Magnetic Moment Of Particle Research Articles

Brownian Dynamics Simulations for Investigating Aggregate Structures in a Suspension Composed of Rod-Like Hematite Particles

We have investigated aggregation phenomena in a suspension composed of rod-like hematite particles, which have a magnetic moment normal to the particle axis direction, by means of Brownian dynamics simulations. The present Brownian dynamics method takes into account the spin rotational Brownian motion around the particle axis in addition to the ordinary translational and rotational Brownian motion. First, the validity of the present simulation method has been clarified by comparing with results obtained by the fully-established Monte Carlo method. Then, from performing the simulations, we have investigated the influences of magnetic particle-field and particle-particle interactions, shear rate and volumetric fraction of particles on particle aggregation phenomena. Snapshots of aggregate structures are used for qualitative discussion and cluster size distribution, radial distribution function and orientational correlation functions of the direction of particle axis and magnetic moment are focused on for quantitative discussion. The main results obtained here are summarized as follows. The raft-like clusters are significantly formed at a magnetic particle-particle interaction much larger than that for a magnetic spherical particle suspension. This is because the rotational Brownian motion has a significant influence on the formation of clusters for a suspension composed of rod-like particles with large aspect ratio. An applied magnetic field enhances the formation of raft-like clusters. A shear flow does not have a significant influence on the internal structures of clusters, but influences the cluster size distribution of raft-like clusters. The volumetric fraction of particles has a significant effect on the growth of raft-like clusters and clear cluster formation appears from a volumetric fraction between 0.01 and 0.05 in the present simulation circumstances.

Time quantification for Monte Carlo modeling of superparamagnetic relaxation

Monte Carlo modeling of superparamagnetic kinetics is addressed with regard to quantification of the simulation steps in units of real time. For the classical N\'eel case (field-free relaxation of the magnetic moment in a single-domain uniaxial particle), a modification of the Nowak-Chantrell-Kennedy formula is proposed enabling the calculations with the variation amplitude of the magnetic moment independent of the particle anisotropy. This relation is obtained from comparison of the Monte Carlo simulated relaxation against the solution of the kinetic equation for the particle magnetic moment. The same idea is applied to a strongly nonequilibrium process: steady re-magnetization of a superparamagnetic particle under an ac field. It is found that in the latter case a linear relationship can be established between the number of simulation steps and the real-time scale (the field period) that holds fairly well in wide frequency and anisotropy parameter intervals.

Magnetic moment nonconservation in magnetohydrodynamic turbulence models

The fundamental assumptions of the adiabatic theory do not apply in the presence of sharp field gradients or in the presence of well-developed magnetohydrodynamic turbulence. For this reason, in such conditions the magnetic moment μ is no longer expected to be constant. This can influence particle acceleration and have considerable implications in many astrophysical problems. Starting with the resonant interaction between ions and a single parallel propagating electromagnetic wave, we derive expressions for the magnetic moment trapping width Δμ (defined as the half peak-to-peak difference in the particle magnetic moments) and the bounce frequency ω(b). We perform test-particle simulations to investigate magnetic moment behavior when resonance overlapping occurs and during the interaction of a ring-beam particle distribution with a broadband slab spectrum. We find that the changes of magnetic moment and changes of pitch angle are related when the level of magnetic fluctuations is low, δB/B(0) = (10(-3),10(-2)), where B(0) is the constant and uniform background magnetic field. Stochasticity arises for intermediate fluctuation values and its effect on pitch angle is the isotropization of the distribution function f(α). This is a transient regime during which magnetic moment distribution f(μ) exhibits a characteristic one-sided long tail and starts to be influenced by the onset of spatial parallel diffusion, i.e., the variance <(Δz)(2)> grows linearly in time as in normal diffusion. With strong fluctuations f(α) becomes completely isotropic, spatial diffusion sets in, and the f(μ) behavior is closely related to the sampling of the varying magnetic field associated with that spatial diffusion.

Effect of distributed particle magnetic moments on the magnetization of NiO nanoparticles

Magnetization of NiO nanoparticles, at different temperatures, is measured and analyzed taking into account a distribution in particle magnetic moment. We find that disregarding this distribution in the analysis is the reason for the many anomalous observations reported on this system in the literature.

Size effects on gamma radiation response of magnetic properties of barium hexaferrite powders

Little is currently known about the effects of gamma-ray irradiation on oxide magnet materials. In particular, the effect of particle size on radiation susceptibility was investigated. Two commercial powders of BaFe12O19 were thoroughly characterized, then exposed to 1 MGy of gamma radiation from a 60Co source. AC susceptibility and DC magnetometry and Mössbauer spectroscopy were performed after irradiation and compared to pre-irradiated measurements. DC magnetization and AC susceptibility decreased for both samples with the relative change of DC magnetization being larger for the micrometer-sized particles and the relative change of the AC susceptibility being larger for the nanometer-sized particles. Mössbauer spectroscopy indicated a decrease in both the hyperfine fields and in their distribution for each Fe site, particularly in the larger particle sample. Decreases in susceptibility are believed to be due to radiation-induced amorphization at the particle surfaces as well as amorphization and nucleation of new crystallites at internal crystallite boundaries, resulting in overall reduction in the particle magnetic moment. This radiation damage mechanism is different than that seen in previous studies of neutron and heavy ion irradiation of BaFe12O19.

Positive magnetoresistance in granular magnetic films with perpendicular anisotropy

Magnetic and magneto-transport properties of granular ferromagnetic Cox-(Al2O3)1-x films with a large range of cobalt concentrations x are investigated. The films with x below the percolation threshold xp demonstrate the presence of oriented uniaxial anisotropy of the granules with the anisotropy field Ha ∼ 2 kOe and easy anisotropy axis perpendicular to the film plane. It is shown that for the values of x close but below xp, field dependencies of the magnetoresistance in the magnetic fields applied in the film plane, possess an anomalous field dependence with a positive magnetoresistance and a maximum in the range of field magnitudes 0 &amp;lt; H &amp;lt; Ha. The positive magnetoresistance effect is isotropic in the film plane and reduces as the field direction is rotated out of the film plane up to the complete quenching for the field perpendicular to the film plane. The effect is not related to an anisotropic magnetoresistance manifestation and is temperature-dependent. We propose a model, which explains the phenomenon and bases on an accounting of thermal fluctuations of uniaxial particle magnetic moments in the fields perpendicular to the particle easy anisotropy axis in the presence of the ferromagnetic interaction between the particles.

Giant magnetostriction in magnetite nanoparticles

Typically the value of the magnetostrictive coefficient (λ) observed for bulk magnetic materials such as cubic ferrites is 10−6. However, giant magnetostriction (λ ≤ 10−3) is only observed in a few bulk intermetallic materials based on alloys of rare earth and iron such as TbFe, TbFe2, DyFe2 and Terefenol-D. While giant magnetostriction is known in nanostructured films, we show for the first time, this phenomenon occurs in magnetic nanoparticles. By using in-field small angle X-ray scattering (SAXS) as a tool, we demonstrate that a 4% relative change in dimension of the particle can be observed in 5.0 nm Fe3O4 nanoparticles at room temperature with 1 kG magnetic field. Also, we propose that the observed values are due to interaction effects and magnetoelastic coupling of particle magnetic moments and external magnetic field.

PEG coating reduces NMR relaxivity ofMn0.5Zn0.5Gd0.02Fe1.98O4hyperthermia nanoparticles

To investigate both T1 and T2 MR relaxation enhancement of Gd substituted Zn-Mn ferrite magnetic nanoparticles. Both uncoated and polyethylene glycol (PEG) coated particles were used. Chemical co-precipitation was used to synthesize particles in the form Mn(0.5)Zn(0.5)Gd(0.2)Fe(1.98)O(4) suitable for hyperthermia applications. Physical characterization of the magnetic nanoparticles included SEM, TEM, ICP, and SQUID. T1 and T2 measurements were performed at 1.5 Tesla (T). The saturation magnetization was 12.86 emu/g while the particle's magnetic moment was 1.86 × 10(-19) J/T. The particle size increased due to coating, while 1/T1 and 1/T2 relaxivities (26°C) decreased from 2.5 to 0.7 and from 201.3 to 76.6 s(-1) mM(-1), respectively, at a magnetic field 1.5T. The reduction in both 1/T1 and 1/T2 is attributed to increased distance of closest approach between the protons and the magnetic core caused by the shielding provided by the high molecular weight PEG. 1/T2 data are compared with existing theoretical models using a modified radius that takes into account both possible agglomeration of the particles and increased inter-particle separation induced by PEG coating.

Magnetized particle capture cross section for braneworld black hole

Capture cross section of magnetized particle (with nonzero magnetic moment) by braneworld black hole in uniform magnetic field is studied. The magnetic moment of particle was chosen as it was done by de Felice and Sorge (Class. Quantum Gravity 20:469, 2003) and for the simplicity particle with zero electric charge is chosen. It is shown that the spin of particle as well as the brane parameter are to sustain the stability of particles circularly orbiting around the black hole in braneworld i.e. spin of particles and brane parameter try to prevent the capture by black hole.

The application of the spin label method for studying the adsorption of macromolecules on magnetic nanoparticles

A procedure for using spin-label EPR spectroscopy in studies of the adsorption of macromolecular systems on magnetic nanoparticles and quantitative characterization of adsorption layers was developed. The procedure was based on measurements of the intensity of EPR signals of stable nitroxide radical labels covalently bound with macromolecules. The adsorption of spin-labeled macromolecules on the surface of nanoparticles caused broadening of EPR lines as a result of the dipole interaction of unpaired radical electrons with the magnetic moment of particles, which decreased signal intensity. Multilayer adsorption in aqueous media of biological fibrinogen molecules and synthetic polymer (polyethyleneimine) on magnetite particles with a mean diameter of 17 nm was studied. The method was shown to be applicable to the estimation of the amount of adsorbed macromolecules in multilayer coatings up to ∼40 nm thick.

Elaboration of superparamagnetic cobalt–ferrite nanocomposites from films of chitosan chelates

AbstractCobalt–ferrite nanocomposites were synthesized from polymeric films of chelates of Co(II) and Fe(III) ions within a chitosan matrix by a solid‐state coprecipitation reaction with weight content ratios of chitosan to cobalt–ferrite of 50/50 and 25/75 w/w. Morphological and crystalline studies of the composites were performed by high‐resolution transmission electron microscopy, X‐ray diffraction, and selected area electron diffraction with a nanobeam diffraction probe. The results show nanoparticles around 4 nm with a spinel structure, consistent with the cobalt–ferrite phase. The magnetic behavior was evaluated with curves of the applied‐field‐dependent magnetization [M(H)] and the temperature‐dependent magnetization [M(T)]. Both the M(H) and M(T) curves showed typical superparamagnetic behavior, depicting an absence of hysteretic characteristics and the characteristic peak at blocking temperature in the zero‐field‐cooled curve. There was also evidence of strong interparticle and intraparticle interactions, which suggested magnetic frustration in the particle magnetic moment alignment with the applied field. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Unusual magnetism in templated NiS nanoparticles

Nanostructured NiS was prepared by inclusion into anodic alumina templates. Theresultant particles were found to be stoichiometric and highly crystalline. The particlesdisplayed small particle superparamagnetism, and a low temperature (at 48 K(Tsg)) spin-freezing phenomenon (a spin-glass) and higher temperature (170 K) thermal blockingof small particle magnetic moment fluctuations were both observed for the firsttime for a sulfide material. Very unusually, these NiS materials are quite distinctfrom antiferromagnetic nanoparticulate sulfide materials, as they display a hightemperature ferromagnetic-like phase. The saturation magnetization, the remanentmagnetization, the coercivity and the ferromagnetic mass susceptibility were measured as0.58 emu g − 1 (at 100 K),0.19 emu g − 1, 219.5 Oe (at170 K) and ∼ 900 × 10 − 6 emu Oe − 1 g − 1 respectively and these are consistent with a moderately strong ferromagnetism. Thematerials had an unexpectedly high Curie temperature of 390 K. The decrease of thesaturation magnetization value at 30 K suggests that the ferromagnetic response is a surfacephenomenon and the high coercivity of the paramagnetic component well aboveTsg suggests that the core can be described as superparamagnetic.

Bidisperse monolayers: Theory and computer simulations

We try to elucidate the microstructure formation in a bidisperse ferrofluid monolayer and understand in detail the difference brought by the geometrical constrains. The system under study consists of soft-sphere magnetic dipolar particles confined to a thin fluid layer. The positions of the particles are constraint to a 2D geometry, whereas the particle magnetic dipole moments are not fixed to the body systems, and are free to rotate in 3 dimensions, hence forming in what we call a quasi-2D geometry (q2D). Unlike the q2D monodisperse case studied in [1] we discover that the presence of small particles inhibit ring formation. Unlike the bidisperse system in bulk thoroughly investigated in [2], small particles can form clusters and can appear in various amounts in the clusters formed by large particles. Finally we come to the conclusion, that geometrical constraints play a crucial role in determining the ferrofluid microstructure, and thus, the direct extrapolation of experimental results obtained for q2D systems to the bulk magnetic fluids might be misleading.

Stochastic dynamics of magnetization in a ferromagnetic nanoparticle out of equilibrium

We consider a small metallic particle (quantum dot) where ferromagnetism arises as a consequence of Stoner instability. When the particle is connected to electrodes, exchange of electrons between the particle and the electrodes leads to a temperature- and bias-driven Brownian motion of the direction of the particle magnetic moment. Under certain conditions this Brownian motion is described by the stochastic Landau-Lifshitz-Gilbert equation. As an example of its application, we calculate the frequency-dependent magnetic susceptibility of the particle in a constant external magnetic field, which is relevant for ferromagnetic resonance measurements.

Magnetization dynamics of non-dilute ferrofluids

The paper is devoted to theoretical studies of the magnetodipolar interparticle interaction effect on magnetization dynamics in moderately concentrated ferrofluids. We consider a homogeneous (without particle aggregates) ferrofluid consisting of identical spherical particles and employ a rigid dipole model, where the particle magnetic moment is fixed with respect to the particle itself. For the magnetization relaxation after an instant change of the external field, we show that the magnetodipolar interaction leads to the increase of the initial magnetization relaxation time. For the complex ac-susceptibility χ we find that the this interaction leads to an overall increase of its imaginary part χ" and shifts the χ" – peak towards lower frequencies. Comparing results obtained with our analytical approach to numerical simulation data, we demonstrate that the theory provides good quantitative agreement with numerical simulations up to the particle volume fraction ϕ ∼ 6 – 10%

Elementary transitions and magnetic correlations in two-dimensional disordered nanoparticle ensembles

The magnetic relaxation processes in disordered two-dimensional ensembles of dipole-coupled magnetic nanoparticles are theoretically investigated by performing numerical simulations. The energy landscape of the system is explored by determining saddle points, adjacent local minima, energy barriers, and the associated minimum energy paths (MEPs) as functions of the structural disorder and particle density. The changes in the magnetic order of the nanostructure along the MEPs connecting adjacent minima are analyzed from a local perspective. In particular, we determine the extension of the correlated region where the directions of the particle magnetic moments vary significantly. It is shown that with increasing degree of disorder, the magnetic correlation range decreases, i.e., the elementary relaxation processes become more localized. The distribution of the energy barriers and their relation to the changes in the magnetic configurations are quantified. Finally, some implications for the long-time magnetic relaxation dynamics of nanostructures are discussed.

Superparamagnetic iron oxide nanoparticles – Proton Nuclear Magnetic Resonance Dispersion curves

Superparamagnetic nanoparticles are widely used as contrast agents for magnetic resonance imaging. We present the Proton Nuclear Magnetic Resonance Dispersion curves for colloidal suspension of iron oxide nanoparticles used as contrast agents. The systems studied are composed of iron oxide nanoparticles of two different sizes, 80–150 nm dextran coated and 300–400 nm silicon coated. Previous studies show that the longitudinal relaxation time dispersion as a function of the proton Larmor frequency is not easily obtained for the aqueous colloidal suspension of 300–400 nm diameter nanoparticles. We obtained this system longitudinal relaxation time dispersion over a broad range of magnetic fields in a viscous medium. A theoretical model that accounts for the relaxation rate of water protons under the influence of such colloidal superparamagnetic nanoparticles was fitted to the experimental data of both systems. The fit allows access to characteristic parameters of superparamagnetic nanoparticles such as the Neel relaxation time, the nanoparticle radius, particle's magnetic moment and translational correlation time, important parameters for the contrast agent efficiency.

Magneto-orientational behavior of a suspension of antiferromagnetic particles

A theory describing magneto-orientational properties of suspensions containingantiferromagnetic nanoparticles is developed. Due to their small size, these particlespossess, apart from an anisotropic magnetic susceptibility pertinent to antiferromagnets, aspontaneous magnetic moment caused by sublattice decompensation. In a colloid subjectedto a DC field of increasing strength an orientational crossover takes place: the particlemagnetic moments, initially aligned along the field, turn to the transverse orientation. Thisbehavior considerably changes the observable characteristics of the system: the spectrum oflinear dynamic susceptibility and the integral time of magnetic relaxation under a pulsefield.

Magnetically induced hyperthermia: size-dependent heating power ofγ-Fe2O3 nanoparticles

By combining magnetic properties with nanosized biocompatible materials,superparamagnetic nanoparticles may serve as colloidal heating mediators for cancertherapy. This unique potential has attracted attention for designing new magneticnanoparticles with high efficiency heating properties. Their heating power underhigh frequency magnetic field is governed by the mechanisms of magnetic energydissipation for single-domain particles due both to internal Néel fluctuations ofthe particle magnetic moment and to the external Brownian fluctuations. Thesemechanisms are highly sensitive to the crystal size, the particle material, and the solventproperties. Here we explore the heating properties of maghemite particles with largeparticle sizes, in the range 15–50 nm, synthesized through a new procedure whichincludes a hydrothermal process. Particle shape and size distribution, hydrodynamicvolume, and magnetic anisotropy are characterized, respectively, by transmissionelectron microscopy, dynamic magnetically induced birefringence, and ferromagneticresonance. Together with our previous data on low diameter particles (Fortin J Pet al 2007 J. Am. Chem. Soc 129 2628–35), this study provides the whole sizedependence of heating efficiency in the range 5–50 nm and assesses the balance betweenNéel and Brownian contributions to thermal losses. In agreement with theoreticalpredictions, the heating efficiency shows a maximum for an optimal size of about 15 nm.

On the scattering of charged particles in complex current configurations

The scattering of charged particles in the magnetic fields of current sheets with bell-shaped and double-humped current density profiles has been studied analytically and numerically. The particle scattering processes are shown to depend significantly on the structure of the current sheet. Thus, for example, when particles are scattered in double current sheets, a regime under which the compensation of two successive jumps in particle magnetic moments can lead to a quasi-regular type of motion, instead of a chaotic one, can be maintained in the system.