Apparently enhanced magnetization of Cu(I)-modified <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> based nanoparticles

Using a chemically induced transition method in FeCl₂ solution, γ-Fe₂O₃ based magnetic nanoparticles, in which γ-Fe₂O₃ crystallites were coated with FeCl₃?6H₂O, were prepared. During the synthesis of the γ-Fe₂O₃ nanoparticles Cu(I) modification of the particles was attempted. According to the results from both magnetization measurements and structural characterization, it was judged that a magnetic silent “dead layer”, which can be attributed to spin disorder in the surface of the γ-Fe₂O₃ crystallites due to breaking of the crystal symmetry, existed in the unmodified particles. For the Cu(I)-modified sample, the CuCl thin layer on the γ-Fe₂O₃ crystallites incurred the crystal symmetry to reduce the spin disorder, which “awakened” the “dead layer” on the surface of the γ-Fe₂O₃ crystallites, enhancing the apparent magnetization of the Cu(I)-modified nanoparticles. It was determined that the surface spin disorder of the magnetic crystallite could be related to the coating layer on the crystallite, and can be modified by altering the coating layer to enhance the effective magnetization of the magnetic nanoparticles.     

Mössbauer Spectroscopy Study of the Superparamagnetism of Ultrasmall ϵ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanoparticles

The superparamagnetism of an ensemble of ?-Fe₂O₃ nanoparticles with a mean size of 3.9 nm dispersed in a xerogel SiO₂ matrix is studied by the Mössbauer spectroscopy method. It is shown that most nanoparticles at room temperature are in the superparamagnetic (unblocked) state. As the temperature decreases, the progressive blocking of the magnetic moments of the particles occurs, which is manifested in the Mössbauer spectra as the transformation of the quadrupole doublet into a Zeeman sextet. The analysis of the relative intensity of the superparamagnetic (quadrupole doublet) and magnetically split (sextets) spectral components in the range of 4–300 K provides the particle size distribution, which is in agreement with the transmission electron microscopy data. The values of the effective magnetic anisotropy constants (K_eff) are determined, and the contribution of surface anisotropy (K_S) is estimated for particles of various sizes. It is shown that the quantity K_eff is inversely proportional to the particle size, which indicates the significant contribution of the surface to the magnetic state of the ?-Fe₂O₃ nanoparticles with the size of several nanometers.     

Surface functionalization of silica-coated magnetic nanoparticles for covalent attachment of cholesterol oxidase

A systematic approach towards the fabrication of highly functionalized silica shell magnetic nanoparticles, presently used for enzyme immobilization, is herein fully presented. The synthesis of bare maghemite (γ-Fe₂O₃) nanoparticles was accomplished by thermal co-precipitation of iron ions in ammonia alkaline solution at harsh reaction conditions, respectively. Primary surface engineering of maghemite nanoparticles was successfully performed by the proper deposition of silica onto nanoparticles surface under strictly regulated reaction conditions. Next, the secondary surface functionalization of the particles was achieved by coating the particles with organosilane followed by glutaraldehyde activation in order to enhance protein immobilization. Covalent immobilization of cholesterol oxidase was attempted afterwards. The structural and magnetic properties of magnetic silica nanocomposites were characterized by TEM and vibrating sample magnetometer (VSM) instruments. X-ray diffraction measurements confirmed the spinel structure and average size of uncoated maghemite nanoparticles to be around 20 nm in diameter. SEM-EDS spectra indicated a strong signal for Si, implying the coating procedure of silica onto the particles surface to be successfully accomplished. Fourier transform infrared (FT-IR) spectra analysis confirmed the binding of amino silane molecules onto the surface of the maghemite nanoparticles mediated Si-O-Si chemical bonds. Compared to the free enzyme, the covalently bound cholesterol oxidase retained 50% of its activity. Binding of enzyme onto chemically modified magnetic nanoparticles via glutaraldehyde activation is a promising method for developing biosensing components in biomedicine.     

Mössbauer spectra of iron (III) sulfide particles

Trivalent iron sulfide (Fe₂ S ₃) particles were synthesized using a modified polyol method. These particles exhibited a needle-like shape (diameter =?10-50 nm, length =?350-1000 nm) and generated a clear XRD pattern. Mössbauer spectra of the product showed a paramagnetic doublet at room temperature and distributed hyperfine magnetic splitting at low temperature. The Curie temperature of this material was determined to be approximately 60 K. The data suggest that the Fe₂ S ₃ had a structure similar to that of maghemite (γ-Fe₂ O ₃) with a lattice constant of a =?10.6 Å. The XRD pattern calculated from this structure was in agreement with the experimental pattern and the calculated hyperfine magnetic field was also equivalent to that observed in the experimental Mössbauer spectrum.     

Magnetically coupled clusters in aggregated maghemite ferrofluid: Mössbauer and magnetization study

Mössbauer spectroscopy in a weak static magnetic field and measurements of isothermal magnetization loops were used to study the effect of polymer coating of the γ-Fe ₂ O ₃ nanoparticles on the magnetic properties of concentrated ensembles of such nanoparticles. It was found that the individual coating of the nanoparticles by a ～ 1 nm layer of the polymer leads to the observable changes in the shapes of the Mössbauer spectra and the magnetization curves of the ensembles. Modeling of the experimental magnetization curves in the classical Langevin model and analysis of the Mössbauer spectra in the generalized multi-level relaxation model revealed that the establishment of interparticle magnetic dipole interactions leads to both a ～ 30 % increase in the magnetic anisotropy constant and a ～ 35 % increase in the width of the hysteresis loop.     

Experimental and <Emphasis Type="Italic">ab initio</Emphasis> study of the hyperfine parameters of ZnFe <Subscript>2</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript> with defects

We present a combined Mössbauer and ab initio study on the influence of oxygen-vacancies on the hyperfine and magnetic properties of the ZnFe ₂ O ₄ spinel ferrite. Samples with different degree of oxygen-vacancies were obtained from zinc ferrite powder that was thermally treated at different temperatures up to 650 ^°C under vacuum.Theoretical calculations of the hyperfine parameters, magnetic moments and magnetic alignment have been carried out considering different defects such as oxygen vacancies and cation inversion. We show how theoretical and experimental approaches are complementary to characterize the local structure around Fe atoms and interpret the observed changes in the hyperfine parameters as the level of defects increases.     

Magnetism of gadolinium nanoparticles near <Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>

Influence of temperature and magnetic field H on magnetism of spherical Gd nanoparticles of different sizes (89, 63, 47, 28, and 18 nm) was studied in the temperature range 250 K < T < 325 K. The particles were obtained by metal vapor condensation in the flow of helium. The particles with d = 18 nm did not show a magnetic transition; their structure is a combination of two cubic phases (FCC1 and FCC2). Large particles remained in the HCP phase and had an admixture of the FCC1 phase, the amount of which decreased as the particle sizes increased; magnetic transition took place at T _c = 293 K. The admixture of O₂ did not alter the structure but decreased the magnetization σ and magnetic permeability μ. An orientation transition in polycrystalline gadolinium initiated by the magnetic field H was proved in an experiment. The orientation transition in Gd particles smaller than 63 nm, the magnetic structure of which is close to the single-domain structure, occurred near T _c without the influence of H.     

Magnetic iron oxide nanopowders produced by CO2 laser evaporation—‘In situ’ coating and particle embedding in a ceramic matrix

The CO₂ laser evaporation technique is not only well suited for the production of magnetic iron oxide nanopowders, but also allows for their conditioning. Two optional methods, the ‘in situ’ coating and the co-laser evaporation, are introduced. Laser-generated magnetic Fe_xO_y nanoparticles very frequently form chain-like structures, which were stabilized by ‘in situ’ coating with stearic acid. A first attempt was made to align these chains in a magnet field before the coating process. Homogeneous hematite/silica mixtures were co-laser evaporated in order to embed Fe_xO_y nanoparticles in a silica matrix. The produced nanopowders were analyzed with TEM, X-ray diffraction (XRD), and magnetic measurements.     

Formation of Silica-Embedded Iron-Oxide Nanoparticles in Low-Pressure Flames

The formation of γ-Fe₂O₃ nanoparticles embedded in a silica matrix has been studied in low-pressure premixed flames of hydrogen and oxygen. The organometallic precursors iron-pentacarbonyl (Fe(CO)₅) and tetramethylsilane (Si(CH₃)₄) were used as starting materials for the core particles and matrix material, respectively. Fe₂O₃ particles with a diameter of about 3–7nm were successfully embedded in a surrounding silica matrix of about 9–13nm in diameter. The iron oxide particle kernels are superparamagnetic at room temperature.     

Structure and magnetic properties of iron–platinum particles with <Emphasis Type="Italic">γ</Emphasis>-ferric-oxide shell

Nanoparticles of solid solution Fe _x Pt_1?x , where 0.25≥x≥0 (?fcc lattice) with γ-Fe₂O₃ shell (lattice of the spinel type) were synthesised and characterised by high-resolution transmission electron microscopy, energy dispersive X-ray analysis, electron energy loss spectroscopy, Mössbauer spectroscopy and magnetometry. From the point of view of magnetic properties, such two-phase particles are interesting because their core is antiferromagnetic or paramagnetic (at very small values of x) whereas the shell is ferrimagnetic. The size of the particles was in the range of several nanometers. The Mössbauer measurements revealed a blocking temperature of about 100 K above which the particles are superparamagnetic. Towards lower temperatures, the magnetic characteristics of an ensemble of such particles show an increase of magnetic rigidity.     

Distribution of cations in magnetite prepared by mechanochemical synthesis

The distribution of iron cations in the crystal lattice of the Fe_3?vO₄ (v=0.153) cation-deficient spinel produced by mechanical dispersion of α-Fe₂O₃ hematite in water is investigated using x-ray diffraction and Mössbauer spectroscopy. Analysis of the Mössbauer data shows that the Fe_2.847O₄ magnetite prepared by mechanochemical synthesis is a chemically heterogeneous compound. The crystal structure of Fe_2.847O₄ is characterized by local environments of the (Fe^2.5+)₀ cations at v₀≤0.1, v₁?0.12, v₂?0.18, and v₃?0.26, which are responsible for a broad distribution of magnetic hyperfine fields with the P(H) probability maxima near 37.0, 36.0, 34.0, and 30.0 MA m^?1.     

Isotropic positive magnetoresistance in Co-Al<Subscript>2</Subscript>O<Subscript>n</Subscript> nanocomposites

The magnetotransport properties of Co_x(Al₂O_n)_{100 ? x} nanocomposites were studied in a wide concentration range (34 ≤ x ≤ 74 at %). Negative tunnel magnetoresistance reaching 6.5% in a field of 10 kOe was established. In addition to the negative magnetoresistance, the Co_x(Al₂O_n)_{100 ? x} composites were found to exhibit positive magnetoresistance reaching 1.5% in fields of 10 kOe over the concentration range corresponding to the percolation threshold (54 ≤ x ≤ 67 at %). The positive magnetoresistance is assumed to be due to the simultaneous existence in the composite structure of clusters and individual nanoparticles characterized by different values of the magnetic anisotropy and due to the dipole-dipole interaction between the clusters and nearest neighbor particles.     

Simulation of the magnetic properties of manganese oxide Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The magnetic susceptibility, heat capacity, and spin-spin correlation functions of manganese oxide Pb₃Mn₇O₁₅ are calculated by the Monte Carlo method. Two critical temperatures are determined: T ₁ ≈ 20 K, above which a modulated structure along the hexagonal axis is formed, and T ₂ ≈ 70 K, at which the long-range magnetic order disappears. The antiferromagnetic exchange interaction constant in a hexagonal plane is estimated to be J ₁ ～ 7 K, and the antiferromagnetic and ferromagnetic exchange interaction constants between hexagonal planes are calculated to be J ₂ ～ 3 K and K ～ 50 K, respectively.     

Magnetic nanoparticles coated with polysaccharide polymers for potential biomedical applications

This study reports a two-steps route for obtaining magnetic nanoparticles–polysaccharide hybrid materials consisting of Fe₃O₄, NiFe₂O₄ and CuFe₂O₄ nanoparticles synthesis by coprecipitation method in the presence of a soft template followed by coating of ferrite nanoparticles of 8–10-nm size range with polysaccharide type polymers—sodium alginate or chitosan. Magnetic oxide nanoparticles and the corresponding hybrid materials were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, atomic absorption spectroscopy (AAS), FTIR spectroscopy, scanning and transmission electron microscopy (SEM and TEM) and specific surface area measurements. The vibrating sample magnetometry confirms the superparamagnetic properties of the synthesized ferrites and hybrids. Using this route, the percent of magnetic nanoparticles retained in chitosan-based hybrid materials is nearly double in comparison with that of sodium alginate–based materials. The biological activity tests on Escherichia coli ATCC 25922, Pseudomonas aeroginosa ATCC 27853, Staphylococcus aureus ATCC 25923 and Candida scotti microorganisms show the non-toxic properties of prepared hybrid materials.     

On the hyperfine field of γ-Fe2O3 small particles

The smaller magnetic hyperfine field of γ-Fe₂O₃ small particles below the superparamagnetic blocking temperature compared to that found in larger crystals is discussed in terms of surface and of intrinsic size effects by using Mössbauer spectroscopy.     

Superparamagnetic properties of <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> particles: Mössbauer spectroscopy and d.c. magnetic measurements

Particles of Fe oxide were prepared by chemical coprecipitation and their sizes were shown by TEM and confirmed by XRD to be in the range of 5 nm. The Mössbauer spectra at 120K clearly indicated the absence of magnetite and presence of the maghemite (γ-Fe₂O₃) phase.We studied the transition of the system to superparamagnetic behaviour, which strongly depends on the relevant time window amounting to ～ 10^?7 s for Mössbauer spectroscopy of ⁵⁷Fe and units of seconds for d.c. magnetic measurements. From the temperature dependences of magnetic moments of zero-field-cooled (ZFC)) and field-cooled (FC) samples, the distributions of blocking temperatures were determined. The comparison of the transition temperatures derived from these two types of measurements gave an independent estimate for the pre-exponential factor and the energy barrier and thus magnetocrystalline anisotropy in an order-of-magnitude agreement with the published data for bulk γ-Fe₂O₃.     

Interparticle interaction effects on magnetic behaviors of hematite (α-Fe2O3) nanoparticles

The interparticle magnetic interactions of hematite (α-Fe₂O₃) nanoparticles were investigated by temperature and magnetic field dependent magnetization curves. The synthesis were done in two steps; milling metallic iron (Fe) powders in pure water (H₂O), known as mechanical milling technique, and annealing at 600 °C. The crystal and molecular structure of prepared samples were determined by X-ray powder diffraction (XRD) spectra and Fourier transform infrared (FTIR) spectra results. The average particle sizes and the size distributions were figured out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The magnetic behaviors of α-Fe₂O₃ nanoparticles were analyzed with a vibrating sample magnetometer (VSM). As a result of the analysis, it was observed that the prepared α-Fe₂O₃ nanoparticles did not perform a sharp Morin transition (the characteristic transition of α-Fe₂O₃) due to lack of unique particle size distribution. However, the transition can be observed in the wide temperature range as “a continuously transition”. Additionally, the effect of interparticle interaction on magnetic behavior was determined from the magnetization versus applied field (σ(M)) curves for 26±2 nm particles, dispersed in sodium oxalate matrix under ratios of 200:1, 300:1, 500:1 and 1000:1. The interparticle interaction fields, recorded at 5 K to avoid the thermal interactions, were found as ∼1082 Oe for 26±2 nm particles.     

Magnetic nanoparticles coated with polyaniline to stabilize immobilized trypsin

It is reported the synthesis of magnetic nanoparticles via the chemical co-precipitation of Fe ³⁺ ions and their preparation by coating them with polyaniline. The electronic micrograph analysis showed that the mean diameter for the nanoparticles is ～15 nm. FTIR, powder X-ray diffraction and Mössbauer spectroscopy were used to understand the chemical, crystallographic and ⁵⁷Fe hyperfine structures for the two samples. The nanoparticles, which exhibited magnetic behavior with relatively high spontaneous magnetization at room temperature, were identified as being mainly formed by maghemite (γFe₂O₃). The coated magnetic nanoparticles (sample labeled “mPANI”) presented a real ability to bind biological molecules such as trypsin, forming the magnetic enzyme derivative (sample “mPANIG-Trypsin”). The amount of protein and specific activity of the immobilized trypsin were found to be 13±5 μg of protein/mg of mPANI (49.3 % of immobilized protein) and 24.1±0.7 U/mg of immobilized protein, respectively. After 48 days of storage at 4 ^°C, the activity of the immobilized trypsin was found to be 89 % of its initial activity. This simple, fast and low-cost procedure was revealed to be a promising way to prepare mPANI nanoparticles if technological applications addressed to covalently link biomolecules are envisaged. This route yields chemically stable derivatives, which can be easily recovered from the reaction mixture with a magnetic field and recyclable reused.     

Magnetic properties of maghemite nanoparticles

Magnetic particles of maghemite (spinel γ-Fe₂O₃) are synthesized by means of aerosol pyrolysis, making it possible to produce chemically uniform highly-dispersed single-phase materials. The magnetic properties of synthesized particles for temperatures ranging from helium temperature up to room temperature and higher are investigated using a SQUID magnetometer. The experimental curves are compared to the results from calculations performed by the Monte Carlo method. It is found that the Curie temperature is lower for γ-Fe₂O₃ nanoparticles than for bulk samples. Several parameters of the material are estimated by comparing the experimental and calculated results.     

Role of <Emphasis Type="Italic">N</Emphasis>-methyl-2-pyrrolidone for preparation of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@SiO<Subscript>2</Subscript> controlled the shell thickness

We developed a simple and novel approach for the synthesis of Fe₃O₄@SiO₂ nanoparticles with controlled shell thickness, and studied the mechanism. The introduction of N-methyl-2-pyrrolidone (NMP) led to trapping of monomer nuclei in single shell and controlled the shell thickness. Fe₃O₄@SiO₂ controlled the shell thickness, showing a high magnetization value (64.47 emu/g). Our results reveal the role and change in the chemical structure of NMP during the core-shell synthesis process. NMP decomposed to 4-aminobutanoic acid in alkaline condition and decreased the hydrolysis rate of the silica coating process.