Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

Study of cerium doped magnetite (Fe3O4:Ce)/PMMA nanocomposites

The paper presents the synthesis and properties of polymer nanocomposite material based on cerium doped magnetite (Fe₃O₄) as filler material and poly methyl methacrylate (PMMA) as host matrix. The magnetite (Fe₃O₄) particles were synthesized by co-precipitation route using stable ferrous and ferric salts with ammonium hydroxide as precipitating agent. Further, they doped by cerium oxide (CeO₂) non-stoichiometrically. The composite material was fabricated by solvent evaporation method. Here 2.4 GHz microwaves were used to study the effect of microwaves heating on polymerization. The phase and crystal structure is determined by X-ray diffraction (XRD). The average crystallite size of the composites varies from 28 to 35 nm. The chemical structure is confirmed by Fourier transform infrared (FTIR) spectroscopy. The magnetic and thermal properties are investigated by vibrating sample magnetometer (VSM) and differential scanning calorimetry (DSC). The thermal study shows that the microwave heated samples possess higher glass transition temperature (T_g). The magnetic results suggest that coercivity (H_C) and squareness (M_r/M_s) of the loop increases with increasing doping percent of cerium.     

Preparation of poly(styrene-glucidylmethacrylate)/Fe3O4 composite microspheres with high magnetite contents

Magnetic polymer composite microspheres with high magnetite contents were prepared by dispersion polymerization of styrene (St) and glucidylmethacrylate (GMA), in which Fe₃O₄ nanoparticles were co-stabilized by oleic acid and silane surfactants. The microstructure of the composite microspheres was characterized by Fourier transform infrared (FTIR) spectrometry, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results demonstrated the presence of a hybrid morphology with organic polymer-encapsulated inorganic particles. Subsequently, thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM) were used to evaluate the magnetite content of the microspheres. It was found that an accordant magnetite content of about 70 wt%, could be obtained for the magnetic polymer microspheres, a value significantly higher than those reported thus far. The possible mechanism for the formation of the microspheres was proposed.     

Synthesis and properties of Au-Fe3O4 and Ag-Fe3O4 heterodimeric nanoparticles

Monodisperse Au-Fe 3 O 4 heterodimeric nanoparticles (NPs) were prepared by injecting precursors into a hot reaction solution.The size of Au and Fe 3 O 4 particles can be controlled by changing the injection temperature.UV-Vis spectra show that the surface plasma resonance band of Au-Fe 3 O 4 heterodimeric NPs was evidently red-shifted compared with the resonance band of Au NPs of similar size.The as-prepared heterodimeric Au-Fe 3 O 4 NPs exhibited superparamagnetic properties at room temperature.The Ag-Fe 3 O 4 heterodimeric NPs were also prepared by this synthetic method simply using AgNO 3 as precursor instead of HAuCl 4.It is indicated that the reported method can be readily extended to the synthesis of other noble metal conjugated heterodimeric NPs.     

Microwave dielectric and magnetic properties of superparamagnetic 8-nm Fe3O4 nanoparticles

The superparamagnetic 8-nm Fe₃O₄ nanoparticles were successfully prepared by chemical oxidation process. For the complex permittivity, the dual dielectric relaxation processes have been proved by two overlapped Cole–Cole semicircles, and the natural resonance frequency is 3.03 GHz for the complex permeability. The maximum reflection loss value reaches −55.5 dB at 6.11 GHz with 3.85 mm in the thickness of the absorbers for the superparamagnetic 8-nm Fe₃O₄ nanoparticles which is better than that of 150 nm and 30 nm Fe₃O₄ nanoparticles. It is believed that the superparamagnetic 8-nm Fe₃O₄ nanoparticles can be used as a kind of candidate for microwave absorber.     

Ferroelectric relaxor behaviour in Pb(Fe0.5Ta0.5)O3

The relaxor ferroelectric lead iron tantalate, Pb(Fe_0.5Ta_0.5)O₃ (PFT) is synthesized by Coulombite precursor method. The X-ray diffraction pattern of the sample at room temperature shows a cubic phase. The field dependence of dielectric response is measured in a frequency range 0.1 kHz — 1 MHz and in a temperature range from 173–373 K. The temperature dependence of permittivity (ɛ′) shows broad maxima at various frequencies. The frequency dependence of the permittivity maximum temperature (T _m ) has been modelled using Vogel-Fulcher relation.      

Optical emissions of products sputtered from Fe, Fe2O3 and Fe3O4 powders

Powder iron has been bombarded by a 5 keV Kr⁺ ions in a vacuum better than 10^-7 torr and under few 10^-6 torr ultra pure oxygen partial pressure. The optical spectra of the sputtered particles were recorded between 340.0 nm and 410.0 nm. These spectra exhibit discrete lines, which are attributed to neutral excited atoms of iron. Two iron oxides, namely hematite (Fe₂O₃)_{3}) and magnetite (Fe₃O₄)_{4}), in powder form, were studied under the same experimental conditions and identical lines were observed in the obtained spectra. The absolute intensities of the spectral lines in all spectra were measured and the differences in the recorded yield photons were discussed in term of electron-transfer processes between the excited sputtered atom and the bombarded surface. In accordance with the proposed interpretation, we suggest values for the energy gaps and electronic affinities for the studied oxides and for the oxide layer that might be formed by the adsorption of oxygen atoms.     

Dependence of pH and surfactant effect in the synthesis of magnetite (Fe3O4) nanoparticles and its properties

Nanoparticles of Fe₃O₄ were synthesized by co-precipitation in an aqueous solution containing ferrous and ferric salts (1:2) at varying pH with ammonia as a base. It was found that the value of pH influences the reaction mechanism for the formation of Fe₃O₄. Furthermore, the addition of mercaptoethanol significantly reduced the crystalline size of Fe₃O₄ nanoparticles from 15.03 to 8.02 nm. X-ray diffraction (XRD) spectra revealed that the synthesized nanoparticles were ε-Fe₂O₃ or Fe₃O₄ phase. To further prove the composition of the product, as-prepared Fe₃O₄ were examined by X-ray photoelectron spectroscopy (XPS). Magnetic properties of the obtained particles were determined by vibrating sample magnetometer (VSM). Further analysis of the X-ray studies shows that while maintaining a pH value of 6 and 9 in a solution containing iron salts II and III ions produces ε-Fe₂O₃. Whereas a pH value of 11 produces magnetite (Fe₃O₄) phase. All of these results show that the pH has a major role in the observed phase formation of (Fe₃O₄) nanoparticles.     

Field-induced microwave absorption in Fe3O4 nanoparticles and Fe3O4/polyaniline composites synthesized by different methods

Three kinds of nanoscale powders containing Fe₃O₄ nanoparticles (NPs) have been studied by ferromagnetic resonance (FMR): (i) Fe₃O₄ NPs grown and then covered with polyaniline (PANI), (ii) unclad Fe₃O₄ NPs, and (iii) Fe₃O₄ NPs grown “in situ” with the PANI. In every case, there is no low field microwave absorption, rather a single FMR line is observed. However, the half-power widths are of order of 1 kOe presumably due to a distribution of internal fields. For type I particles with a low concentration (below 40%) of Fe₃O₄, the observed resonance fields (H_r) are close to those expected for spheres with negligible magnetocrystalline anisotropy. For all other cases, H_r values are significantly lower. Such shortfalls can be roughly understood by invoking dipolar interactions between the grains, stresses frozen in grains during manufacture (method III), as well as anisotropy fields when the specimens are prepared in an aligning field.     

X-ray photoelectron spectroscopy of Co3O4, Fe3O4, Mn3O4, and related compounds

The metal 2p region spectra of the mixed valence spinels, Co₃O₄, Fe₃O₄, Mn₃O₄, and related compounds were studied. The satellite splittings of Co 2p_{$\text{3}\text{2}$} for the octahedrally coordinated cobaltous ions are 6.2 eV and those for the tetrahedrally coordinated ones are about 5.3 eV. The Co 2p spectrum for Co₃O₄ is considered to be the sum of spectra of magnetic cobaltous ions and low-spin cobaltic ions. In the cases of Fe₃O₄ and Mn₃O₄, the oxidation states were not clearly distinguished because both the divalent and trivalent ions of iron and manganese are high-spin.     

Growth of single-crystal magnetite nanowires from Fe3O4 nanoparticles in a surfactant-free hydrothermal process

Single-crystal magnetite nanowires with average diameter of ca. 20 nm and length of up to several micrometers were prepared by a simple alkaline surfactant-free hydrothermal process. The crystallinity, purity, morphology, and structural features of the as-prepared magnetite nanowires were investigated by powder X-ray diffraction, transmission electron microscopy (TEM) and selected area electron diffraction. The composition and length of nanowires depends on the pH, with higher pH favoring longer nanowires composed entirely of Fe₃O₄. A mechanism for nanowire growth is proposed.     

Defect structure and migration in Fe3O4

We present the results of a computer simulation study of the defect formation and migration energies in Fe₃O₄. Calculated Frenkel and Schottky energies are found to be similar in magnitude and both are considerably reduced by screening due to electron redistribution around the charged-defect species. The calculations also suggest that cation diffusion is mainly effected by a colinear interstitialcy mechanism at low P_o2 and by a simple octahedral vacancy mechanism at higher P_o2.     

Silane treatment of Fe3O4 and its effect on the magnetic and wear properties of Fe3O4/epoxy nanocomposites

In this study, the effect of silane treatment of Fe₃O₄ on the magnetic and wear properties of Fe₃O₄/epoxy nanocomposites was investigated. Fe₃O₄ nanopowders were prepared by coprecipitation of iron(II) chloride tetrahydrate with iron(III) chloride hexahydrate, and the surfaces of Fe₃O₄ were modified with 3-aminopropyltriethoxysilane. The magnetic properties of the powders were measured on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites using SQUID magnetometer. Wear tests were performed on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites under the same conditions (sliding speed: 0.18 m/s, load: 20 N).The results showed that the saturation magnetization (M_s) of surface-modified Fe₃O₄/epoxy nanocomposites was approximately 110% greater than that of unmodified Fe₃O₄/epoxy nanocomposites. This showed that the specific wear rate of surface-modified Fe₃O₄/epoxy nanocomposites was lower than that of unmodified Fe₃O₄/epoxy nanocomposites. The decrease in wear rate and the increase in magnetic properties of surface-modified Fe₃O₄/epoxy nanocomposites occurred due to the improved dispersion of Fe₃O₄ into the epoxy matrix.     

One-dimensional structural and electronic properties of magnetite Fe3O4(1 1 0)

We study the structure and the electronic properties of the (1 1 0) surfaces of magnetite Fe₃O₄ thin films by scanning tunneling microscopy (STM) and spectroscopy (STS). The STM images show a surface reconstruction consisting of ridges along the direction. Based on atomically resolved STM images we present a model for the observed ridge reconstruction of the surface, in agreement with a bulk-truncated layer containing both octahedral and tetrahedral iron ions. The metallic and semiconductor-like shapes of the measured current-voltage (I-V) curves indicate a non-uniform segregation of magnesium through the film. The weak contrast between the tops and valleys of ridges measured in the STS current maps is attributed to tetrahedral and octahedral coordination at the tops and the valleys, respectively. This attribution is in agreement with the proposed structure model. We observe a contrast enhancement at a tip change accompanied by a corrugation enhancement. This tip change is induced by picking up material from the sample, resulting in a magnetic tip. Thus, the contrast enhancement is attributed to detection of spin polarized current.     

Surface termination, composition and reconstruction of Fe3O4(001) and γ-Fe2O3(001)

We address a current controversy concerning the nature of the surfaces of Fe₃O₄(001) and γ-Fe₂O₃(001) grown on MgO(001) by molecular beam epitaxy. Despite recent claims to the contrary, we show that γ-Fe₂O₃(001) unambiguously exhibits a (1×1) surface net, in contrast to Fe₃O₄(001), which assumes a R45 reconstruction. In addition, we present high-energy-resolution Fe 2p and O 1s core-level photoelectron spectra obtained at both normal and grazing emission for γ-Fe₂O₃(001) and Fe₃O₄(001). These spectra show that the Fe₃O₄(001) surface has a higher Fe(III)/Fe(II) ratio than the bulk, and that the asymmetry in the O 1s line shape for Fe₃O₄(001) is due to final state effects rather than the presence of a surface oxygen or hydroxyl species.     

Electro-precipitation of Fe3O4 nanoparticles in ethanol

The preparation of superparamagnetic magnetite (Fe₃O₄) nanoparticles by electro-precipitation in ethanol is proposed. Particle average size can be set from 4.4 to 9 nm with a standard deviation around 20%. Combination of wide-angle X-ray scattering (WAXS), Electron energy loss spectroscopy (EELS) and Mössbauer spectroscopy characterizations clearly identifies the particles as magnetite single-crystals (Fe₃O₄).     

The properties of acicular particles of (γ-Fe2O3)x(Fe3O4)1-x

It is known that acicular particles, initially of Fe₃O₄, which have been partially oxidised, show anomalous properties e.g. in the variation of H_c with composition. It is shown that these can be explained by considering the particles to comprise an inner core of Fe₃O₄ with an outer layer of γ-Fe₂O₃. Owing to the large change in volume which occurs on oxidation, these mutually stress each other. Over a certain range of composition, the stress field interacts with the magnetization to increase H_c, and it is also responsible for H_c increasing with time after the oxidation process. A similar situation applies to partially reduced γ-Fe₂O₃.     

空心Fe_3O_4纳米微球的制备及超顺磁性

采用水热控制合成法,以六水三氯化铁、柠檬酸三钠和尿素为原料,聚丙烯酰胺为稳定剂, 200?C下反应12 h制备得到了超顺磁性空心Fe_3O_4纳米微球.通过X射线衍射仪、扫描电子显微镜、透射电子显微镜对样品的结构和形貌进行表征,并采用振动样品磁强计测试了样品的磁性能.结果表明:所得样品为具有尖晶石结构的Fe_3O_4纳米微球,尺寸为160 nm左右,呈分等级结构,即整个微球由粒径约18 nm的初级晶粒自组装堆叠而成;室温下表现为典型的超顺磁性,且饱和磁化强度为73.3 emu/g (1 emu/g=1 A·m~2/kg),这种高饱和磁化强度可以由其初级晶粒晶化程度高且粒径较大以及这种特殊的二次自组装结构进行解释.这种Fe_3O_4纳米微球为疏松多孔的空心球状结构,具有粒径分布均匀、分散性良好和超顺磁性的特点,在药物靶向输运和肿瘤热疗中有潜在的应用.     

Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe₃O₄ nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N₂ and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N₂ atmosphere are almost pure Fe₃O₄ phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.