samples have a cubic spinel structure with no impurity phase. As the Cr³⁺ content x increases, bulk density and crystallite size decrease, whereas porosity increases. The saturation magnetization decreases linearly from 58.31 to 42.90 emu/g with increasing Cr³⁺ content. However, coercivity increases with increasing Cr³⁺ substitution. The magnetic moments calculated from Neel's molecular-field model are in agreement in the experiment results. The initial permeability (μ_i) decreases with increasing Cr³⁺ substitution. The decrease in initial permeability (μ_i) is attributed to decrease in magnetization on addition of Cr³⁺. The real part of the permeability decreases gradually with increasing frequency in accordance with Snoek's law. The Curie temperature decreases linearly with increasing Cr³⁺ content.     

Perpendicular magnetic anisotropy in 70 nm CoFe2O4 thin films fabricated on SiO2/Si(1 0 0) by the sol-gel method

Cobalt ferrite CoFe₂O₄ films were fabricated on SiO₂/Si(1 0 0) by the sol-gel method. Films crystallized at/above 600 °C are stoichiometric as expected. With increase of the annealing temperature from 600 °C to 750 °C, the columnar grain size of CoFe₂O₄ film increases from 13 nm to 50 nm, resulting in surface roughness increasing from 0.46 nm to 2.55 nm. Magnetic hysteresis loops in both in-plane and out-of-plane directions, at different annealing temperatures, indicate that the films annealed at 750 °C exhibit obvious perpendicular magnetic anisotropy. Simultaneously, with the annealing temperature increasing from 600 °C to 750 °C, the out of plane coercivity increases from 1 kOe to 2.4 kOe and the corresponding saturation magnetization increases from 200 emu/cm³ to 283 emu/cm³. In addition, all crystallized films exhibit cluster-like structured magnetic domains.     

Synthesis and characterization of dl-thioctic acid (DLTA)-Fe3O4 nanocomposite

dl-Thioctic acid (DLTA) coated magnetite (Fe₃O₄) NP's have been prepared by the co-precipitation of iron oxide in the presence of DLTA. The product identified as magnetite, which has an average crystallite size of 7 ± 2 nm as estimated from X-ray line profile fitting. Particle size was estimated as 11 ± 1 nm from TEM micrographs. FT-IR analysis showed that the binding of DLTA on the surface of iron oxide is through carboxyl group is bidentate. VSM analysis explained the super-paramagnetic nature of the nanocomposite. TG analysis showed that the 80% of the nanocomposite was DLTA and 10% was Fe₃O₄, respectively. The conductivity measurements displayed the magnetic transition at ∼60 °C for DLTA-Fe₃O₄ NPs. Analysis of the conductivities reveals the fact that the a.c. conductivity shows a frequency-dependent behavior while d.c. electrical conductivity is strongly temperature dependent and is classified into two regions over a limited temperature range of up to 120 °C. Toxicity was tested measured by LDH assay.     

One-step synthesis of MFe2O4 (M = Fe, Co) hollow spheres by template-free solvothermal method

Monodispersed magnetic MFe₂O₄ (M = Fe, Co) hollow spheres were synthesized by simple template free solvothermal method in ethylene glycol (EG) solution. The hollow spheres were in the same size with an average diameter of about 360 nm and the shells of these spheres were about 80 nm, consisted of closely packed nanocrystallines due to Ostwald ripening. EG plays the key role in the synthesis of hollow spheres in contrast with octahedral crystals synthesized in aqueous solution. The products synthesized in aqueous solution were calcined at 800 °C and 1000 °C. The amount of spinel ferrite products increased monotonically with the increase of temperature and appeared as a single phase at 1000 °C. The saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) for Fe₃O₄ hollow spheres was 74.47 emu/g, 2.59 emu/g and 32.503 Oe respectively whereas the reading of the same indicators for CoFe₂O₄ hollow spheres was 69.07 emu/g, 14.46 emu/g and 242.79 Oe, respectively. The magnetic variation between Fe₃O₄ and CoFe₂O₄ hollow spheres was caused by the radius difference of Fe²⁺ (3d⁶) and Co²⁺ (3d⁷) ions and it was also relevant with nanocrystal sizes of the spin disorder of crystal surface.     

Hydrothermal growth and characterization of magnetite (Fe3O4) thin films

Well-crystallized magnetite (Fe₃O₄) thin films were successfully prepared by a simple hydrothermal process using hydrazine hydrate as the mineralizer. X-ray diffraction and scanning electron microscopy (SEM) and transmission electron microscopy were employed to characterize the products. SEM images show that the uniform Fe₃O₄ film (∼ 3 μm in thickness) is firmly grown on a nickel substrate. The magnetic property of the Fe₃O₄ particles scraped from the film was measured by Physical Property Measurement System (PPMS) at room temperature, and the magnetization curve reveals a soft ferromagnetic behavior with high saturation magnetization of 85 emu/g. Furthermore, the chemical and growth mechanisms for the hydrothermal formation of the Fe₃O₄ film are discussed.     

Direct synthesis and characterization of mesoporous Fe3O4 through pyrolysis of ferric nitrate-ethylene glycol gel

Mesoporous magnetite (Fe₃O₄) was successfully synthesized on a large scale by direct pyrolysis of ferric nitrate-EG (EG = ethylene glycol) gel in a one-end closed horizontal tube furnace in the air without using any template, additions, and carrier gas. The as-synthesized mesoporous Fe₃O₄ were characterized by powder X-ray diffraction (XRD), infrared spectra (IR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and thermal gravimetric analysis (TGA). Results from TEM showed that the as-obtained Fe₃O₄ has mesoporous structure formed by the loose agglomeration of nanoparticles with diameter of about 6 nm, which was also confirmed by small-angle XRD and nitrogen adsorption analysis. Furthermore, vibrating sample magnetometer (VSM) measurements indicated that the saturated magnetization of the as-obtained mesoporous Fe₃O₄ was ferromagnetic with the saturation magnetization (Ms) and coercivity (Hc) of 46 emu/g and 136 Oe, respectively. In addition, a possible growth mechanism of mesoporous Fe₃O₄ was also discussed.     

Ultrasonic-assisted in situ synthesis and characterization of superparamagnetic Fe3O4 nanoparticles

Superparamagnetic Fe₃O₄ nanoparticles were synthesized via a modified coprecipitation method, and were characterized with X-ray diffraction (XRD), vibrating sample magnetometer (VSM), Zeta potential and FT-IR, respectively. The influences of different kinds of surfactants (sodium dodecyl benzene sulfonate, polyethyleneglycol, oleic acid and dextran), temperatures and pH values on the grain size and properties were also investigated. In this method, Fe³⁺ was used as the only Fe source and partially reduced to Fe²⁺ by the reducing agent with precise content. The following reaction between Fe³⁺, Fe²⁺ and hydroxide radical brought pure Fe₃O₄ nanoparticles. The tiny fresh nanoparticles were coated in situ with surfactant under the action of sonication. Comparing with uncoated sample, the mean grain size and saturation magnetization of coated Fe₃O₄ nanoparticles decrease from 18.4 nm to 5.9-9.0 nm, and from 63.89 emu g⁻¹ to 52-58 emu g⁻¹ respectively. When oleic was used as the surfactant, the mean grain size of Fe₃O₄ nanoparticles firstly decreases with the increase of reaction temperature, but when the temperature is exceed to 80 °C, the continuous increase of temperature resulted in larger nanoparticles. the grain size decreases gradually with the increasing of pH values, and it remains unchanged when the PH value is up to 11. The saturation magnetization of as-prepared Fe₃O₄ nanoparticles always decreases with the fall of grain size.     

Synthesis and characterization of polypropiolate sodium (PPNa)-Fe3O4 nanocomposite

Polypropiolate sodium (PPNa)-Fe₃O₄ nanocomposites were successfully synthesized by the precipitation of Fe₃O₄ in the presence of sodium polypropiolate and followed by reflux route. Structural, morphological, electrical and magnetic properties evaluation of the nanocomposite were performed by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), vibrating scanning magnetometry (VSM) and conductivity measurements. Crystalline phase was identified as magnetite with an average crystallite size of 7 ± 3 nm as estimated from X-ray line profile fitting. Particle size estimated from TEM, by log-normal fitting, is ∼9 ± 1 nm. FT-IR analysis shows that the binding of PPNa on the surface of iron oxide is through bidentate linkage of carboxyl group. TGA analysis showed the presence of 20% PPNa around 80% magnetic core (Fe₃O₄)…PPNa-Fe₃O₄ nanocomposite show superparamagnetic characteristics at room temperature. It is found that the a.c. conductivity of the nanocomposites obeys the well-known power law of frequency in which it also depends on temperature. Additionally, its d.c. conductivity showed that two operating regions of the activation energy. Both real and imaginary parts of either permittivity exhibit almost the same attitudes which are the indication of the same ability in the stored energy, and dissipation of energy within the PPNa and PPNa-Fe₃O₄ nanocomposites.     

Magnetic properties of a new iron lead vanadate Pb2FeV3O11

Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb₂FeV₃O₁₁ iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie-Weiss temperature, T_CW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at T_N = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb₂FeV₃O₁₁ at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at g_eff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4-300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb₂FeV₃O₁₁.     

Low temperature synthesis of Fe3O4 micro-spheres and its application in lithium ion battery

Fe₃O₄ micro-spheres with nanoparticles close-packed architectures were synthesized via a simple chemical method using (NH₄)₂Fe(SO₄)₂·6H₂O, hexamethylenetetramine, and NaF as reaction materials. This chemical synthesis took place in a vitreous jar under low temperature (90 °C) and atmospheric pressure. The morphology and structure of the as-synthesized products were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and Raman spectrum. Electrochemical properties of the as-synthesized Fe₃O₄ micro-spheres as anode electrode of lithium ion batteries were studied by conventional charge/discharge tests, which exhibit steady charge/discharge platforms at different current densities. The as-prepared Fe₃O₄ electrode shows high initial discharge capacity of 1166 and 1082 mAh g⁻¹ at current density of 0.05 and 0.1 mA cm⁻², respectively.     

Negative magnetization, magnetic anisotropy and magnetic ordering studies on Al-substituted copper ferrite

Detailed magnetic properties of Al³⁺-modified CuFe₂O₄ spinel ferrite system: CuAl_xFe_2−xO₄; x = 0.0, 0.2, 0.4 and 0.6, have been studied by means of X-ray powder diffraction, field cooled (FC) and zero field cooled magnetization (ZFC) (H = 10 mTesla, T = 4-325 K), magnetic hysteresis (H_max = 2 Tesla, T = 10 and 300 K) and low field (40 A/m) ac susceptibility (T = 300-750 K) measurements. The system exhibits canted spin structure. It has been shown that the observed features of the FC-ZFC magnetization and ac susceptibility curves arise due to the low magneto crystalline anisotropy, not due to the cluster spin-glass-like magnetic ordering. The interesting features like low temperature cusp in the ZFC magnetization for all the compositions and negative magnetization for x = 0.6 composition have been observed. An attempt has been made to explain the negative magnetization within the framework of available models.     

Synthesis, structural and magnetic characterization of nanocrystalline nickel ferrite—NiFe2O4 obtained by reactive milling

Nanocrystalline nickel ferrite (NiFe₂O₄) has been synthesized from a stoichiometric mixture of oxides NiO and α-Fe₂O₃ in a high energy planetary mill. An annealing at 350 °C, after milling, was used to improve the solid state reaction. The obtained powders were investigated by X-ray diffraction, magnetic measurements, scanning electron microscopy, X-ray microanalysis and differential scanning calorimetry. The particles size distribution was analyzed using a laser particle size analyser. The nickel ferrite begins to form after 4 h of milling and continuously form up to 16 h of milling. The obtained nickel ferrite has many inhomogeneities and a distorted spinel structure. The mean crystallites size at the final time of milling is 9 ± 2 nm and the lattice parameter increases with increase the milling time. DSC measurements revealed a large exothermic peak associated with cations reordering in the crystalline structure. The magnetization of the obtained powder depends on the milling time and annealing. After the complete reaction between the starting oxides the milling reduces the magnetization of the samples. The magnetization increases after annealing, due to the reorganization of the cations into the spinel structure.     

Preparation of Ni0.5Zn0.5Fe2O4/SiO2 nanocomposites and their adsorption of bovine serum albumin

The magnetic nanocomposites of (1 − x)Ni_0.5Zn_0.5Fe₂O₄/xSiO₂ (x = 0-0.2) were synthesized by the citrate-gel process and their absorption behavior of bovine serum albumin (BSA) was investigated by UV spectroscopy at room temperature. The gel precursor and resultant nanocomposites were characterized by FTIR, XRD, TEM and BET techniques. The results show that the single ferrite phase of Ni_0.5Zn_0.5Fe₂O₄ is formed at 400 °C, with high saturation magnetization and small coercivity. A porous, amorphous silica layer is located at the ferrite nanograin boundaries, with the silica content increasing from 0 to 0.20, the average grain size of Ni_0.5Zn_0.5Fe₂O₄ calcined at 400 °C reduced from about 18-8 nm. Consequently, the specific surface area of the nanocomposites ascends clearly with the increase of silica content, which is largely contributed by the increase in the thickness of the porous silica layer. The Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites demonstrate a better adsorption capability than the bare Ni_0.5Zn_0.5Fe₂O₄ nanoparticles for BSA. With the increase of the silica content from 0 to 0.05 and the specific surface area from about 49-57 m²/g, the BSA adsorption capability of the Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites calcined at 400 °C improve dramatically from 22 to 49 mg/g. However, with a further increase of the silica content from 0.05 to 0.2, the specific surface area increase from about 57-120 m²/g, the BSA adsorption for the nanocomposites remains around 49 mg/g, owing to the pores in the porous silica layer which are too small to let the BSA protein molecules in.     

One-step solvothermal approach for preparing soft magnetic hydrophilic PFR coated Fe3O4 nanocrystals

The hydrophilic phenol formaldehyde resin coated Fe₃O₄ nanocrystals are prepared via a novel one-step solvothermal approach at 160 °C for 6-9 h without inert gas protection. Water-glycol mixture is used as solvent in common air surrounding. FeSO₄·7H₂O, hexamethylenetetramine and phenol are used as resource materials without any others additives or surfactants. The transmission electronic microscope images show the samples are composed of sphere-like particles with sizes about 10-20 nm. The X-ray diffraction data indicate cube-phase Fe₃O₄ nanocrystals are obtained at given conditions. Fourier transform infrared spectra further reveal the samples are consisted of Fe₃O₄ and PFR. Without modified pH and added surfactants, the solubility of the obtained sample is over 1% in water, which is far more than its solubility in toluene. Room-temperature hysteresis loop indicate that the as-obtained nano-crystals possess soft magnetic properties with high saturated mass magnetization (50.6 emu/g) and negligible coercivity.     

Electrochemical syntheses of soft and hard magnetic Fe50Pd50-based nanotubes and their magnetic characterization

Near-equiatomic Fe-Pd-based nanotubes with diameters of 200 nm and lengths of 1 μm were directly electrodeposited from a single electrolyte into polycarbonate templates. The as-deposited Fe₅₀Pd₅₀ nanotubes were then characterized compositionally, structurally and magnetically. The as-deposited Fe₅₀Pd₅₀ tubes had an fcc crystal structure and were magnetically soft (H_C ≈ 10 kA/m), with the easy axis of the magnetization being parallel to the axes of the tubes. Angular-dependence measurements of the coercivity, where the hysteresis loops were measured as a function of the angle (θ) of the applied demagnetizing field, revealed a combination of magnetization reversal mechanisms, consisting of the curling mechanism, which dominates at low angles, with a transition to coherent rotation at angles ≥70°. The development of the coercivity with annealing temperature due to the L1₀ ordering was also investigated. For this purpose the as-deposited nanotubes were annealed at temperatures from 400 °C to 650 °C for 1 h in Ar + 7% H₂ and the phase formation, the microstructure and the magnetic properties were analyzed. A maximum in the coercivity of 135 kA/m was achieved upon annealing at 550 °C.     

Microstructure and magnetic properties of HVOF thermally sprayed Fe75Si15B10 coatings

Partially amorphous Fe₇₅Si₁₅B₁₀ coatings were prepared from nanostructured feedstock powders by using high velocity oxy-fuel spraying. Scanning electron microscopy, X-ray diffraction, Vickers indenter and magnetic measurements were used to investigate microstructural, structural, microhardness and magnetic properties of the coatings. The Rietveld refinement of the X-ray diffraction patterns reveals the presence of an amorphous phase, nanocrystalline α-Fe(Si,B) structure having a lattice parameter close to 0.2841 nm and an average crystallite size of about 78-83 nm in addition to small amounts of Fe₃O₄ oxide (104 nm) and Fe₂B boride (151 nm), which disappear completely with increasing coating thickness. Coercivity and microhardness values are 15.5 Oe and 478 Hv, respectively, for 84 μm thickness.     

Synthesis and characterization of MWCNTs/Co1−xZnxFe2O4 magnetic nanocomposites and their use in hydrogels

A novel magnetic nanocomposite of multiwalled carbon nanotubes (MWCNTs) decorated with Co_1−xZn_xFe₂O₄ nanocrystals was synthesized successfully by an effective solvothermal method. The as-prepared MWCNTs/Co_1−xZn_xFe₂O₄ magnetic nanocomposite was used for the functionalization of P/H hydrogels as a prototype of device to show the potential application of the nanocomposites. The nanocomposites were characterized by X-ray diffraction analysis, transmission electron microscopy and vibrating sample magnetometer. The results show that the saturation magnetization of the MWCNTs/Co_1−xZn_xFe₂O₄ magnetic nanocomposites increases with x when the Zn²⁺ content is less than 0.5, but decreases rapidly when the Zn²⁺ content is more than 0.5. The saturation magnetization as a function of Zn²⁺ substitution reaches a maximum value of 57.5 emu g⁻¹ for x = 0.5. The probable synthesis mechanism of these nanocomposites was described based on the experimental results.     

Strong correlation between structural, magnetic and transport properties of non-stoichiometric Sr2FexMo2−xO6 (0.8 ≤ x ≤ 1.5) double perovskites

Sr₂Fe_xMo_2−xO₆ (x = 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5 wt.%) (SFMO) double perovskite oxides of different compositions have been prepared by sol-gel method. These materials were subjected to X-ray diffraction and found that crystal structure changes from tetragonal to cubic around x = 1.2 wt.%. Lattice parameters and unit cell volume have been calculated using X-ray diffraction data. Magnetization studies have been carried out using Vibrating Sample Magnetometer ranging from −15 kOe to +15 kOe and saturation magnetization (M_s) has been determined. Electrical resistivity and magnetoresistance studies have been carried out in the magnetic field range of −40 kOe to +40 kOe keeping the temperature constant at 5, 150 and 300 K using standard four-probe method. Resistivity studies have also been carried out in the temperature ranging from 5 to 300 K keeping the magnetic field constant at 0, 10, 20 and 40 kOe. Maximum degree of Fe/Mo ordering (η_max) of SFMO has been calculated and compared with magnetic and transport properties. It has been found that there is a strong correlation between 3 parameters η_max, M_s and MR (%), i.e. all of them show a maximum at x = 1.0 wt.% and decreases as x deviates from 1.0 in SFMO. It has been also found that there is a different resistivity behavior between x ≤ 1.2 wt.% and x > 1.2 wt.% samples of SFMO. Semiconductor metal transition temperature was found to be maximum at x = 1.0 wt.%.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.