Enhanced high frequency properties of FeSiBPC amorphous soft magnetic powder cores with novel insulating layer

Fe-based amorphous magnetic powder cores (AMPCs) with excellent comprehensive properties were successfully fabricated via using the uniform double insulating layer core-shell structured FeSiBPC/Fe₃O₄@Epoxy resin (EP). The effects of the in-situ hydrothermal oxidized time on the magnetic properties of the AMPCs have been systematically investigated on the basis of the growth mechanism of the insulating layer of spherical amorphous powder in alkaline environment. The hydrothermal oxidation process could well ensure the uniformity of the oxide layer composed of Fe₃O₄ nanoparticle. The evolution of the insulating layer is a process of homogeneous nucleation and growth at different hydrothermal oxidation time. After 10 h of hydrothermal oxidation, a thin and dense layer composed of Fe₃O₄ nanoparticle was formed on the surface of the amorphous powder. As a consequent, the FeSiBPC AMPCs exhibit excellent performance such as stable effective permeability of 49.5 at 2 MHz, a very low core loss of 187 mW/cm³ at 100 kHz@0.05 T and high-quality factor of 160 at 600 kHz. The results indicate that the thickness of the Fe₃O₄ insulating layer can be completely controlled via the reaction parameters, and can effectively suppress the eddy current loss, which is promising for high-frequency electromagnetic systems.     

Controllable SiO2 insulating layer and magnetic properties for intergranular insulating Fe-6.5wt.%Si/SiO2 composites

In this study, the intergranular insulating Fe-6.5wt.%Si/SiO₂ soft magnetic composites (SMCs) were prepared successfully using in-situ chemical deposition followed by the spark plasma sintering (SPS) process. The effects of ammonia concentration on the microstructure and magnetic properties of the composites have been studied systematically. The Fe-6.5wt.%Si alloy particles could be well insulated by the uniform SiO₂ insulating layer, and its thickness increases with increasing the ammonia concentration from 0 to 0.02?ml/g. However, further increasing the ammonia concentration to 0.03 and 0.04?ml/g would result in the discontinuous and uneven SiO₂ insulating layer. Correspondingly, the saturation magnetization and effective permeability of the composite compacts first decrease and then increase with increasing the ammonia concentration from 0.00 to 0.04?ml/g, whereas the coercivity and resistivity vary in the opposite tendency. Note that the overall performances such as the frequency stability of effective permeability, higher resistivity and lower total core loss, reach the optimal value for the sample with the ammonia concentration of 0.02?ml/g.     

Evolution of coating layers during high-temperature annealing and their effects on magnetic behavior of Fe(Si) soft magnetic composites

Low-loss Fe(Si) soft magnetic composites (SMCs) with atomic-layer-deposition coated layer were successfully prepared in this work. The continuous, compact and uniform Al₂O₃ layer is found to form tight bonding with the powder base. Evolution of the coating layers has been investigated under different annealing temperatures and closely linked to the magnetic performance of the composites. Results indicate that 1100 °C was the optimal annealing temperature, at which the Fe(Si) SMCs showed lowest core loss of 1237 mW/cm³ and the highest permeability of 99.7 simultaneously (100 mT/100 kHz). Integrity of the coating layer ensures the maximum grain growth and stress removal of powder particles at the highest possible temperature, so as to reduce the hysteresis loss. Formation of high resistivity oxides and silicates (Al₂O₃, Fe₂SiO₄ and 3Al₂O₃·SiO₂) after annealing result in a low inter-particle eddy current loss. Besides, high temperature vacuum annealing is helpful to eliminate impurity atoms and decrease the anomalous eddy current loss. The improvement in effective permeability caused by high-temperature annealing is mainly attributed to the relaxation of residual stresses and the reduction of defects. While excessive temperature leads to the decomposition and destruction of the insulation layer, resulting in a significant increase in hysteresis loss, inter-particle and anomalous eddy current loss.     

On the initial growth of atomic layer deposited TiO2 films on silicon and copper surfaces

Atomic layer deposition (ALD) of TiO₂ using tetrakis(diethylamino)titanium precursor and H₂O was studied on silicon and copper surfaces in order to examine differences in nucleation. Both surfaces were patterned on the same substrate to assure identical deposition conditions. Spectral ellipsometry, X-ray photoelectron spectroscopy and surface profilometry were used to probe nucleation phenomena, growth rates, and surface morphology on both surfaces. The TiO₂ deposition on copper was found to exhibit a significant induction period of about 20-25 ALD cycles with no observable TiO₂ during the first 10-15 cycles on the copper side; in contrast, no such inhibited growth was observed in the TiO₂ deposition on silicon. This result opens up potential for selective ALD of TiO₂ films on silicon-based substrates patterned with a metal without the use of a mask, a self-assembled monolayer or soft lithography which is impractical for some nanoscale semiconductor fabrication processes. After film nucleation, the TiO₂ growth rate on both surfaces was found to be 0.10 nm/cycle.     

Etch characteristics of CoFeB magnetic thin films using high density plasma of a H2O/CH4/Ar gas mixture

Etch characteristics of CoFeB magnetic thin films patterned with TiN hard masks were investigated using inductively coupled plasma reactive ion etching in H₂O/Ar and H₂O/CH₄ gas mixes. As the H₂O concentration in the H₂O/Ar gas increased, the etch rates of CoFeB and TiN films decreased simultaneously, while the etch selectivity increased and etch profiles improved slightly without any redeposition. The addition of CH₄ to the H₂O gas resulted in an increase in etch selectivity and a higher degree of anisotropy in the etch profile. X-ray photoelectron spectroscopy was performed to understand the etch mechanism in H₂O/CH₄ plasma. A good pattern transfer of CoFeB films masked with TiN films was successfully achieved using the H₂O/CH₄ gas mix.     

Insights into the synthesis optimization of Fe@SiO2 Core-Shell nanostructure as a highly efficient nano-heater for magnetic hyperthermia treatment

Maintaining the intact iron core, protecting the extra iron ion release, and generating biologically sufficient heat, are the critical aspects for magnetic hyperthermia treatment (MHT). Thus, the composition of silica-coated nanoscale zero-valent iron (nZVI) was optimized by applying response surface methodology (RSM) and changing the molar ratio of tetraethyl orthosilicate (TEOS) and iron(II) sulfate (FeSO₄·7H₂O) as the silica and iron precursors, respectively. The TEOS/FeSO₄·7H₂O molar ratio of 1.67 results in the maximized saturation magnetization (99.3 emu/g) and the crystalline phase of pure iron. In comparison with the previously reported studies, the synthesized core–shell nanostructures demonstrate superior heat production features. As silica coating protects the inner core from oxidation and results in more effective heat-generating seeds, nanostructures with a higher amount of silica precursors, i.e., 10.3 mm, demonstrate an efficient specific absorption rate (SAR). Moreover, the medium and higher TEOS amounts represent acceptable cytocompatibility up to 125 μg/mL and 250 μg/mL, respectively. In vitro hyperthermia evaluation depicts the cancer cell viability reduction indicating the hyperthermia-induced apoptosis. Based on the data mentioned above, we could introduce a potential successful nanoparticle for magnetic hyperthermia treatment.     

High frequency magnetic mechanism of Ni-substituted Co2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co₂Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba₃Ni_xCo_2−xFe₂₄O₄₁ (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co₂Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co₂Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.     

Iron-based magnetic nanoparticles for magnetic resonance imaging

Magnetic resonance imaging (MRI) has been an extensive area of research owing to its depth of penetration for clinical diagnosis. Signal intensity under MRI is related to both T₁, spin-lattice relaxation, and T₂, spin-spin relaxation. To increase the contrast variability under MRI, several contrast agents are being used, i.e. T₁ contrast agents (e.g. gadolinium) and T₂ contrast agents (e.g. iron-based magnetic nanoparticles). These contrast agents are administered prior to scanning to increase contrast visibility. They reduce the T₁ and T₂ relaxation times to produce hyperintense and hypointense signals, respectively. Tunable properties of iron-based magnetic nanoparticles and several coating materials provide a platform to get superb MRI contrast in T₂ weighted images. It has been found that contrast enhancement by iron-based magnetic nanoparticles is dependent on the size, shape, composition, surface, and magnetic properties which can be tuned with the synthesis method and coating material. Therefore, understanding the synthesis method and properties of magnetic nanoparticles is vital to contribute to MR signal enhancement which is directing the scientist to design engineered iron-based magnetic nanoparticles. This paper introduces the concept of MRI contrast enhancement. We mainly discuss the synthesis of T₂ contrast agents, i.e. iron-based magnetic nanoparticles and the modification of these T₂ contrast agents by coating followed by their biomedical applications.     

Structure and magnetic properties of FeCo-SiO2 nanocomposite synthesized by a novel wet chemical method

A novel soft magnetic nanocomposite with FeCo particles encapsulated by amorphous SiO₂ was synthesized using a co-precipitation combined H₂ reduction method. The saturation magnetization of the (Fe₇₀Co₃₀)₉₀/(SiO₂)₁₀ nanocomposite is as high as 200 emu/g, which is 4-5 times larger than that of traditional spinel ferrites. The frequency dependence of the complex initial permeability is intensely dependent upon the content of SiO₂ insulating phase. With increasing the content of SiO₂ to 10 wt.%, the cut-off frequency is drastically increased to over 1 GHz. The results show that a new high-frequency soft magnetic material with high saturation magnetization (M_s) can be achieved by introducing FeCo/SiO₂ nanocomposite.     

Enhanced photocatalytic degradation performance of organic contaminants by heterojunction photocatalyst BiVO4/TiO2/RGO and its compatibility on four different tetracycline antibiotics

Photocatalytic performance of four tetracycline antibiotics using BiVO₄/TiO₂/RGO composites was investigated. To make full use of catalysis, optimum preparation conditions involved RGO content, solution pH and hydrothermal temperature on the structure forming of BiVO₄/TiO₂/RGO composites were investigated. Subsequently, the obtained visible light-driven photocatalyst was used to degrade four kinds of tetracycline antibiotics involved tetracycline (TC), chlortetracycline (CTC), oxytetracycline (OTC) and doxycycline (DXC) for wastewater treatment. Results showed that BiVO₄/TiO₂/RGO photocatalyst exhibited excellent photocatalytic activity and high compatibility due to the enhanced separation efficiency of photo-generated carriers with high reduction and oxidation capability. The degradation process of four kinds of tetracycline antibiotics was traced and detected through identifying intermediates produced in the reaction system. And a possible catalytic mechanism for BiVO₄/TiO₂/RGO photocatalyst was put forward based on band gap structure of BiVO₄ and TiO₂.     

Change in reaction pathway of nickel(II) complex induced by magnetic fields

Three identical hydrothermal self-assembly reactions in a mixture of nickel nitrate hexahydrate, nicotinic acid, sodium azide were carried out with the applied magnetic fields of 0, 0.15, 0.3 T, respectively. It was found that [Ni_1.5(N₃)(nic)₂(Hnic)]_n can be obtained at 160 °C for 48 h without an applied magnetic field, while the final product totally changed into [Ni₂(nic)₄(H₂O)] as a 0.3 T weak magnetic field was employed in the reaction. Both of the two products show three-dimensional frameworks, however, they display two different coordination geometries with different ligands. We consider the mechanism base on changes of magnetic susceptibilities and structures of [Ni₂(nic)₄(H₂O)], and propose a view that the magnetic field can reduce the activation energy of reaction pathways according to transition state theory. The possible reason of the decrease of activation energy is that a magnetic field can mediate coupling among the Ni²⁺ ions during the formation of [Ni₂(nic)₄(H₂O)], which is indicated by the slight changes of microstructure of crystal, such as bond angle and length.     

Porous soft magnetic material: The maghemite microsphere with hierarchical nanoarchitecture and its application in water purification

Porous soft magnetic material is a member of soft magnetic material family having large surface area. This kind of material has vast potential in the surface-related applications. In this paper, a kind of maghemite (γ-Fe₂O₃) with large surface area, 82.7 m²/g, specifically, was obtained by the solvothermal method and subsequent calcining process. As shown in the test results, the maghemite microsphere has no magnetic memory in the magnetic field since the residual magnetization is 0.7 emu/g. In the waste water treatment process, Rhodamine B, a model organic pollutant in the water, was removed through the adsorption and desorption cycles by the maghemite microspheres. The maghemite microspheres in the water can be recycled easily by a magnetic separation procedure and regenerated in ethanol at room temperature.     

One-step hydrothermal process to prepare highly crystalline Fe3O4 nanoparticles with improved magnetic properties

Hydrothermal process was successfully used to synthesize Fe₃O₄ powder using ferrous chloride (FeCl₂) and diamine hydrate (H₄N₂·H₂O) as starting materials by carefully controlling the reaction conditions. The as-prepared Fe₃O₄ sample was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and its magnetic properties were evaluated on a vibrating sample magnetometer (VSM). The nanoscale (40 nm) Fe₃O₄ powder obtained at 140 °C for 6 h possessed a saturation magnetization of 85.8 emu/g, a little lower than that of the correspondent bulk Fe₃O₄ (92 emu/g). It is suggested that the well-crystallized Fe₃O₄ grains formed under appropriate hydrothermal conditions should be responsible for the increased saturation magnetization in nanosized Fe₃O₄.     

Highly enhancing electromagnetic properties in Fe-Si/MnO-SiO2 soft magnetic composites by improving coating uniformity

Fe-Si/MnO-SiO₂ soft magnetic composites (SMCs) are prepared by sintering ball milled Fe-Si/MnO₂ core–shell structured composites. The correlation between the coating uniformity and electromagnetic properties have been investigated via adjusting ball milling parameters in detail. The results indicate that uniform MnO₂ coating can be transformed into MnO-SiO₂ composite coatings with high insulation due to the high temperature reaction between MnO₂ and Si. Agate ball is more effective than stainless steel ball to improve the uniformity of MnO₂ coating as well as the electromagnetic properties such as significantly higher resistivity, lower core loss and better frequency stability of permeability. Moreover, increasing the ball milling time from 4 h to 24 h can obviously improve the coating uniformity and thus result in the remarkable increase of the resistivity from 2.4 mΩ·cm to 356.9 mΩ·cm. And the core loss and dynamic loss decrease rapidly while the M_s shows a slight decline. When the ball milling time reaches 24 h, the Fe-Si SMCs exhibits superior magnetic properties such as high M_s (181.0 emu/g), very low core loss (361.5 kW/m³ at 100 kHz) and good frequency stability of permeability (65) from 50 Hz to 1000 kHz.     

Structural,optical, and magnetic properties of polycrystalline Co-doped TiO2 synthesized by solid-state method

We have used a solid-state method to synthesize polycrystalline Co-doped TiO₂ diluted magnetic semiconductors (DMSs) with Co concentrations of 0, and 0.5 at.%. X-ray diffraction patterns reveal that Co doped TiO₂ crystallizes in the rutile tetragonal structure with no additional peaks. Transmission electron microscopy (TEM) did not indicate the presence of magnetic parasitic phases and confirmed that Co ions are uniformly distributed inside the samples. Optical absorbance measurements showed an energy band gap which decreases after doping with the Co atoms into the TiO₂ matrix. Magnetization measurements revealed a paramagnetic behavior for the as-prepared Co-doped TiO₂ and a ferromagnetic behavior for the same samples after annealed under a mixture of H₂/N₂ atmosphere.     

Photocatalytic degradation of dimethyl phthalate in an aqueous solution with Pt-doped TiO2-coated magnetic PMMA microspheres

This study investigates the photocatalytic degradation of dimethyl phthalate (DMP) with both the titanium dioxide-coated magnetic poly(methyl methacrylate) (TiO₂/mPMMA) and platinum-doped TiO₂/mPMMA (Pt–TiO₂/mPMMA) microspheres. The TiO₂/mPMMA and Pt–TiO₂/mPMMA microspheres are employed as novel photocatalysts that offer high photocatalytic activity, magnetic separability and good durability. The photocatalytic experiments of DMP under various conditions are conducted to examine the effects of the initial DMP concentration, photocatalyst dosage, UV radiation intensity and Pt doping content on the degradation of DMP. In addition, the correlations of the photocatalytic kinetics and quantum yield for DMP removal are proposed associated with the system parameters. According to the experimental results, there exists a distinct relationship between the reduction percentages of total organic carbons and DMP. Furthermore, the photodegradation mechanism of DMP in the photocatalytic process is established based on the identification of the intermediates. Moreover, the good repeatability of the photocatalytic performance with the use of the Pt–TiO₂/mPMMA microspheres has also been demonstrated in the multi-run experiments. Therefore the Pt–TiO₂/mPMMA microspheres are considered as a practical and promising photocatalyst in a suspension reaction system and they can be effectively recovered after use. This study provides useful information about the applications of the TiO₂/mPMMA and Pt–TiO₂/mPMMA microspheres for the photodegradation of DMP.     

Structure, morphology and magnetic properties of Fe-B-Si-Nb glassy alloy thin film prepared by a pulsed laser deposition method

With the aim of applying to a soft magnetic underlayer of the double-layered perpendicular magnetic recording media, an Fe_74.9B_17.5Si_2.5Nb_5.1 alloy thin film was fabricated on Si substrate by a pulsed laser deposition method. The Fe-based alloy thin film of 200 nm in thickness was confirmed as a glassy structure. The thermal properties of the thin film have similar features to those for the melt-spun glassy alloy ribbon. The glassy alloy thin film exhibits good soft magnetic properties, i.e., high B_s of 1.2 T and in-plane low H_c of 134 A/m. The Fe-B-Si-Nb glassy alloy thin film is expected to be suitable for the soft magnetic underlayer material in the double-layered perpendicular magnetic recording media.     

Atomic layer deposition of ferromagnetic cobalt doped titanium oxide thin films

TiO₂ thin films doped or mixed with cobalt oxide were grown by atomic layer deposition using titanium tetramethoxide and cobalt(III)acetylacetonate as metal precursors. The films could be deposited using both O₃ and H₂O as oxygen precursors. The films grown using water exhibited considerably smoother surface than those grown with ozone. The TiO₂:Co films with Co/(Co + Ti) cation ratio ranging from 0.01 to 0.30 were crystallized by annealing at 650 °C, possessing mixed phase composition comprising rutile and anatase and, additionally, CoTiO₃ or CoTi₂O₅. The annealed films demonstrated magnetic response expressed by magnetization curves with certain hysteresis and coercive fields.     

Preparation of graphene/TiO2 anode materials for lithium-ion batteries by a novel precipitation method

This paper reports a large-scale production route for graphene/TiO₂ nanocomposites using water-based in situ precipitation method. In this method, freshly prepared graphene oxides/TiO₂ obtained by precipitating Ti(SO₄)₂ with NH₃H₂O was subjected to heat treatment in the presence of N₂, which resulted in the formation of graphene/TiO₂ nanocomposites. Graphene/TiO₂ composites prepared by our method were found to be suitable as anode materials for lithium ion batteries because of its stable cycling performance and high capacity.     

Vanadium dioxide nanobelts: Hydrothermal synthesis and magnetic properties

VO₂ (B) nanobelts were prepared by a hydrothermal method at 180 °C using V₂O₅·nH₂O sol and H₂C₂O₄·2H₂O as starting agents. The obtained nanobelts have diameters ranging from 50 to 100 nm in width, 20-30 nm in thickness with lengths up to 1.5 μm. Measurements of the static magnetic susceptibility provide evidence for two phase transitions at T₁ = 225 K and T₂ = 290 K, respectively. Below T₁, the data suggest the presence quasi-free as well as of strongly antiferromagnetic correlated spins associated to V⁴⁺-ions.