Viscous effects on nanoparticle magnetization harmonics

Dynamic magnetization of nanoparticles is a promising means of functional biosensing. We demonstrate a method for rapid monitoring of relative changes in the viscous environment of superparamagnetic nanoparticles using a ratio of magnetization harmonics. Initial results are capable of detecting changes in viscosity on the order of 0.05 cP. This technique should allow for real-time monitoring of many of the dynamic magnetization applications which have been proposed using AC susceptibility. Further, incorporation of this technique into a harmonic-based imaging system like magnetic particle imaging would allow for in vivo functional imaging capabilities.     

Superparamagnetic silica nanoparticles with immobilized metal affinity ligands for protein adsorption

Superparamagnetic silica-coated magnetite (Fe₃O₄) nanoparticles with immobilized metal affinity ligands were prepared for protein adsorption. First, magnetite nanoparticles were synthesized by co-precipitating Fe²⁺ and Fe³⁺ in an ammonia solution. Then silica was coated on the Fe₃O₄ nanoparticles using a sol–gel method to obtain magnetic silica nanoparticles. The condensation product of 3-Glycidoxypropyltrimethoxysilane (GLYMO) and iminodiacetic acid (IDA) was immobilized on them and after charged with Cu²⁺, the magnetic silica nanoparticles with immobilized Cu²⁺ were applied for the adsorption of bovine serum albumin (BSA). Scanning electron micrograph showed that the magnetic silica nanoparticles with an average size of 190 nm were well dispersed without aggregation. X-ray diffraction showed the spinel structure for the magnetite particles coated with silica. Magnetic measurement revealed the magnetic silica nanoparticles were superparamagnetic and the saturation magnetization was about 15.0 emu/g. Protein adsorption results showed that the nanoparticles had high adsorption capacity for BSA (73 mg/g) and low nonspecific adsorption. The regeneration of these nanoparticles was also studied.     

Magnetic susceptibility curves of a nanoparticle assembly, I: Theoretical model and analytical expressions for a single magnetic anisotropy energy

We study a model system made of non-interacting monodomain ferromagnetic nanoparticles, considered as macrospins, with a randomly oriented uniaxial magnetic anisotropy. We derive a simple differential equation governing the magnetic moment evolution in an experimental magnetic susceptibility measurement, at low field and as a function of temperature, following the well-known Zero-Field Cooled/Field Cooled (ZFC/FC) protocol. Exact and approximate analytical solutions are obtained, together for the ZFC curve and the FC curve. The notion of blocking temperature is discussed and the influence of various parameters on the curves is investigated. A crossover temperature is defined and a comparison is made between our progressive crossover model (PCM) and the crude “two states” or abrupt transition model (ATM), where the particles are assumed to be either fully blocked or purely superparamagnetic. We consider here the case of a single magnetic anisotropy energy (MAE), which is a prerequisite before considering the more realistic and experimentally relevant case of an assembly of particles with a MAE distribution (cf. part II that follows).     

Superparamagnetic nanocomposites based on poly(styrene-b-ethylene/butylene-b-styrene)/cobalt ferrite compositions

Magnetic nanoparticles were created in or around the sulfonated (s) polystyrene domains in a phase separated poly[styrene-b-(ethylene-co-butylene)-b-styrene)] block copolymer (BCP) using an in situ inorganic precipitation procedure. The sBCP was neutralized with a mixed iron/cobalt chloride electrolyte and the doped samples were converted to their oxides by reaction with sodium hydroxide and further washing with water. Transmission electron microscopy indicated the presence of nanoparticles in the 5–25 nm size range. The metal oxide particle structures were studied using select area electron diffraction, which revealed that they are of the cobalt iron oxide composition (CoFe₂O₄). These nanocomposites were shown, using a superconducting quantum interference device magnetometer, to be superparamagnetic at 300 K and ferrimagnetic at 5 K. Nanocomposites consisting of smaller particles have a blocking temperature of 70 K, whereas it was 140 K for larger particles.     

Magnetic susceptibility curves of a nanoparticle assembly II. Simulation and analysis of ZFC/FC curves in the case of a magnetic anisotropy energy distribution

Starting from the theoretical results established in Tournus and Bonet (2010 [1]) to describe ZFC/FC (zero-field cooled/field cooled) susceptibility curves, we examine the limitations of the widely used two states model (or abrupt transition model) where the magnetic particles are supposed to be either fully blocked or fully superparamagnetic. This crude model appears to be an excellent approximation in most practical cases, i.e. for particle assembly with broad enough size distributions. We improve the usual model by taking into account the temperature sweep existing in experimental measurements. We also discuss a common error made in the use of the two states model. We then investigate the relation between the ZFC peak temperature and the particle anisotropy constant, and underline the strong impact of the size dispersion. Other useful properties of ZFC/FC curves are discussed, such as invariance properties, the reversibility of the FC curve and the link between the susceptibility curves and the magnetic anisotropy distribution. All these considerations lay solid bases for an accurate analysis of experimental magnetic measurements.     

Effect of hexane on magnetic blocking behavior of FePt nanoparticles

In this work effect of the carrier fluid, hexane, on the magnetic properties of 4.7 nm sized FePt nanoparticles is investigated. Nanoparticles are synthesized by chemical method. Structural and magnetic characterizations confirmed that samples are monodispersed with disordered face centered cubic (fcc) crystal structure and, magnetically, exhibit two blocking behaviors; the first is at 27 K and second at 110 K. Carrier fluid of particles, hexane, is found to influence the blocking of 7% of the total magnetic moments in the system by freezing at low temperatures resulting in a two blocking phenomena even for nanoparticles that are monodispersed with narrow particle size distribution.     

Stability of the magnetic domain structure of nanoparticle thin films against external fields

We investigate the effect of external magnetic fields on the magnetic structure of thin films from magnetic nanoparticles (MNP) with dipolar interaction. Such fields are present, for example, if samples are scanned with magnetic probes. Numerical simulations and experimental magnetic force microscopy (MFM) studies are presented. Numerically, we have calculated the magnetization pattern of single-layer and multilayer MNP thin films. The calculations show that unperturbed single-layer MNP films have an in-plane orientation of the magnetization with a flux-closure-domain pattern. An external field generated by a point dipole above the film induces locally an out-of-plane configuration of the magnetization. In the corresponding MFM images, the domain pattern in the film is erased and a stripe-like contrast enhancement at the edges appears. Multilayer films are found to be more robust against external fields than monolayers.     

Heating in the MRI environment due to superparamagnetic fluid suspensions in a rotating magnetic field

In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4 and 7 degree Centigrade above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B₀. Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B₀. Results are presented for the expected temperature increase in small tumors (approximately 1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002-0.01 solid volume fraction) and nanoparticle radii (1-10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful selection of the rotating or sinusoidal field parameters (field frequency and amplitude). The work indicates that it may be feasible to combine low-field MRI with a magnetic hyperthermia system using superparamagnetic iron oxide nanoparticles.     

Interaction effects in magnetic oxide nanoparticle systems

The interaction effects in magnetic nanoparticle system were studied through a Monte Carlo simulation. The results of simulations were compared with two different magnetic systems, namely, iron oxide polymer nanocomposites prepared by polymerization over core and nanocrystalline cobalt ferrite thin films prepared by sol-gel process. The size of the particles in the nanocomposites were estimated to be ∼15 nm with very little agglomeration. The low values of the coercivity obtained from the hysteresis measurements performed confirm that the system is superparamagnetic. SEM studies showed the cobalt ferrite films to have a nanocrystalline character, with particle sizes in the nanometer range. Hysteresis measurements performed on the thin films coated on silicon do not give evidence of the superparamagnetic transition up to room temperature and the coercivity is found to increase with decreasing film thickness. Comparison with simulations indicate that the nanocomposites behave like a strongly interacting array where exchange interactions lead to high blocking temperatures, whereas the films are representative of a semi-infinite array of magnetic clusters with weak interactions and thickness-dependent magnetic properties.     

Preparation of a biocompatible magnetic film from an aqueous ferrofluid

Very promising nanoparticles for biomedical applications or in medical drug targeting are superparamagnetic nanoparticles based on a core consisting of iron oxides (SPION) that can be targeted through external magnets. Polyvinyl alcohol (PVA) is a unique synthetic biocompatible polymer that can be chemically cross-linked to form a gel. Biotechnology applications of magnetic gels include biosensors, targeted drug delivery, artificial muscles and magnetic buckles. These gels are produced by incorporating magnetic materials in the polymer composites. In this paper we report the synthesis of an aqueous ferrofluid and the preparation of a biocompatible magnetic gel with polyvinyl alcohol and glutharaldehyde (GTA). HClO₄ was used to induce the peptization since this kind of ferrofluid does not have surfactant. The magnetic gel was dried to generate a biocompatible film.     

Effect of Sn doping on the magnetic and transport properties of LaMnO3+δ

We report the influence of the Sn doping on the magnetotransport properties of the LaMnO_3+δ perovskite. Two series of samples with nominal LaSn_xMn_1−xO_3+δ (I series) and La_{(1−x)/(1+x)}Sn_xMn_1−xO_3+δ (II series) compositions (x=0, 0.025, 0.05 and 0.10) were prepared at T_s=750°C. The M(T) data under 0.01 and 0.5 T for the I series reveal a depressed magnetization as the Sn content increases suggesting the presence of magnetic clusters with a superparamagnetic behavior. Resistivity measurements indicate an insulator material for all Sn content independently of the applied magnetic field. On the contrary, for the II series the M(T) and M(H) data reveal FM behavior and an improvement of the magnetization as Sn increases. These samples show magnetoresistance. The magnetotransport properties are discussed in terms of the presence of A-site cation vacancies.     

Magnetic nanofibers with core (Fe3O4 nanoparticle suspension)/sheath (poly ethylene terephthalate) structure fabricated by coaxial electrospinning

One-dimensional magnetic nanostructures have recently attracted much attention because of their intriguing properties that are not realized by their bulk or particle form. These nanostructures are potentially useful for the application to ultrahigh-density data storages, sensors and bulletproof vest. The magnetic particles in magnetic nanofibers of blend types cannot fully align along the external magnetic field because magnetic particles are arrested in solid polymer matrix. To improve the mobility of magnetic particles, we used magneto-rheological fluid (MRF), which has the good mobility and dispersibility. Superparamagnetic core/sheath composite nanofibers were obtained with MRF and poly (ethylene terephthalate) (PET) solution via a coaxial electrospinning technique. Coaxial electrospinning is suited for fabricating core/sheath nanofibers encapsulating MRF materials within a polymer sheath. The magnetic nanoparticles in MRF were dispersed within core part of the nanofibers. The core/sheath magnetic composite nanofibers exhibited superparamagnetic behavior at room temperature and the magnetic nanoparticles in MRF well responded to an applied magnetic field. Also, the mechanical properties of the nanofiber were improved in the magnetic field. This study aimed to fabricate core/sheath magnetic composite nanofibers using coaxial electrospinning and characterize the magnetic as well as mechanical properties of composite nanofibers.     

Role of dipole-dipole interactions in half-field quantum transitions in magnetic nanoparticles

In order to better understand the origin of “forbidden” quantum transitions observed in superparamagnetic nanoparticles at low magnetic fields, electron magnetic resonance (EMR) studies have been performed at room temperature on iron oxide nanoparticles assembled inside parallel nanosized channels penetrating the anodic alumina membrane. The positions of both the main resonance and “forbidden” (2Q) transitions observed at the half-field demonstrate the characteristic angular dependence with the line shifts proportional to 3cos²θ−1, where θ is the angle between the channel axis and external magnetic field B. This result can be attributed to the inter-particle dipole-dipole interactions within elongated aggregates inside the channels. The angular dependence of the 2Q intensity is found to be proportional to sin²θcos²θ, that is consistent with the predictions of quantum-mechanical calculations with the account for the mixing of states by non-secular inter-particle dipole-dipole interactions. Good agreement is obtained between different kinds of measurements (magnetization curves, line shifts and 2Q intensity), pointing to possibility of the quantum approach to the magnetization dynamics of superparamagnetic objects.     

Structural and magnetic properties of nano nickel-zinc ferrite synthesized by reverse micelle technique

Nanocrystalline nickel-zinc ferrites (Ni_0.58Zn_0.42Fe₂O₄) at different pH values (less than 9.6, 9.6, 10.96, and 11.40) for the alkali-precipitating reaction were synthesized by reverse micelle technique. X-ray diffraction reveals a well-defined nickel-zinc ferrite crystal phase at pH=9.6. Increase in pH value obstructs pure-phase formation and results in partial formation of α-Fe₂O₃. The magnetic behaviour of the samples was studied by superconducting quantum interference device. All the samples show superparamagnetic behaviour at room temperature (300 K) and negligible hysteresis at low temperature (5 K). The low value of saturation magnetization is explained on the basis of spin canting. The high-field irreversibility and shifting of the hysteresis loop detected in single-phase sample has been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 42 K and other two samples have an antiferromagnetic phase (α-Fe₂O₃) coupled to the ferromagnetic phase.     

纳米Ni/SiO2介孔复合体的制备与超顺磁性

采用溶胶－凝胶法制备了Ni^2+的SiO2干凝胶,再通过化学还原得到了纳米Ni/SiO2介孔复合体。从样品的透射电子显微镜观测结果可估算出,介孔复合体中Ni粒子的尺寸约为11－12nm。样品的磁性测量结果表明,与通常的Ni纳米颗粒相比,纳米Ni/SiO2介孔复合体中纳米Ni粒子的粒径在大于理论计算的纳米Ni粒子的临界尺寸时,仍能够保持超顺磁状态。在一定温度范围内,提高还原温度有利于复合体中纳米Ni粒子向超顺磁状态转变。     

Effect of the number of iron oxide nanoparticle layers on the magnetic properties of nanocomposite LbL assemblies

Aqueous colloidal suspension of iron oxide nanoparticles has been synthesized. Z-potential of iron oxide nanoparticles stabilized by citric acid was −35±3 mV. Iron oxide nanoparticles have been characterized by the light scattering method and transmission electron microscopy. The polyelectrolyte/iron oxide nanoparticle thin films with different numbers of iron oxide nanoparticle layers have been prepared on the surface of silicon substrates via the layer-by-layer assembly technique. The physical properties and chemical composition of nanocomposite thin films have been studied by atomic force microscopy, magnetic force microscopy, magnetization measurements, Raman spectroscopy. Using the analysis of experimental data it was established, that the magnetic properties of nanocomposite films depended on the number of iron oxide nanoparticle layers, the size of iron oxide nanoparticle aggregates, the distance between aggregates, and the chemical composition of iron oxide nanoparticles embedded into the nanocomposite films. The magnetic permeability of nanocomposite coatings has been calculated. The magnetic permeability values depend on the number of iron oxide nanoparticle layers in nanocomposite film.     

Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size

In order to better understand the transition from quantum to classical behavior in spin system, electron magnetic resonance (EMR) is studied in suspensions of superparamagnetic magnetite nanoparticles with an average diameter of ∼9 nm and analyzed in comparison with the results obtained in the maghemite particles of smaller size (∼5 nm). It is shown that both types of particles demonstrate common EMR behavior, including special features such as the temperature-dependent narrow spectral component and multiple-quantum transitions. These features are common for small quantum systems and not expected in classical case. The relative intensity of these signals rapidly decreases with cooling or increase of particle size, marking gradual transition to the classical ferromagnetic resonance (FMR) behavior.