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.     

Influence of doping concentration on room-temperature ferromagnetism for Fe-doped BaTiO3 ceramics

Ba(Ti_1−xFe_x)O₃ ceramics (x=7, 30 and 70 at%) were prepared by solid-state reaction. All samples are single-phase with 6H-BaTiO₃-type hexagonal perovskite structure. Mössbauer spectra show all Fe atoms to be present as Fe³⁺ in BaTiO₃ lattice, occupying M1 octahedral and pentahedral sites. Room-temperature ferromagnetism is exhibited and saturation magnetization gradually decreases with increasing Fe content. The observed ferromagnetism is considered to be an intrinsic property of Ba(Ti_1−xFe_x)O₃, originating from super-exchange interactions between Fe³⁺ in different occupational sites associated with oxygen vacancies. The variation in magnetization with Fe content is related to the ratio of pentahedral to octahedral sites and oxygen vacancies.     

Magnetic and structural properties of Fe ion-implanted GaN

The magnetic and structural properties of Fe ion-implanted GaN was investigated by various measurements. XRD results did not show any peaks associated with second phase formation. The magnetization curve at 5 K showed ferromagnetic behavior for 900 °C-annealed sample. In zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, the irreversibility and a cusp-like behavior of the ZFC curve were observed for 900 °C-annealed sample. These behaviors are typically observed in superparamagnetic or spin glass phase. While the temperature dependence magnetization of 800 °C-annealed sample showed non-Brillouin-like curve and it is not exhibited ferromagnetic hysteresis at 5 K. In XPS measurement, the coexistence of metallic Fe (Fe⁰) and Fe–N bond (Fe²⁺ and Fe³⁺) for Fe 2p core level spectra is observed in as-implanted sample. But 700–900 °C-annealed samples showed only Fe–N bond (Fe²⁺ and Fe³⁺) spectra. For Ga 3d core level spectra only Ga–N bonds showed for as implanted with 700–900 °C-annealed samples. From XPS results, it could be explained that magnetic property of our films originated from FeN structures.     

Magnetic iron particles with high magnetization useful for immunoassay

TiO₂-encapsulated metallic Fe particles (Ti-O/Fe) were synthesized through a solid phase reaction. The Ti-O/Fe particles were non-toxic to tumor cells in a cell viability assay. After silica coating using a sol-gel method, streptavidin was covalently bound onto the Ti-O/Fe particles. Thus produced HMMI particles showed higher magnetization (114 Am²/kg) and a larger specific surface area (15 m²/g) than conventional streptavidin-immobilized magnetite particles. The high magnetization allowed for rapid magnetic separation, while the additional large specific surface area improved the detection of the adiponectin antigen both in terms of extended detection range and higher assay speed.     

Fe nanowires in carbon nanotubes as an example of a one-dimensional system of exchange-coupled ferromagnetic nanoparticles

The cooperative phenomena revealed in the field and temperature dependences of the magnetization in a system of iron nanoparticles in carbon nanotubes were studied experimentally. The character of the temperature dependences of the magnetization indicates that the ferromagnetic Fe particles in carbon nanotubes are exchange-coupled. In the region where the magnetization approaches saturation, the magnetization curves reveal the power dependence ΔM～ H^?3/2 typical for a one-dimensional system of exchange-coupled ferromagnetic nanoparticles.     

Preparation and chemical stability of iron-nitride-coated iron microparticles

Iron-nitride-coated iron microparticles were prepared by nitridation of the surface of iron microparticles with ammonia gas at a temperature of 510 °C. The phases, composition, morphology, magnetic properties, and chemical stability of the particles were studied. The phases were α-Fe, ε-Fe₃N, and γ-Fe₄N. The composition varied from the core to the surface, with 99.8 wt% Fe in the core, and 93.8 wt% Fe and 6 wt% N in the iron-nitride coating. The thickness of the iron-nitride coating was about 0.28 μm. The chemical stability of the microparticles was greatly improved, especially the corrosion resistance in corrosive aqueous media. The saturation magnetization and the coercive force were 17.1×10³ and 68 kA/m, respectively. It can be concluded that iron-nitride-coated iron microparticles will be very useful in many fields, such as water-based magnetorheological fluids and polishing fluids.     

Magnetization and specific heat of DyFe3(BO3)4 single crystal

We present thermodynamic and magnetic studies of single crystalline DyFe₃(BO₃)₄. The data indicate an easy axis antiferromagnetic order below T_N~ 38 K which we attribute to the Fe subsystem. The Dy subsystem remains paramagnetic down to the lowest investigated temperatures of 2 K, but it is polarized by the Fe spins due to a f-d interaction. External magnetic field leads to a spin-flop transition in the iron subsystem as well as to superposed magnetization in the Dy subsystem. The repopulation of two low-lying Kramers doublets in Dy³⁺ ions results in well defined Schottky anomalies in specific heat and magnetization.     

Ce 5d and Fe 3d magnetic profiles in CeH2/Fe multilayers probed by XRMS

The element and electronic shell specificities of X-ray resonant magnetic scattering have been used to investigate the magnetization of Ce 5d and Fe 3d states in [CeH₂(19.6 ?)/Fe(25.4 ?)]*38 a multilayer. We show that the measurement of the magnetic contribution to the intensities reflected at low angles at the Ce L₂ and Fe L_2,3 edges allows us to investigate the profile of the Ce 5d and Fe 3d magnetic polarization. The Fe 3d polarization is found to be uniform across the Fe layer and the Ce 5d polarization appears to be restricted close to the interface with Fe. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Magnetization and electron spin resonance of Fe impurities in (TMTSF)2AsF6: magnetic-field-dependent interaction between impurity spins and spin-density waves

A new spin-density-wave (SDW) system with magnetic impurities (TMTSF)₂(AsF₆)_1−x(FeCl₄)_x was prepared and its magnetic properties were studied by means of magnetization and electron-spin-resonance measurements. The anisotropic g-value and comparison of the Fe concentration with the Curie constant indicate that the Fe³⁺ ions are in a low-spin state. We also found that the magnetization curve of the impurity spins in this compound shows an anomalous behavior. This behavior can be explained if one assumes a field-dependent magnetic interaction between the Fe³⁺ spins and the SDW moment. We suppose that the field dependence of the SDW pinning potential is responsible for this phenomenon.     

Effect of annealing atmosphere on magnetism for Fe-doped BaTiO3 ceramic

Ba(Ti_0.3Fe_0.7)O₃ ceramic was prepared by solid-state reaction and post-annealed in vacuum and oxygen, respectively. The as-prepared and annealed samples are all single-phase, crystallizing in a 6H-BaTiO₃-type hexagonal perovskite structure. Room-temperature ferromagnetism is exhibited in all ceramics. For the as-prepared sample, the super-exchange interactions of Fe³⁺ in different occupational sites (pentahedral and octahedral sites) are expected to produce the ferromagnetism observed. After annealing in vacuum, the magnetization is reduced while the exchange mechanism remains unchanged. On the contrary, O₂ annealing can effectively enhance the magnetization due to the presence of Fe⁴⁺, an unusual valence for iron. The simultaneous presence of Fe³⁺ and Fe⁴⁺ allows new exchange mechanism responsible for the ferromagnetic interaction. The exchange couplings of Fe ions with mixed valences (Fe³⁺ and Fe⁴⁺) determine the magnetic behavior.     

Theoretical and experimental study of Fe/Cr nanometric quasiperiodic multilayers

An experimental and theoretical study of magnetization curves of Fe/Cr nanometric magnetic films grown with the structure of the quasiperiodic Fibonacci sequence is presented. Fe ferromagnetic films with interfilm exchange coupling provided by intervening Cr non-ferromagnetic layers were grown on MgO (100) by dc magnetron sputtering at 300 ^°C. The magnetization curves were investigated using the magneto-optical Kerr effect with the external field applied along the easy axis. The theoretical approach for this system is based on a realistic phenomenological model that includes the following contributions to free magnetic energy: Zeeman, cubic magneto-crystalline anisotropy, as well as bilinear and biquadratic exchange energies. Our numerical results are in very good agreement with the experimental data.     

Synthesis and characterization of a POM-based nanocomposite as a novel magnetic photocatalyst

A novel magnetic photocatalyst, prepared by grafting polyoxometalates (POM) anions PW₁₂O₄₀³⁻ onto Fe₃O₄ nanoparticles via a layer of Ag, was synthesized and characterized. The coated Ag layer was used as an intermediate bond for anchoring POM anions onto the magnetite cores. Resulting materials have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption isotherm, magnetization, and inductively coupled plasma (ICP). The activity of the photocatalyst was tested by the photocatalytic degradation of Rhodamine B. It was found that, compared to pure POM, the decolorization fraction of Rhodamine B in 2 h operation was 2.8-3.4 times higher by using the POM-based nanocomposite. ICP analysis of the concentration of Fe, W and P in treated water showed that photodissolution was minimal. In addition, as the synthesized composite possesses a magnetite core, it is possible to retrieve the photocatalyst by exerting an external magnetic field, which is easier than the recovery of conventional TiO₂ fine particles and homogeneous POM photocatalysts. The exhibited photocatalytic activity and magnetization of the novel photocatalyst provide a promising solution for the degradation of water contaminants and photocatalyst recovery.     

Non-Langevin behaviour of the uncompensated magnetization in nanoparticles of artificial ferritin

The magnetic behaviour of nanoparticles of antiferromagnetic artificial ferritin has been investigated by ⁵⁷Fe M?ssbauer absorption spectroscopy and magnetization measurements, in the temperature range 2.5-250 K and with magnetic fields up to 7 T. Samples containing nanoparticles with an average number of ⁵⁷Fe atoms ranging from 400 to 2 500 were studied. By analysing the magnetic susceptibility and zero field M?ssbauer data, the anisotropy energy per unit volume is found, in agreement with some of the earlier studies, to have a value typical for ferric oxides, i.e. a few 10⁵ ergs/cm³. By comparing the results of the two experimental methods at higher fields, we show that, contrary to what is currently assumed, the uncompensated magnetization of the ferritin cores in the superparamagnetic regime does not follow a Langevin law. For magnetic fields below the spin-flop field, we propose an approximate law for the field and temperature variation of the uncompensated magnetization, which was early evoked by Néel but has so far never been applied to real antiferromagnetic systems. More generally, this approach should apply to randomly oriented antiferromagnetic nanoparticles systems with weak uncompensated moments. Received 20 January 2000     

Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and⁵⁷Fe Mössbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 Å. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 Å, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.     

Magnetization of sol-gel prepared zinc ferrite nanoparticles: Effects of inversion and particle size

Nanoparticles of ZnFe₂O₄ have been prepared by using sol-gel method in two different mediums (acidic and basic) in order to observe the influence of the medium on the magnetic properties of the obtained nanoparticles. X-ray diffraction and Mössbauer studies of these samples show the presence of single-phase spinel structure. The average size of the particles as determined by X-ray diffraction increases with the annealing temperature from 18 to 52 nm. With the increase in particle size, magnetization decreases while the magnetization blocking temperature increases. Magnetization studies show that the samples prepared in basic medium have more ferrimagnetic nature as compared to those prepared in acidic medium. We understand this increase in magnetization as reflective of the increased degree of inversion (transfer of Fe³⁺ ions from octahedral to tetrahedral sites) in the particles of smaller size unit cells. From lattice parameter calculations on different particles it is determined that inversion is more favorable in the particles prepared in a basic medium than in the acidic medium due to the smaller cell size in the former.     

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.     

Structural characterization and ferromagnetic behavior of Fe-doped TiO2 powder by high-energy ball milling

Fe-doped TiO₂ powder was prepared by high-energy ball milling, using TiO₂ Degussa P-25 and α-Fe powders as the starting materials. The structure and magnetic properties of the Fe-doped TiO₂ powder were studied by X-ray diffraction, ⁵⁷Fe Mossbauer spectroscopy and vibrating sample magnetometer. The Reitveld refinement of XRD revealed that ball milling not only triggered incorporation of Fe in TiO₂ lattice but also induced the phase transformation from anatase to rutile in TiO₂ and consequently the milled Fe-doped TiO₂ powder contained only rutile.⁵⁷Fe Mössbauer effect measure showed that Fe atoms existed in Fe²⁺ and Fe³⁺ state, which were assigned to the solid solution Fe_xTi_1−xO₂. The magnetization measurements indicated that the milled Fe-doped TiO₂ powder was ferromagnetic above room temperature. The ferromagnetism in our milled Fe-doped TiO₂ powder seemingly does not come from Fe and iron oxides particles/clusters but from the Fe-doped TiO₂ powder matrices.     

Quantum theory of the magneto-optical effect and magnetization of Nd-substituted yttrium iron garnet

The magneto-optical and magnetic properties of Nd ³⁺ ions in Y ₃Fe ₅O ₁₂ garnet are analyzed by using quantum theory. In the spontaneous state, the magneto-optical effects originate mainly from the intra-ionic electric dipole transitions between the 4 f ³ and 4 f ²5d states split by the spin-orbit, crystal field, and superexchange interactions. For the excited configuration, the coupling scheme of Yanase is extended to the Nd ³⁺ ion. The magneto-optical resonance frequencies are mainly determined by the splitting of the 5d states induced by the crystal field. The theoretical results of both Nd magnetization and Faraday rotation are in good agreement with experiments. The observed Faraday rotation is proved to be of the paramagnetic type. Although the value of the magneto-optical resonance frequency derived from a macroscopic analysis is approximately confirmed by our theoretical study, a new assignment about the transitions associated with this resonance is unambiguously determined. The spin-orbit coupling of the ground configuration has a great influence on both the Faraday rotation and magnetization, but, unlike the theoretical results obtained in some metals and alloys, the relation between the Faraday rotation and the spin-orbit coupling strength is more complex than a linear one. The magnitude of the magneto-optical coefficient increases as the spin-orbit interaction strength of the ground configuration decreases when the strength is not very weak. Finally, the temperature dependence of the magneto-optical coefficient and the effect of the mixing of different ground-term multiplets induced by the crystal field are analyzed. Received 8 November 2000     

Structure and property changes and femtosecond laser-induced magnetization dynamics of two-year-old high coercive TbFeCo films

An uncoated TbFeCo film on glass substrate exposed to dry air for 2 years is studied using magneto-optical polar Kerr spectroscopy and vibrating sample magnetometry (VSM). The out-of-plane hysteresis loop by VSM is found to become slanted and meanwhile, an in-plane loop is also observable. The Kerr loop measured from the exposed surface is also slanted compared to that measured from the substrate-contacted surface, which indicates that the exposed surface is oxidized. So a bilayer structure of the film is proposed. When a pump laser is switched on, the Kerr loops measured from both surfaces at the delay time of −5 ps become anomalous, showing the occurrence of magnetization reversal across magnetization compensation temperature. Unlike that measured from the substrate-contacted surface, femtosecond laser-induced magnetization dynamics measured from the exposed surface does not show magnetization reversal crossing zero magnetization. This can be explained by the bilayer structure as the compensation effect of demagnetization recovery of the oxidized layer on magnetization reversal of perpendicularly anisotropic TbFeCo layer across magnetization compensation temperature. Above experimental results show that the uncoated TbFeCo film cannot resist oxidation in dry air at room temperature for 2 years while the SiO₂-coated surface can do so for over 2 years.     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001