Maghemite Interparticle Interactions in γ–Fe2O3/Ag nanocomposites

Maghemite and silver nanoparticles were prepared using the ionic coordination reaction technique. X-ray diffractograms (XRD) show that all the samples have a high degree of purity. The average particle sizes obtained by XRD and transmission electron microscopy (TEM) vary from 9 to 13 nm for the γ–Fe₂O₃ particles and from 130 to 142 nm for the Ag particles. The magnetic properties of the samples have been studied by room-temperature DC magnetization and Mössbauer spectroscopy. These measurements indicate the presence of γ–Fe₂O₃ particles with interparticle interactions of varying strength, resulting in three magnetic regimes: superparamagnetic, superparamagnetic with interactions, and blocked with interactions.     

The magnetic behaviour of nanostructured zinc ferrite

We have investigated a series of nanostructured ZnFe₂O₄ samples produced by mechanical activation (mean particle sizes d ～50-8 nm) by variable temperature neutron diffraction measurements (2-535 K) supported by DC magnetisation measurements (4.2-300 K). The systematic increase in the mean inversion parameter (c ～0.04-0.43) with increasing milling time is accompanied by a gradual decrease in the occurrence of the long range antiferromagnetic ordering observed in the starting ZnFe₂O₄ material, as well as a gradual decrease in the related diffuse short range order peak. The neutron diffraction patterns of particles with d < ～15 nm and c> ～0.2 are consistent with the occurrence of ferrimagnetic order and exchange interactions of the type Fe³⁺A—O^2?—Fe³⁺ [B]. Diagrams summarising the magnetic regions of nanostructured ZnFe₂O₄ are presented. The magnetic behaviour overall agrees well with the enhanced magnetisation and ferromagnetic behaviour reported for nanostructured, ultrafine and thin films of ZnFe₂O₄ by other groups using mainly magnetisation and Mössbauer spectroscopy measurements.     

Synthesis and Magnetic Properties of Bi<Subscript>1.7</Subscript>Pb<Subscript>0.3</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+<Emphasis Type="Italic">δ</Emphasis></Subscript> Added with Nano Y

Bi₁₇Pb_0.3Sr₂Ca₂Cu₃O_10+δ superconductor samples were synthesized by the conventional solid-state reaction method. To study the effects of the addition of yttrium nanoparticles to the superconducting system, nano Y₂O₃ was introduced by small weight percentages (0.2, 0.4, 0.6, 0.8, and 1.0 wt%) in the first step of the synthesis process. Phase identification and microstructural characterization of the samples were investigated using X-ray diffraction and scanning electronic microscopy (SEM). Energydispersive X-ray spectroscopy (EDX) analysis was utilized to confirm the presence of the desired elements in the chemical composition of the samples. Moreover, DC electrical resistivity as a function of the temperature, critical current density (J _c), AC magnetic susceptibility, and DC magnetization measurements were carried to evaluate the relative performance of samples. XRD analysis showed that both (Bi,Pb)-2223 and Bi-2212 phases coexist in the samples having an orthorhombic crystal structure. Both the onset critical temperatures (T _c) (onset) and zero electrical resistivity critical temperatures (T _c) (R = 0) of the samples were determined from the DC electrical resistivity measurements. An improvement of the superconducting transition temperature of 3.0 % was obtained with increasing Y₂O₃ nanoparticles to x = 1.0 wt%, while the critical current density is improved by 200 %. AC magnetic susceptibility measurement showed that the diamagnetic fraction and intergranular coupling of the x = 1.0 wt% sample are greater than those of the others. The variation of magnetization with temperature (M–T curve) of the samples was measured by cooling the sample in zero fields (ZFC) and an applied field of 10 Oe (FC). The results of AC magnetic susceptibility and DC magnetization measurements were in good agreement with DC electrical resistivity measurement.     

Optimization of the Dy Content in Single Crystalline GdBCO Bulk Superconductors Fabricated in Air Via Top-Seeded Infiltration Growth Process

Single-crystalline bulk GdBa₂Cu₃O_7?δ (GdBCO) superconductors were fabricated with top-seeded infiltration-growth technique (TSIG) in air by adding the different amounts of Dy₂BaCuO₅ (Dy-211) particles (5, 10, 15, 20, 25, 30, 35, and 40 wt%). All the samples show superconducting critical temperature at ～93 K with sharp transition irrespective of the chemical composition. X-ray diffraction measurements confirmed that all the samples were single domain with high alignment. Microstructural studies through scanning electron microscopy showed that TSIG-processed GdBCO samples contained 211 phase particles of two different sizes, i.e., ～2–4 and ～10 μ m. The addition of Dy-211 phase particles led to a significant enhancement of critical current density (J _c) at 77 K. GdBCO superconductor with the addition of 20 wt% of Dy-211 showed the highest J _c of 45.5 kA/cm ² at 77 K and self field.     

Effect of the addition of Fe2O3 and heat treatment duration on the magnetic susceptibility of V2O5-P2O5-B2O3 glass system

The effect of the addition of Fe₂O₃ and heat treatment duration on the magnetic susceptibility of vanadium borophosphate glass were studied. The magnetic susceptibility of glass samples was found to increase with increasing Fe₂O₃ content, which may be explained by the formation of the FeO₆ group and the change of Fe²⁺ to Fe³⁺ which has higher paramagnetic properties. No detectable changes in the magnetic susceptibility with heat treatment for the samples containing 0.0, 0.5 and 1.0 mol% Fe₂O₃ was observed. The magnetic susceptibility for the heat treated samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃ decreases sharply with increasing duration of heat treatment up to 6 h and then remains almost constant. The sharp decrease in magnetic susceptibility of 2.5 mol% Fe₂O₃ is attributed to the increase in the number of ferrous ions. The sharp decrease for samples containing 5.0 and 7.5 mol% Fe₂O₃ is attributed to the increase in the number of Fe³⁺ in tetrahedral co-ordination. The rate of crystallization owing to the heat treatment was calculated and was found to increase with increasing iron oxide content. The geometry of crystallization was found to be in three-, two-and one-dimension(s) for samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃, respectively.     

A Study of the Effect of Nanometer Fe3O4 Addition on the Properties of Silicone Oil-based Magnetorheological Fluids

With the aim to improve the performance of magnetorheological fluids (MRFs) for mechanical transmission system, a process to prepare silicone oil-based MRFs with the addition of nanometer Fe₃O₄ particles is presented and five MRFs samples with different mass fraction of nanometer Fe₃O₄ particles have been prepared. The experimental materials, the preparation process, and test methods are elaborated. Moreover, the microstructures of soft magnetic carbonyl iron particles, nanometer Fe₃O₄ particles, and carbonyl iron/nano-sized Fe₃O₄ composites have been characterized via scanning electron microscope (SEM). Finally, test experiments of sedimentation stability, zero field viscosity, and shear yield stress have been carried out. The experimental results show that adding a certain amount of nanometer Fe₃O₄ particles (4 and 6 wt%) into MRFs can improve the performance of MRFs.     

DNA- and Field-Mediated Assembly of Magnetic Nanoparticles into High-Aspect Ratio Crystals

Under an applied magnetic field, superparamagnetic Fe₃O₄ nanoparticles with complementary DNA strands assemble into crystalline, pseudo-1D elongated superlattice structures. The assembly process is driven through a combination of DNA hybridization and particle dipolar coupling, a property dependent on particle composition, size, and interparticle distance. The DNA controls interparticle distance and crystal symmetry, while the magnetic field leads to anisotropic crystal growth. Increasing the dipole interaction between particles by increasing particle size or external field strength leads to a preference for a particular crystal morphology (e.g., rhombic dodecahedra, stacked clusters, and smooth rods). Molecular dynamics simulations show that an understanding of both DNA hybridization energetic and magnetic interactions is required to predict the resulting crystal morphology. Taken together, the data show that applied magnetic fields with magnetic nanoparticles can be deliberately used to access nanostructures beyond what is possible with DNA hybridization alone.     

Magnetic properties of magnesium-cobalt ferrites synthesized by co-precipitation method

The alternating current (a.c.) low field susceptibility vs temperature, magnetization and⁵⁷Fe Mössbauer effect measurements are reported for the spinel solid solution series Mg _x Co_1?x Fe₂O₄ synthesized by a wet-chemical method before and after high temperature annealing. The observed features for the wet samples, such as the coexistence of paramagnetic doublet and magnetic sextets in Mössbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mössbauer spectra of wet samples reveal the presence of superparamagnetic particles which exist simultaneously with ferrimagnetic regions in the materials well supported by a.c. susceptibility data. The high temperature annealing changes the wet-prepared ferrites into the ordered magnetic structure of ceramic ferrites.     

Magnetic Properties of Mg x Mn1?x Fe2O4 Nanoferrites

We report the magnetic properties of Mg _x Mn_1?x Fe₂O₄ (0??x??1.0) nanosize compounds with particle sizes between 8?nm and 15?nm. The evolutions of the properties as a function of composition have been investigated by X-ray diffraction, M?ssbauer, and SQUID magnetometry. Pure cubic spinel could be obtained under a low reaction temperature of 200°C in all the samples. Impurity phases have been observed in compounds annealed at above 900°C. Magnetic relaxation is observed in samples with particles of about 8?nm. The spectra with particle sizes larger than 10?nm could be fitted with at least two sextets attributed to Fe³⁺ ions on tetrahedral (A) and octahedral?(B) sites. The magnetization measurement indicates superparamagnetic behavior in nanosized compounds.     

Magnetization and Mossbauer spectra of non-crystalline Y3Fe5O12

Non-crystalline Y₃Fe₅O₁₂ is studied by X-ray, electron microscope, magnetic susceptibility, and Mössbauer effect techniques. The material appears to consist of X-ray amorphous platelets of several microns, which are agglomerations of particles of about 200 Å. Mössbauer spectra at room temperature show Fe in tetrahedral surrounding, while at liquid helium temperature hyperfine splitting for both octahedral and tetrahedral sites is observed. Magnetization measurements show that individual particles order magnetically at ～850 K, much higher than well-crystallized YIG (560 K). These differences are consistent with a high degree of crystallographic disorder and a small particle size. Crystallographic ordering and particle growth are induced by annealing. At 680°C crystalline YIG is formed.     

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB₂ thin films were deposited on MgO (100) substrate and r-plane Al₂O₃ $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB₂ films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J _c was calculated from M–H loops and also the magnetic field dependence of F _p was compared for the different temperature ranges from 5 to 25 K. The critical current density J _c was found to be around 1.0×10⁶ A/cm² and 1.7×10⁶ A/cm² in zero field at 5 K for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al₂O₃ in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.     

Variation with added material in the effects of reactive mechanical grinding and hydriding–dehydriding cycling on the hydrogen-storage properties of Mg

Samples Mg–14Ni–6Fe₂O₃, Mg–14Ni–3Fe₂O₃–3Ti, and Mg–14Ni–2Fe₂O₃–2Ti–2Fe were prepared by reactive mechanical grinding, and their hydrogen storage properties were examined. The activated Mg–14Ni–2Fe₂O₃–2Ti–2Fe had the highest hydriding rate, absorbing 4.14 wt% H for 5 min, and 4.27 wt% H for 10 min, and 4.42 wt% H for 60 min at 573 K under 12 bar H₂. The activated Mg–14Ni–3Fe₂O₃–3Ti had the highest dehydriding rate, desorbing 3.81 wt% H for 20 min, 3.98 wt% H for 25 min, and 4.15 wt% H for 60 min. Mg–14Ni–6Fe₂O₃ dehydrided at n = 4 contained Mg, Mg₂Ni, MgO, and Mg(OH)₂. Mg(OH)₂ is considered to be formed by the reactions of MgH₂ or Mg with water vapor. The effects of reactive mechanical grinding and hydriding–dehydriding cycling are the creation of defects and cracks, and the reduction of Mg particle size. The addition of a larger amount of Ti and/or Fe has stronger effects of reactive mechanical grinding, whereas the addition of a larger amount of Fe₂O₃ has greater effects of hydriding–dehydriding cycling.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Synthesis and characterization of SrFe12O19 nanoparticles produced by a low-temperature solid-state reaction method

Ultra-fine nanoparticles of strontium hexaferrite, SrFe₁₂O₁₉, have been synthesized by low-temperature solid-state reaction method without ball-milling process. The effects of the preparing conditions of samples such as calcination temperature, Fe³⁺/Sr² and Na¹⁺/Sr²⁺ mole ratio on the phase composition, particle size and shape and magnetic properties of the calcined samples have been investigated by differential thermal analysis, X-ray diffraction, field emission scanning electronic microscopy, and vibrating sample magnetometery. The X-ray diffraction patterns showed the formation of SrFe₁₂O₁₉ single phase at temperatures as low as 750° C. Fine particles with particle size around 30–100 nm obtained at 750° C with Fe³⁺/Sr² mole ratio of 10. The magnetic measurements and structural analysis showed that particles were single domain and exhibited better magnetic properties than those obtained by the ceramic method.     

Magnetic PNIPA hydrogels for hyperthermia applications in cancer therapy

Temperature-sensitive Poly (N-isopropylacrylamide), PNIPA gels were synthesized with micron-sized iron and iron oxide (Fe₃O₄) particles to investigate their viability for combined hyperthermia and drug release applications. The ultimate goal is to combine hyperthermia and triggered drug release. Induction heating of the magnetic hydrogels with varying concentration of magnetic powder was conducted at a frequency of 375 kHz for magnetic field strength varying from 1.7 kA/m to 2.5 kA/m. It was observed that the maximum temperature induced in the magnetic hydrogels increased with the concentration of magnetic particles and magnetic field strength. The PNIPA gel underwent a collapse transition at 34 °C. The best combination was found for the PNIPA–Fe₃O₄ system, 2.5 wt.% Fe₃O₄ in PNIPA–Fe₃O₄ system took 260 s to be heated to 45 °C under a magnetic field strength of 1.7 kA/m and the specific absorption rate (SAR) was found to be 1.83. SAR of iron oxide was found to be higher than the SAR of iron.     

Synthesis of a soluble polyaniline–ferrite composite: magnetic and electric properties

We report the preparation of a processible magnetite/polyaniline (Fe₃O₄/PANI) nanocomposite, containing dodecylbencensulfonic acid (DBSA) as a surfactant and dopant, with both magnetic and conducting properties. Different amounts of Fe₃O₄ nanoparticles were successfully disposed with FeCl₃ solution to prevent their aggregation in the solution by the application of common ion effect. The magnetic properties of the resulting composites were investigated by a quantum design magnetometer (PMPS). The (Fe₃O₄/PANI) nanocomposite showed at 300 K no loop of hysteresis indicating the superparamagnetic nature. The saturation magnetization varies from 0.167 to 28.45 emu/g with increasing Fe₃O₄ content. Zero field cooling (ZFC) and Field cooling (FC) profiles showed that the polyaniline matrix allows each ferrite nanoparticles to behave independently and interparticle interactions are not important for iron oxide content lower than 36 wt.%. The electrical conductivity of composites was found to be higher than that of the pure PANI in spite of the insertion of the insulating material Fe₃O₄ particles. It is noticeable that conductivity increases with low Fe₃O₄ particles content and then decreases. Structural characterization by X-ray diffraction (XRD), UV spectroscopy and thermogravimetric analysis (TGA) have been performed.     

Hydriding and dehydriding rates and hydrogen-storage capacity of Mg–14Ni–3Fe2O3–3Ti prepared by reactive mechanical grinding

The magnesium prepared by mechanical grinding under H₂ (reactive mechanical grinding) with transition elements or oxides showed relatively high hydriding and dehydriding rates when the content of additives was about 20 wt%. Ni (expected to increase hydriding and dehydriding rates) was chosen as transition element to be added. Fe₂O₃ (expected to increase hydriding rate) was selected as an oxide to be added. Ti was also selected since, it was considered to increase the hydriding and dehydriding rates by forming Ti hydride. A sample, Mg–14Ni–3Fe₂O₃–3Ti, was prepared by reactive mechanical grinding and its hydrogen storage properties were investigated. This sample absorbed 4·02 wt% H for 5 min, 4·15 wt% H for 10 min and 4·42 wt% H for 60 min at n?=?2. It desorbed 2·46 wt% H for 10 min, 3·98 wt% H for 30 min and 4·20 wt% H for 60 min at n?=?2.     

合金元素对块体纳米晶Fe3Al材料磁学性能的影响

为了研究合金元素对块体纳米晶Fe3Al材料磁学性能的影响,通过铝热反应熔化法制备了纳米晶Fe3Al以及分别含Ni质量分数10%、Cr质量分数10%、Mn质量分数10%和含Ni质量分数10%-Cu质量分数2%的块体纳米晶Fe3Al.在振动样品磁强计(VSM)上测得合金的磁滞回线,分析其磁性能,采用X射线衍射仪进行结构分析和平均晶粒尺寸计算.结果表明:各样品的磁滞回线呈倾斜状且狭长,磁滞损耗很小;含Ni质量分数10%的样品饱和磁化强度Ms较大,剩余磁化强度Mr和矫顽力Hc较其他样品最小,具有较好的软磁性能;添加合金元素后几种材料的晶粒尺寸变小,磁性能有较大变化,合金元素对纳米晶Fe3Al块体材料的磁性能影响明显.     

Facile preparation of carbon coated magnetic Fe3O4 particles by a combined reduction/CVD process

In this work, we report a simple method for the preparation of magnetic carbon coated Fe₃O₄ particles by a single step combined reduction of Fe₂O₃ together with a Chemical Vapor Deposition process using methane. The temperature programmed reaction monitored by Mössbauer, X-ray Diffraction and Raman analyses showed that Fe₂O₃ is directly reduced by methane at temperatures between 600 and 900 °C to produce mainly Fe₃O₄ particles coated with up to 4 wt% of amorphous carbon. These magnetic materials can be separated into two fractions by simple dispersion in water, i.e., a settled material composed of large magnetic particles and a suspended material composed of nanoparticles with an average size of 100-200 nm as revealed by Scanning Electron Microscopy and High-resolution Transmission Electron Microscopy. Different uses for these materials, e.g., adsorbents, catalyst supports, rapid coagulation systems, are proposed.     

Vortex Flux Dynamics and Harmonic ac Magnetic Response of Ba(Fe<Subscript>0.94</Subscript> Ni<Subscript>0.06</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Bulk Superconductor

Vortex dynamics and nonlinear ac response are studied in a Ba(Fe_0.94 Ni_0.06)₂As₂(T _c = 18.5 K) bulk superconductor in magnetic fields up to 12 T via ac susceptibility measurements of the first ten harmonics. A comprehensive study of the ac magnetic susceptibility and its first ten harmonics finds shifts to higher temperatures with increasing ac measurement frequencies (10 to 10,000 Hz) for a wide range of ac (1, 5, and 10 Oe) and dc fields (0 to 12 T). The characteristic measurement time constant t ₁ is extracted from the exponential fit of the data and linked to vortex relaxation. The Anderson-Kim Arrhenius law is applied to determine flux activation energy E _a /k as a function dc magnetic field. The de-pinning, or irreversibility lines, were determined by a variety of methods and extensively mapped. The ac response shows surprisingly weak higher harmonic components, suggesting weak nonlinear behavior. Our data does not support the Fisher model; we do not see an abrupt vortex glass to vortex liquid transition and the resistivity does not drop to zero, although it appears to approach zero exponentially.