Showing papers on "Saturation (magnetic) published in 2010"

Preparation and characterization of magnetic TiO2 nanoparticles and their utilization for the degradation of emerging pollutants in water

Abstract: Magnetic TiO 2 /Fe 3 O 4 and TiO 2 /SiO 2 /Fe 3 O 4 nanoparticles (NPs) were prepared by an ultrasonic-assisted sol–gel method using a commercial nanosized magnetic iron oxide as a support. Magnetic NPs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption (BET surface area) and SQUID magnetometer. Structure analyses indicated that TiO 2 /Fe 3 O 4 NPs presented a core–shell structure with a TiO 2 (anatase) coating wrapped around the magnetic iron oxide surface. TiO 2 /SiO 2 /Fe 3 O 4 NPs showed a ternary structure with a core of Fe 3 O 4 , a SiO 2 mesosphere and a TiO 2 (anatase) crust. Both types of NPs exhibited magnetic properties with saturation magnetization about 40 emu/g and low remanent magnetization and coercivity. Stability of TiO 2 /SiO 2 /Fe 3 O 4 NPs in an aqueous solution under UV illumination was better than that of TiO 2 /Fe 3 O 4 NPs as the presence of the insulation SiO 2 layer prevented photodissolution of iron. Both catalysts showed the ability to catalyze the photodegradation of acetaminophen and other four pharmaceutical and personal care products (PPCPs) (antipyrine, caffeine, metoprolol and bisphenol A) from an aqueous solution. In terms of catalytic activity the synthesized magnetic NPs were almost comparable to the commercial Degussa P25 TiO 2 photocatalyst. In addition, the easily recoverable magnetic photocatalysts showed good reusability, especially in the case of TiO 2 /SiO 2 /Fe 3 O 4 NPs.

Spin-glass-like freezing and enhanced magnetization in ultra-small CoFe2O4 nanoparticles

Abstract: The magnetic properties of ultra-small (3 nm) CoFe2O4 nanoparticles have been investigated by DC magnetization measurements as a function of temperature and magnetic field. The main features of the magnetic behaviour are blocking of non-interacting particle moments (zero-field-cooled magnetization Tmax≈40 K), a rapid increase of saturation magnetization (up to values higher than for the bulk material) at low T and an increase in anisotropy below 30 K due to the appearance of exchange bias. The low temperature behaviour is determined by a random freezing of surface spins. Localized spin-canting and cation distribution between the two sublattices of the spinel structure account quantitatively for the observed increase in saturation magnetization.

Controlled synthesis of iron oxide nanoparticles over a wide size range.

Abstract: We report on the effect of using decanoic acid as capping ligand on the synthesis of iron oxide nanoparticles by thermal decomposition of an organic iron precursor in organic medium. This procedure allowed us to control the particle size within 5 nm and about 30 nm by modifying the precursor-to-capping ligand ratio in a systematic fashion and to further expand the particle size range up to about 50 nm by adjusting the final synthesis temperature. The nanoparticles also showed high saturation magnetization of about 80−83 emu/g at low temperature, almost size-independent and close to the value for the bulk counterpart. Decanoic acid-coated nanoparticles were transferred to water by using tetramethylammonium hydroxide, which allowed further coating with silica in a tetraethyl orthosilicate solution. Consequently, these iron oxide nanoparticles are tunable in size and highly magnetic, and they could become suitable candidates for various biomedical applications such as contrast agents for magnetic resonance i...

Effects of oxidation on the magnetization of nanoparticulate magnetite.

Abstract: Synthetic nanomagnetite has been suggested as a potential reactant for the in situ treatment of contaminated groundwater. Although the application of magnetite nanoparticles for environmental remediation is promising, a full understanding of particle reactivity has been deterred by the propensity of the nanoparticles to aggregate and become colloidally unstable. Attractive magnetic interactions between particles are partially responsible for their aggregation. In this study, we characterized the magnetic behavior of magnetite by determining the saturation magnetization, coercivity, remanent magnetization, susceptibility, and blocking temperature of synthetic magnetite using a superconducting quantum interference device (SQUID). We show how these properties vary in the presence of surface-associated solutes such as tetramethylammonium (TMA(+)) and ferrous (Fe(II)) cations. More importantly, because magnetite readily reacts with O(2) to produce maghemite, we analyzed the effect of oxidation on the magnetic properties of the particles. Because maghemite has a reported magnetic saturation that is less than that of magnetite, we hypothesized that oxidation would decrease the magnitude of the magnetic attractive force between adjacent particles. The presence of TMA(+) and Fe(II) caused a change in the magnetic properties of magnetite potentially because of alterations in its crystalline order. Magnetite oxidation caused a decrease in saturation magnetization, resulting in less significant magnetic interactions between particles. Oxidation, therefore, could lead to the decreased aggregation of magnetite nanoparticles and a potential enhancement of their colloidal stability.

Spin and Orbital Magnetic Moment Anisotropies of Monodispersed Bis(Phthalocyaninato)Terbium on a Copper Surface

Abstract: The magnetic properties of isolated TbPc2 molecules supported on a Cu(100) surface are investigated by X-ray magnetic circular dichroism a t8Ki nmagnetic fields up to 5 T. The crystal field and magnetic properties of single molecules are found to be robust upon adsorption on a metal substrate. The Tb magnetic moment has Ising-like magnetization; XMCD spectra combined with multiplet calculations show that the saturation orbital and spin magnetic moment values reach 3 and 6 µB, respectively.

Hysteresis and unusual magnetic properties in the singular Heusler alloy Ni45Co5Mn40Sn10

Abstract: The alloy Ni45Co5Mn40Sn10 is shown to be singular relative to nearby alloys in three following ways: (1) The austenite has remarkably high magnetization (1170 emu/cm3) and low magnetic anisotropy. (2) The thermal hysteresis is near minimum. (3) The transformation temperature ∼135 °C is unusually high. Because the unusually large magnetization and low hysteresis is seen at relatively small applied fields, applications such as magnetic shape memory, energy conversion, and solid state refrigeration may become practical.

Effect of doping on the morphology and multiferroic properties of BiFeO3 nanorods

Abstract: In this study we report the synthesis of BiFeO3 nanorods using a sonochemical technique. The nanorods had a diameter of 20–50 nm, a length of 100–500 nm and exhibit aspect ratios in the range of 5–10. However, after doping, the TEM images of Bi0.9Ba0.1Fe0.9Mn0.1O3 and Bi0.9Ca0.1Fe0.9Cr0.1O3 samples show that the aspect ratios of both the double doped samples have reduced considerably, while retaining the crystallinity of the particles. BiFeO3 nanorods show a weak ferromagnetic order at room temperature, which is quite different from the linear M–H relationship reported for bulk BiFeO3. The saturation magnetization of these BiFeO3 nanostructures has been found to increase on doping with various metal ions (Ba2+, Ca2+, Mn2+, Cr3+), reaching a maximum value of 1.35 emu g−1 for the Bi0.9Ba0.1Fe0.9Mn0.1O3 nanostructures. However, saturation of electric polarization was observed only in case of the Bi0.9Ca0.1Fe0.9Cr0.1O3 nanostructures.

Liquid state DNP using a 260 GHz high power gyrotron.

Abstract: Dynamic nuclear polarization (DNP) at high magnetic fields (9.2 T, 400 MHz 1H NMR frequency) requires high microwave power sources to achieve saturation of the EPR transitions. Here we describe the first high-field liquid-state DNP results using a high-power gyrotron microwave source (20 W at 260 GHz). A DNP enhancement of −29 on water protons was obtained for an aqueous solution of Fremy's Salt; in comparison the previous highest value was −10 using a solid-state microwave power source (maximum power 45 mW). The increased enhancements are partly due to larger microwave saturation and elevated sample temperature. These experimentally observed DNP enhancements, which by far exceed the predicted values extrapolated from low-field DNP experiments, demonstrate experimentally that DNP is possible in the liquid state also at high magnetic fields.

Inverted hysteresis and giant exchange bias in La0.7Sr0.3MnO3/SrRuO3 superlattices

Abstract: The magnetization reversal mechanisms in a La0.7Sr0.3MnO3/SrRuO3 superlattice with ultrathin individual layers were studied. Due to the strong exchange bias between La0.7Sr0.3MnO3 and SrRuO3 layers inverted hysteresis loops were observed at temperatures below 62 K; at higher temperatures the superlattice showed an unconventional reversal mechanism with the magnetically hard SrRuO3 layers switching first on reducing the magnetic field from saturation. These observations were corroborated by micromagnetic simulations and were interpreted as arising from interfacial Bloch walls.

Enhanced hardening of soft self-assembled copolymer gels under homogeneous magnetic fields

Abstract: The rheological response of highly swollen physical gels obtained by self-assembling of triblock copolymers containing low remanence ferromagnetic particles was investigated in the presence of external homogeneous magnetic fields. Three different types of sample geometries with distinctive magnetic particle orderings were investigated: isotropic (no magnetic field present during synthesis), parallel to the plane of the gel film and perpendicular to the plane of the gel film. Both the storage and loss moduli exhibit a strong increase with magnetic field strength for all geometries. Dependence of the rheological response on particle volume fraction was also investigated. The strength of such rheological hardening, as well as its saturation behaviour, depend strongly on the relative orientation between particle strings, shear and external field. In some cases a very strong relative increase of storage modulus, up to 6000% was obtained. Further transient rheological studies suggest that strong rearrangement of the particle network is largely responsible for the enormous increase in elastic modulus. Parallel to that, a maximum in the loss factor was observed as a function of particle volume fraction and field strength and it was interpreted in terms of a competition between an increase in string (clusters) hardening and a decrease in their ability to deform and flow. These results suggest that magnetorheological gels are an intermediate system between magnetorheological elastomers (MREs) and magnetorheological fluids (MRFs) with directional dependent rheological response and partial rearrangement of the particle network.

Role of vacancies in transport and magnetic properties of nickel ferrite thin films

Abstract: Nickel ferrite thin films were synthesized by pulsed laser deposition. It was determined that the monotonic increase in saturation magnetization and the non-monotonic increase in electrical conductivity depend on the oxygen partial pressure during the growth of the thin films. A substantial reduction in magnetization was found which ranged between 0.4% and 40% of the bulk value as the oxygen partial pressure increased from 0.2 × 10−6 Torr to 500 mTorr during the deposition of the films. There was a three orders of magnitude increase in conductivity for the sample prepared under the most oxygen deficient environment (partial pressure of oxygen 0.2 × 10−6 Torr). These variations in saturation magnetization and conductivity are described within the framework of cation/oxygen vacancies in an inverse spinel nickel ferrite structure. The changes in the electronic structure due to the presence of the vacancies were investigated using x-ray photoelectron spectroscopy, which confirmed the formation of lower valent Ni for the samples prepared in an oxygen deficient atmosphere.

New high magnetic field phase of the frustrated $S=1/2$ chain compound LiCuVO$_4$

Abstract: Magnetization of the frustrated $S=1/2$ chain compound LiCuVO$_4$, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2).

High-performance Fe/SiO2 soft magnetic composites for low-loss and high-power applications

Abstract: High-performance Fe/SiO2 soft magnetic composites (SMCs) by coating pure iron particles with amorphous SiO2 layers have been fabricated by controlled hydrolysation of tetraethyl orthosilicate (TEOS). Microstructural studies show that the pure iron particles are uniformly and entirely coated with the SiO2 insulating layers to form Fe/SiO2 core–shell structure. The dielectric SiO2 layers lead to the electrical insulation behaviour of the SiO2-coated iron particles, which results in low core loss of the compacted Fe/SiO2 SMCs in application of alternating current. The core loss measured at a frequency of 50 Hz and an induction level of 1 T is 3.5 W kg−1, which is much lower than that of the commercial SMCs prepared by coating iron particles with phosphate layers. The Fe/SiO2 SMCs exhibit nearly the same coercivity as that of pure iron, and saturation magnetization of 18.2 kG, a little lower than that of pure iron due to the nonmagnetic SiO2 layers. The low core loss and high saturation magnetization promise the great potential of Fe/SiO2 SMCs in high-power applications. High-frequency measurements reveal that the working frequency of the Fe/SiO2 SMCs can reach up to 20 MHz.

Magnetic and optical studies on polyvinylpyrrolidone thin films doped with rare earth metal salts

Abstract: A casting technique was used to prepare new nanocomposite thin films of pure and GdCl3- or HoCl3-doped polyvinylpyrrolidone (PVP). Transmission electron microscopy (TEM) was used to study the particle size and dispersibility in the investigated nanocomposites. Magnetic measurements, using a vibrating sample magnetometer (VSM), were carried out at room temperature and applied magnetic fields up to 10 kOe. The appearance of hysteresis loops with small values of saturation flux density (Bs), remanent flux density (Br), squareness ratio (SQR) and coercive field (Hc), along with permeability (μ) values relatively higher than 1, may be taken, as a matter of experimentation, to consider the investigated composites as soft magnet materials. In this study, PVP-HoCl3 films showed slightly better magnetic properties compared with PVP-GdCl3 films. A comparative analysis of infrared (IR) and ultraviolet-visible spectra of the parent components and their composites indicated the effective role of dopant nature and concentration in morphological and microstructural changes occurring in the PVP matrix. Optical constants, such as the energy gap (Eg) and the Urbach tail (Ee), were determined and discussed in terms of a model based on the electronic transition between localized states in the band structure. Cast technique was used to prepare new nanocomposite thin films of pure and GdCl3- or HoCl3-doped polyvinylpyrrolidone (PVP) with different dopant concentrations. During magnetic measurements, using a vibrating sample magnetometer, carried out at room temperature and applied magnetic fields up to 10 kOe, thin films of the system PVP-HoCl3 showed slightly better magnetic properties than PVP-GdCl3. A comparative infrared and ultraviolet-visible spectral analysis of the parent components and their nanocomposites provided an indication to the effective role of dopant nature and concentration in morphological and microstructural changes occurring in the PVP matrix.

Magnetic response of nanoscale left-handed metamaterials

Abstract: Using detailed simulations we investigate the magnetic response of metamaterials consisting of pairs of parallel slabs or combinations of slabs with wires including the fishnet design as the length scale of the structures is reduced from millimeter to nanometer. We observe the expected saturation of the magneticresonance frequency when the structure length scale goes to the submicron regime, as well as weakening of the effective permeability resonance and reduction in the spectral width of the negative permeability region. All these results are explained by using an equivalent resistor-inductor-capacitor circuit model, taking into account the current-connected kinetic energy of the electrons inside the metallic parts through an equivalent inductance, added to the magnetic field inductance in the unit cell. Using this model we derive simple optimization rules for achieving optical negative permeability metamaterials with improved performance. Finally, we analyze the magnetic response of the fishnet design and we explain its superior performance regarding the high attainable magnetic-resonance frequency, as well as its poor performance regarding the width of the negative permeability region.

Design of Magnetic Actuator With Nonlinear Ferromagnetic Materials Using Level-Set Based Topology Optimization

Abstract: In the practical design of magnetic actuators, the effect of magnetic saturation usually plays an important role. This paper proposes a new computation approach for identifying the optimal configuration of a magnetic actuator to deal with saturation of the ferromagnetic material. A level-set method for topology optimization in magnetic fields is employed to represent the material boundary considering nonlinear B-H characteristics. Design of magnetic actuators is mathematically formulated as a general optimization problem for maximizing magnetic energy in the air gap between armature and yoke under the limited usage of ferromagnetic material. The nonlinear magnetostatic finite element analysis where transient eddy current effects are ignored and the associated design sensitivity analysis are performed. The movement of the implicit boundaries of the ferromagnetic material is driven by the normal velocity derived from optimality and convergence conditions of level-set equation. The validity and effectiveness of the proposed method are illustrated with 2D examples that are widely used in the literature.

Comparison of Nonoriented and Grain-Oriented Material in an Axial Flux Permanent-Magnet Machine

Abstract: The performance and iron losses of an axial flux permanent-magnet synchronous machine (AFPMSM) using nonoriented (NO) steel are compared with the performance and iron losses of an AFPMSM using grain-oriented (GO) material. The machine is modeled by several 2-D finite element models in circumferential direction, at different radii. The material model for the GO material is an anhysteretic anisotropic model based on the magnetic energy. The magnetic energy is computed by using several measured quasi-static BH-loops on an Epstein frame in seven directions starting from the rolling direction to the transverse direction. The losses are calculated a posteriori, based on the principles of loss separation and dynamic loop measurements. A loss model was made for each of the seven directions, assuming unidirectional fields. In comparison with the more usual NO material, both the saturation induction and the torque are higher with GO material. The magnetic field in the GO material is lower than for NO material in the major part of the iron, but higher in the tooth tips where the field is not in the rolling direction. The stator iron losses are about 7 times lower for the considered GO compared to the NO material.

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Abstract: Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.

Experimental evidence for ferromagnetism at room temperature in MgO thin films

Abstract: Ferromagnetic ordering at room temperature (RTFM) in MgO thin films deposited by RF magnetron sputtering under various atmospheric conditions and temperatures is reported A saturation magnetizatio