Investigation of magnetic properties of interacting Fe2O3 nanoparticles

The magnetic properties of Fe₂O₃ nanoparticles (average diameter ∅≅3 nm) in alumina (68% Fe₂O₃ in weight) have been investigated by magnetization measurements. The results indicate a superparamagnetic behavior of interacting particles, which block with decreasing temperature (the zero-field-cooled susceptibility shows a maximum at T≅145 K) with a distribution of relaxation times. A change of magnetic regime is observed below ∼60 K, due to the increasing interparticle interactions and local surface anisotropy.     

Magnetic behaviour of a magnetite/silicon nanocomposite

Mesoporous silicon is utilized to infiltrate quite monodisperse iron oxide nanoparticles into the pores. This semiconducting matrix exhibits oriented pores, clearly separated from each other, with an average pore diameter of 55 nm. Iron oxide nanoparticles of 8 nm and 5 nm in size which are coated with a surfactant are prepared by high temperature decomposition in the presence of an organic precursor. The achieved nanocomposite consists of dispersed Fe₃O₄-nanoparticles within the pores and offers magnetic properties which are determined by the morphology of the silicon matrix as well as by the distribution of the particles within the individual pores. Thus, the change of regime between a superparamagnetic and a blocked state of the system can be tuned. Furthermore, magnetic anisotropy between the two magnetization directions, normal and parallel to the sample surface, is observed due to the oriented and separated pores of the template which are quasi-regular arranged. This porous silicon/magnetite composite with its adjustable magnetic properties is also of interest for possible applications in biomedicine due to the low toxicity of both materials.     

Preparation of monodisperse magnetite nanoparticles under mild conditions

In this paper, we reported a method to prepare monodisperse magnetite nanoparticles at mild temperature using cheap and non-toxic precursors. It overcomes the shortages of chemical co-precipitation method and thermal decomposition method and combines the advantages of facile, cheap, large-scale, monodisperse, nanosize, and low synthesis temperature and low toxic. In this method, FeCl₃ · 6H₂O, FeCl₂ · 4H₂O and sodium oleate were mixed in toluene/ethanol/water mixture solvent and refluxed at 74 °C to prepare magnetite nanoparticles directly. The nanoparticles were characterized by transmission electron microscopy, dynamic light scattering, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer and thermogravimetric analysis. The magnetic properties of nanoparticles were measured by superconducting quantum interference device. The results showed that the nanoparticles are well-monodisperse with about 4–5 nm of average diameter. The nanoparticles were proved to be superparamagnetic with saturated magnetization 23.6 emu/g and blocking temperature 24.4 K. A possible formation mechanism of monodisperse magnetite nanoparticles was presented at the same time.     

Effects of dipolar interactions on magnetic properties of granular solids

The magnetic behavior of superparamagnetic Co nanoparticles (2–4 nm in diameter) dispersed in an amorphous, insulating SiO₂ matrix was studied. Conventional fittings of magnetization curves present mean magnetic moments which diminish with decrease in temperature. In order to treat this anomalous behavior, we have applied the interacting superparamagnetic model (ISP). Mean diameters obtained from transmission electron microscopy (TEM) were compared with values obtained applying ISP model.     

Surface and frustration evidence in Co–Ni–B nanoparticles by FMR measurements

We present ferromagnetic resonance (FMR) measurements on ∼3 nm amorphous magnetic nanoparticles of Co–Ni–B as a function of temperature (T). The particles were studied in powder form and dispersed in a polymer matrix to study the interparticle interaction effect. In both samples the FMR responses are similar down to T∼10 K, where the powder sample shows an intensity increase not followed by the dispersed sample. We argue that the general characteristics are compatible with previous magnetization measurements and Monte Carlo simulations indicating large surface contributions to the effective anisotropy. In this case the frustration of the single-particle behavior observed in the powder sample at very low T (T ⩽ 10 K) is due to interparticle interactions.     

A Mössbauer study of the chemical stability of iron oxide nanoparticles in PMMA and PVB beads

We have prepared magnetic beads consisting of iron oxide nanoparticles in a polymethyl methacrylate (PMMA) and a polyvinyl butyral (PVB) matrix. High-field Mössbauer studies show that the particles have an almost perfect collinear spin structure and magnetization measurements show that they are superparamagnetic at room temperature at a time scale of seconds. We have followed the oxidation of the particles, which initially have a stoichiometry close to magnetite. The oxidation is fast during the first 2–3 weeks and then continues slowly such that even after 30 weeks the particles have not completely transformed to maghemite. The PVB beads are hydrophilic and biocompatible and are therefore well suited for applications in medicine and biology.     

Magnetic properties of marine tertiary tephra investigated over a wide temperature range

We report on a detailed investigation of the magnetic properties of tertiary tephra as a function of temperature and magnetic field, over a wide temperature. The tertiary tephra were chosen from two marine sites in the South Indian Ocean. The measurements were made using the electron spin resonance (ESR) technique in the temperature range 4–1100 K and a SQUID magnetometer at temperatures from 4 to 300 K.We show that the magnetic properties of the studied ashes are mainly due to two contributions, a ferrimagnetic one and a paramagnetic one. At moderate magnetic fields, B<1 T, in the large temperature interval, 20 K<T<1000 K, the magnetic response is dominated by the ferrimagnetic component present in the ashes. The experimental data concerning the hysteresis cycles and the Curie temperatures allow us to suggest that (titano-)magnetite and a haematite-like phase could be responsible for the observed ferrimagnetic behaviour. In addition, by electron microscopy investigations, two types of morphologies containing Fe ions have been detected: vitreous splinters, in which embedded crystallised nanoparticles (60–200 nm) are present, and smectite flakes which are crystallised and in which the Fe concentration is higher than in the splinters. We speculate that the ferrimagnetic contribution is due to the nanoparticles and the paramagnetic one results from lamellar flake-like particles.     

Preparation of superparamagnetic magnetite nanoparticles by reverse precipitation method: Contribution of sonochemically generated oxidants

Magnetic iron oxide nanoparticles were successfully prepared by a novel reverse precipitation method with the irradiation of ultrasound. TEM, XRD and SQUID analyses showed that the formed particles were magnetite (Fe₃O₄) with about 10 nm in their diameter. The magnetite nanoparticles exhibited superparamagnetism above 200 K, and the saturation magnetization was 32.8 emu/g at 300 K. The sizes and size distributions could be controlled by the feeding conditions of FeSO₄ · 7H₂O aqueous solution, and slower feeding rate and lower concentration lead to smaller and more uniform magnetite nanoparticles. The mechanisms of sonochemical oxidation were also discussed. The analyses of sonochemically produced oxidants in the presence of various gases suggested that besides sonochemically formed hydrogen peroxide, nitrite and nitrate ions contributed to Fe(II) ion oxidation.     

Study of structural and magnetic properties of superparamagnetic Fe3O4/SiO2 core–shell nanocomposites synthesized with hydrophilic citrate-modified Fe3O4 seeds via a sol–gel approach

This paper describes a simple way for the coating of magnetite nanoparticles (MNPs) with amorphous silica. First, MNPs were synthesized by controlled co-precipitation technique under N₂ gas and then their surface was modified with trisodium citrate in order to achieve particles with improved dispersibility. Afterward, magnetite-silica core/shell nanocomposites were prepared by a sol–gel approach, using magnetic fluid including electrostatically stabilized MNPs as seeds. The prepared samples were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, zeta potential analysis and vibrating sample magnetometer (VSM) in order to study their structural and magnetic properties. FT-IR and XRD results imply that resultant nanocomposites are consisted of two compounds; Fe₃O₄ and SiO₂ and TEM images confirm formation of their core/shell structure. TEM images also show increase in silica shell thickness from ∼5 to ∼24 nm with increase in amount of tetraethyl orthosilicate (TEOS) used during the coating process from 0.1 to 0.3 mL. Magnetic studies indicate that Fe₃O₄ nanoparticles remain superparamagnetic after coating with silica although their M_s values are significantly less than pristine MNPs. These core/shell nanocomposites offer a high potential for different biomedical applications due to having superparamagnetic property of magnetite and unique properties of silica.     

Effect of Ni doping on the properties of fine magnetite particles

Vibrating sample magnetometer (VSM) and Mössbauer spectroscopy are used to characterize the magnetic behaviour of fine magnetite particles obtained from (i) pure goethite and (ii) Ni-doped goethite, in ammoniacal solution. The latter sample has 0.4 wt% Ni which has significantly changed the properties of the sample. The Ni-doped magnetite shows a much higher overoccupancy of tetrahedral sites by iron atoms as compared to the undoped sample. TEM study shows that presence of Ni ions leads to narrower size distribution of magnetite particles as compared to the magnetite obtained from undoped goethite. The coercive field is also affected by presence of Ni, being only 105 Oe for the Ni-doped magnetite as against 170 Oe in the undoped sample.     

Synthesis of magnetoliposomes with monodisperse iron oxide nanocrystal cores for hyperthermia

Heating rates generated by superparamagnetic particles deteriorate quickly with particle polydispersity. We prepared highly uniform, monodisperse, single-crystal magnetite nanoparticles of tailorable size via organometallic decomposition. As-synthesized nanocrystals were coated with phospholipids to form biocompatible magnetoliposomes. Modeling of AC-magnetic field parameters indicates that 11 nm nanocrystals have maximum heating rates within the biologically safe frequency range.     

Superparamagnetic properties of C60Co3 complexes

Preparation of fullerites containing cobalt and analyses of reactions based on semiempirical quantum calculations are described. The magnetic properties of thermally treated C₆₀Co₃ samples: Curie constant (C≈3500 emu K/mol Oe) temperature and field dependencies of magnetization and nonequilibrium effects of magnetization are interpreted in terms of superparamagnetic blocking model of the compound.     

Magnetostriction and magnetization measurements on a DyNi2B2C single crystal in a magnetic field

In order to investigate the interactions between lattice properties, magnetic ordering and superconductivity of DyNi₂B₂C, thermal expansion, magnetostriction and magnetization measurements were performed for T=2–15 K and for μ₀H=0–3 T on a single crystal in the crystallographic [1 1 0] direction. A magnetic phase diagram is derived that shows two phases (AF1 and AF2) in the narrow region between the zero-field antiferromagnetic AF and the induced ferromagnetic state FM. Moreover, it is characterized by a large-field hysteresis. This behaviour can be described by a two domain magnetic state. The metamagnetic structure AF1 with about a quarter of the saturated magnetization is responsible for suppressing the superconductivity in DyNi₂B₂C because of its ferromagnetic component.     

Micro Raman,Mossbauer and magnetic studies of manganese substituted zinc ferrite nanoparticles: Role of Mn

A series of Mn–Zn Ferrite nanoparticles (<15 nm) with formula Mn_xZn_1−xFe₂O₄ (where x=0.00, 0.35, 0.50, 0.65) were successfully prepared by citrate-gel method at low temperature (400 °C). X-ray diffraction analysis confirmed the formation of single cubic spinel phase in these nanoparticles. The FESEM and TEM micrographs revealed the nanoparticles to be nearly spherical in shape and of fairly uniform size. The fractions of Mn²⁺, Zn²⁺ and Fe³⁺ cations occupying tetrahedral sites along with Fe occupying octahedral sites within the unit cell of different ferrite samples are estimated by room temperature micro-Raman spectroscopy. Low temperature Mossbauer measurement on Mn_0.5Zn_0.5Fe₂O₄ has reconfirmed the mixed spinel phase of these nanoparticles. Room temperature magnetization studies (PPMS) of Mn substituted samples showed superparamagnetic behavior. Manganese substitution for Zn in the ferrite caused the magnetization to increase from 04 to18 emu/g and Lande's g factor (estimated from ferromagnetic resonance measurement) from 2.02 to 2.12 when x was increased up to 0.50. The FMR has shown that higher Mn cationic substitution leads to increase in dipolar interaction and decrease in super exchange interaction. Thermomagnetic (M–T) and magnetization (M–H) measurements have shown that the increase in Mn concentration (up to x=0.50) enhances the spin ordering temperature up to 150 K (blocking temperature). Magnetocrystalline anisotropy in the nanoparticles was established by Mossbauer, ferromagnetic resonance and thermomagnetic measurements. The optimized substitution of manganese for zinc improves the magnetic properties and makes these nanoparticles a potential candidate for their applications in microwave region and biomedical field.     

Numerical micromagnetics of interacting superparamagnetic nanoparticles assembled in clusters with different dimensionalities

We analyze here the equilibrium magnetization state of densely packed interacting superparamagnetic nanoparticles assembled in clusters of various sizes and dimensionalities by comparison with the non-interacting case. We demonstrate that the average magnetization of individual particles is strongly increased in linear chains aligned parallel with the external magnetic field. Two-dimensional (2D) distributions of superparamagnetic nanoparticles present weaker increases of their average magnetization with respect to the non-interacting approximation whereas volume distributions (3D) are almost equivalent with the non-interacting case. A large number of nanoparticles densely packed in 2D superparamagnetic clusters present almost the same magnetic moment as infinite superparamagnetic chains. The effect of mutual interactions on the total magnetic moment of 3D surfaces (spheroids with various aspect ratios) uniformly covered with densely packed monolayers of superparamagnetic nanoparticles is also investigated.     

Synthesis of CoFe2O4 nanoparticles embedded in a silica matrix by the citrate precursor technique

A metals–citrate–silica gel was prepared from metallic salts, citric acid and tetraethylorthosilicate by sol–gel method (citrate precursor technique) and it was further used to prepare magnetic nanocomposites. The gel was dried at 100 °C and then calcined at temperatures between 600 and 1000 °C to obtain powder samples. The nanocomposites were characterized by XRD, IR, VSM and TEM techniques. The diffraction patterns show the formation of a single magnetic phase identified as CoFe₂O₄. Magnetic nanoparticles with average size less than 50 nm were obtained which are well dispersed in the silica matrix. The combination of different metals concentrations and calcining temperatures allowed obtaining samples with magnetization ranging from 3.6 to 25.3 emu/g.     

Synthesis and investigation of magnetic properties of Gd-substituted Mn–Zn ferrite nanoparticles as a potential low-TC agent for magnetic fluid hyperthermia

Gd-substituted Mn–Zn ferrite nanoparticles of different compositions were synthesized by chemical co-precipitation method. To study the reduction of the Curie temperature (T_C) for different samples, their magnetic properties in dependence from the composition and cationic distribution were investigated. An attempt to lower the T_C of superparamagnetic particles to the optimal temperature required in magnetic fluid hyperthermia (44–47 °C) was made.     

Physicotechnological principles of the development of biocompatible magnetic nanoparticles for medicine

Nanodispersive powder of a zinc-substituted magnetite was developed. Functional characteristics (biocompatibility, dispersity, magnetic state) allow us to recommend it for approbation in medical and biological applications. The nature of the investigated field dependencies of magnetization indicates that for particles of 3–13 nm, a superparamagnetic state is realized at room temperature, reflecting the specificity of the small particles’ magnetism.     

Magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in silicon oxide

The magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in the amorphous silicon oxide matrix are investigated as a function of the implantation dose. The analysis of the field dependences of the magnetization and the magneto-optical Faraday and Kerr effects demonstrates that, as the ion implantation dose increases, the superparamagnetic behavior of an ensemble of cobalt nanoparticles at room temperature gives way to a ferromagnetic response with the anisotropy characteristic of a thin magnetic film. The magnetization curves for the superparamagnetic and ferromagnetic ensembles of cobalt nanoparticles are simulated to determine their average sizes and the filling density in the irradiated layer of the silicon dioxide matrix. It is revealed that the spectral dependences of the Faraday and Kerr effects for ion-synthesized cobalt nanoparticles differ substantially from those for continuous cobalt films due to the localized excitations of free electrons in the nanoparticles.     

Magnetic properties of self-assembled nanostructure films on the base of amorphous Ni granules

We report on structural and magnetic properties of granular films consisting of 2.5 nm Ni nanoparticles. The films are fabricated by the original laser electrodispersion technique, which allows producing nearly monodisperse and amorphous particles. Atomic force microscopy (AFM) study shows that in 8 nm thickness films the particles are self-assembled in clusters with the lateral size 100-150 nm and the height of about 8 nm. Performed by SQUID, the films magnetization measurements reveal superparamagnetic behaviour, characteristic for an ensemble of non-interacting single domain magnetic particulates. It is found that the magnetic moment of the particulate is equal to that of about 3000 individual Ni nanoparticles and the blocking temperature is close to room temperature. Defined from magnetic measurements, the size of single domain particulates correlates well with the size of the clusters determined from AFM images. We propose that exchange interaction plays an important role in the formation of the particulates by aligning the magnetic moments of the individual Ni nanoparticles inside the clusters. Presence of magnetic clusters with high blocking temperature makes the fabricated films potentially useful for high-density magnetic data storage applications.