Magnetic properties of electroplated wires coated by ferrofluid

Low-frequency magnetic properties of ferromagnetic composite wires were studied with and without coating by ferrofluid. Non-magnetic CuBe wires of 0.1 mm diameter were electroplated with FeCoNi layer of 1 μm thickness. Magnetization curves were measured in the frequency range of 10 Hz–3 kHz. The composite CuBe/FeCoNi/ferrofluid material shows a hysteretic behaviour in a small field. The hysteresis loop of ferrofluid covered electroplated wire is not a simple sum of the ferrofluid “wire” plus non-covered wire signals. It indicates an interaction between magnetic wire and ferrofluid which can be revealed by low-frequency measurements. The combination “electroplated wire/ferrofluid” can be considered as a new type of composite magnetic material consisting of solid magnetic core coated by complementary liquid magnetic material. Low-frequency measurements in presence of ferrofluid can be a useful method to study magnetic properties of ferromagnets.     

X-ray microtomography of field-induced macro-structures in a ferrofluid

X-ray microtomography is used to visualize, in-situ, the three-dimensional nature of the magnetic field induced macro-structures (>1 μm) inside a bulk (∼1 mm diameter) magnetite-particle-mineral oil ferrofluid sample. Columnar structures of ∼10 μm diameter were seen under a 0.35 kG applied magnetic field, while labyrinth type structures ∼4 μm in width were seen at 0.55 kG. The structures have height/width aspect ratios >100. The results show that the magnetite volume fraction is not constant within the structures and on average is considerably less than a random sphere packing model.     

The colloidal stability and core-shell structure of magnetite nanoparticles coated with alginate

The adsorption of alginate (Alg) onto the surface of in water dispersed Fe₃O₄ nanoparticles and zeta potential of alginate-coated Fe₃O₄ nanoparticles have been investigated to optimize the colloidal stability of Alg-coated Fe₃O₄ nanoparticles. The adsorption amount of Alg increased with the decrease of adsorption pH. The zeta potential of Fe₃O₄ nanoparticles shifted to a lower value after adsorption of Alg. The lower adsorption pH was the lower zeta potential of Fe₃O₄ nanoparticles became. The Alg-coated Fe₃O₄ nanoparticles were found to be stabilized by steric and electrostatic repulsions. Those prepared at pH 6 were not stable around pH 5, and those prepared at pH 4 became unstable at pH below 3.5. Alg of M_w 45 kDa was a little bit more adsorbed onto nanoparticles surface than that of M_w 24 kDa. An average Fe₃O₄ core size of 9.3 ± 1.7 nm was found by transmission electronic microscopy. An average hydrodynamic diameter of 30-150 nm was measured by photon correlation spectroscopy. However, an average core size of 10 nm and an average hydrodynamic diameter of 38 nm were estimated from the magnetization curve of the concentrated magnetic fluids (MFs). The maximum available saturation magnetization of MFs was about 3.5 kA/m.     

Suitability of commercial colloids for magnetic hyperthermia

Commercial nanoparticles supplied by Chemicell, Micromod and Bayer-Schering were characterised with regard to their nanocrystalline diameter, hydrodynamic diameter, total iron content and relative ferrous iron content. Additionally, calorimetric measurements were taken using a 900 kHz AC magnetic field of amplitude 5.66 kA/m. It was found that those samples containing relatively high (>18%) ferrous content generated a substantially smaller (12% on average) intrinsic loss power (ILP) than those samples with a lower ferrous content. Two nominally identical Chemicell samples that differed only in their production date showed significantly different ILPs, attributed to a variation in batch-to-batch crystallite sizes. The highest ILP values in the cohort, ca. 3.1 nHm²/kg, were achieved for particles with hydrodynamic diameters of ca. 70 nm and nanocrystalline diameters of ca. 12 nm. These compare favourably with most samples prepared in academic laboratories, although they are not as high as the ca. 23.4 nHm²/kg reported for naturally occurring bacterial magnetosomes.     

Ionic magnetic fluid based on cobalt ferrite nanoparticles: Influence of hydrothermal treatment on the nanoparticle size

Magnetic fluid based on cobalt ferrite nanoparticles was obtained using a hydrothermal treatment added to the Massart procedure. This treatment increases the average size of the nanoparticles from 11.9 to 18.7 nm and also improves the dispersity and crystallinity of the cobalt ferrite particles. The nanoparticles obtained after the hydrothermal treatment were dispersed in aqueous solvent by the classical procedure for ionic magnetic fluids. The ferrofluid thus obtained is stable at pH 7 and may be useful for hyperthermia applications.     

Core-shell structure and magnetic properties of magnetite magnetic fluids stabilized with dextran

The adsorption process of different dextran molecules onto the surface of in water dispersed magnetite nanoparticles has been investigated to optimize the preparation of magnetite magnetic fluids (MMFs). An average magnetite core size of 7.1 nm was found by X-ray diffraction and that of 8 nm was found by transmission electron microscopy for the samples prepared at 90 °C. An average hydrodynamic diameter of 25 nm was observed by scanning electron microscopy and that of 25-300 nm was obtained by photon correlation spectroscopy. The dextran was adsorbed by physical adsorption, a molecular weight of 20 kDa gave the best stability of these MMFs. The shell layer of the particles was weakly negatively charged in buffer solutions of pH values between 5.5 and 9.5. The particles seem to be mainly stabilized by sterical repulsion. The maximum available saturation magnetization of the MMFs was 3.5 kA/m.     

Specific binding of magnetic nanoparticle probes to platelets in whole blood detected by magnetorelaxometry

The binding of monoclonal antibodies labelled with magnetic nanoparticles to CD61 surface proteins expressed by platelets in whole blood samples was measured by magnetorelaxometry. This technique is sensitive to immobilization of the magnetic labels upon binding. Control experiments with previous saturation of the epitopes on the platelet surfaces demonstrated the specificity of the binding. The kinetics of the antibody antigen reaction is accessible with a temporal resolution of 12 s. The minimal detectable platelet concentration is about 2000 μL⁻¹ (sample volume 150 μL). The proportionality of the magnetic relaxation amplitude to the number of bound labels allows a quantification of the antibody binding capacity.     

Synthesis of magnetic rhenium sulfide composite nanoparticles

Rhenium sulfide nanoparticles are associated with magnetic iron oxide through coprecipitation of iron salts with tetramethylammonium hydroxide. Sizes of the formed magnetic rhenium sulfide composite particles are in the range 5.5-12.5 nm. X-ray diffraction and energy-dispersive analysis of X-rays spectra demonstrate the coexistence of Fe₃O₄ and ReS₂ in the composite particle, which confirm the formation of the magnetic rhenium sulfide composite nanoparticles. The association of rhenium sulfide with iron oxide not only keeps electronic state and composition of the rhenium sulfide nanoparticles, but also introduces magnetism with the level of 24.1 emu g^-1 at 14 kOe. Surface modification with monocarboxyl-terminated poly(ethylene glycol) (MPEG-COOH) has the role of deaggregating the composite nanoparticles to be with average hydrodynamic size of 27.3 nm and improving the dispersion and the stability of the composite nanoparticles in water.     

Characterization of energy barrier and particle size distribution of lyophilized ferrofluids by magnetic relaxation measurements

The magnetic properties of a ferrofluid are strongly influenced by its particle size distribution. We analyzed a ferrofluid with an unknown particle size distribution as well as fractionated samples of the original material. The ferrofluid in our investigations consists of a mixture of maghemite and magnetite. We investigated these different samples using temperature-dependent magnetorelaxometry method. The evaluation of the Néel relaxation signal allows us a direct determination of the energy barrier distribution, which is one of the most important parameters of such systems of magnetic nanoparticles. The calculated particle volumes were compared with particle sizes determined by transmission electron microscopy.     

Zn and Zr co-doped M-type strontium hexaferrite: Synthesis,characterization and hyperthermia application

In the present study, hard ferromagnetic (M-type strontium hexaferrite) SrFe₁₂O₁₉ was co-doped by Zn and Zr for magnetic hyperthermia applications. As a result of the high concentration of single domain SrFe₁₂O₁₉ nanoparticles (suspended in the ferrofluid), they found a large hydrodynamic diameter, which caused a long-time Brownian relaxation under the AC magnetic field. On the other hand, increasing the Zn-Zr content (low concentration of SrFe₁₂O₁₉) led to a drop in anisotropy, which coincided with a short-time N´eel relaxation. All of the substituted samples with a multi-disperse state in ferrofluid exhibited an almost equal amount of the N´eel and Brownian effects. Consequently, the magnetic saturation (M_s) was considered as the dominant factor in the specific absorption rate (SAR) of the substituted samples. Transformation to the mono-disperse state was followed by the decrease of the Brownian relaxation time and hence the increase of the SAR. The interesting point in mono-disperse state was the heat generation of pure SrFe₁₂O₁₉ under the AC magnetic field as a result of the decrement of the Brownian relaxation time.     

Magnetite ferrofluid with high specific absorption rate for application in hyperthermia

A magnetite ferrofluid coated by dextran with a high specific absorption rate (SAR) of 75 W/g in an AC field of 55 kHz and 200 Oe was prepared by the gel crystallization method with ultrasonic treatment. For comparison, uncoated magnetite particles with a mean diameter of 50 nm were also fabricated. Several possible mechanisms such as Brownian, Neel and diffusion relaxation processes on heating effects and their influence on SAR are discussed. Several factors which can increase the value of SAR were discussed, including dextran coating, ultrasonic treatment, proper particle size and the presence of defects and disorder in the particles.     

Synthesis of magnetite nanoparticles for AC magnetic heating

Magnetite particles with different average diameter (D_m) suitable for magnetic fluid hyperthermia (MFH) were synthesized by controlled coprecipitation technique. In this method, the reaction pH was stabilized using the pH buffer and the average particle diameter decreased with increasing reaction pH. The size-dependent magnetic behavior of the magnetite nanoparticles was studied and the optimum size range required for magnetic fluid hyperthermia (MFH) has been arrived at. Among the samples studied, the maximum specific absorption rate of 15.7 W/g was recorded for the magnetite sample with D_m of 13 nm, when exposed to an AC magnetic field strength of 3.2 kA/m and a frequency of 600 kHz. The AC magnetic properties suggested that the size distribution of the sample was bimodal with average particle size less than ∼13 nm.     

Magnetic properties of nanoscale crystalline maghemite obtained by a new synthetic route

In this work we describe the synthesis and characterization of maghemite nanoparticles obtained by a new synthetic route. The material was synthesized using triethylamine as a coprecipitation agent in the presence of the organic ligand N,N′-bis(3,5-di-tert-butyl-catechol)-2,4-diaminotoluene (LCH₃). Mössbauer spectrum at 4 K shows typical hyperfine parameters of maghemite and Transmission Electron Microscopy images reveal that the nanoparticles have a mean diameter of 3.9 nm and a narrow size distribution. AC magnetic susceptibility in zero field presents an Arrhenius behavior with unreasonable relaxation parameters due to the strong influence of dipolar interaction. In contrast when the measurements are performed in a 1 kOe field, the effect of dipolar interactions becomes negligible and the obtained parameters are in good agreement with the static magnetic properties. The dynamic energy barrier obtained from the AC susceptibility results is larger than the expected from the average size observed by HRTEM results, evidencing the strong influence of the surface contribution to the anisotropy.     

Ultrasound velocimetry of ferrofluid spin-up flow measurements using a spherical coil assembly to impose a uniform rotating magnetic field

Ferrofluid spin-up flow is studied within a sphere subjected to a uniform rotating magnetic field from two surrounding spherical coils carrying sinusoidally varying currents at right angles and 90° phase difference. Ultrasound velocimetry measurements in a full sphere of ferrofluid shows no measureable flow. There is significant bulk flow in a partially filled sphere (1-14 mm/s) of ferrofluid or a finite height cylinder of ferrofluid with no cover (1-4 mm/s) placed in the spherical coil apparatus. The flow is due to free surface effects and the non-uniform magnetic field associated with the shape demagnetizing effects. Flow is also observed in the fully filled ferrofluid sphere (1-20 mm/s) when the field is made non-uniform by adding a permanent magnet or a DC or AC excited small solenoidal coil. This confirms that a non-uniform magnetic field or a non-uniform distribution of magnetization due to a non-uniform magnetic field are causes of spin-up flow in ferrofluids with no free surface, while tangential magnetic surface stress contributes to flow in the presence of a free surface.Recent work has fitted velocity flow measurements of ferrofluid filled finite height cylinders with no free surface, subjected to uniform rotating magnetic fields, neglecting the container shape effects which cause non-uniform demagnetizing fields, and resulting in much larger non-physical effective values of spin viscosity η′∼10⁻⁸−10⁻¹² N s than those obtained from theoretical spin diffusion analysis where η′≤10⁻¹⁸ N s. COMSOL Multiphysics finite element computer simulations of spherical geometry in a uniform rotating magnetic field using non-physically large experimental fit values of spin viscosity η′∼10⁻⁸−10⁻¹² N s with a zero spin-velocity boundary condition at the outer wall predicts measureable flow, while simulations setting spin viscosity to zero (η′=0) results in negligible flow, in agreement with the ultrasound velocimetry measurements. COMSOL simulations also confirm that a non-uniform rotating magnetic field or a uniform rotating magnetic field with a non-uniform distribution of magnetization due to an external magnet or a current carrying coil can drive a measureable flow in an infinitely long ferrofluid cylinder with zero spin viscosity (η′=0).     

Interaction and size effects in magnetic nanoparticles

Magnetic nanoparticles dispersed as a ferrofluid with volumetric concentrations in the range 0.4 to 10% and sizes ranging from 59–77 Å have been studied via magnetic measurements at room and low temperatures. Remanence measurements have been used to determine switching and coupling effects. Particle size and concentration effects have been investigated and we find that the samples with higher concentrations have larger coercivities than expected due to coupling effects. Interactions have been found to be demagnetising overall as expected for dipolar interacting systems. Surface effects become evident below 10 K when thermal effects are weak.     

Broadband spectroscopy of the electromagnetic properties of aqueous ferrofluids for biomedical applications

This paper presents the results of a broadband spectroscopy study, over the frequency range 1 MHz-2 GHz, of the electromagnetic properties of a ferrofluid consisting of magnetite nanoparticles, with a mean magnetic size of 10 nm, dispersed in water. An innovative measurement approach and apparatus, allowing an accurate determination of the permeability, even in presence of a large permittivity, have been developed to characterize the suspension. The results obtained show a significant magnetic response over the whole analyzed frequency range, with a good agreement with the theoretical models describing the magnetization dynamics of these systems. Moreover, a strong dielectric response has been detected, which is in satisfactory agreement with the models developed to describe the dielectric behavior of charged nanoparticles suspended in aqueous solution. This result implies that measurement techniques able to determine both the permittivity and permeability become mandatory for a reliable determination of the magnetic properties of aqueous ferrofluids. The accuracy of the determined permeability spectrum is estimated to be of the order of few percent, so these results provide a reliable experimental basis to estimate how fruitful the use of magnetic nanoparticles can be in relevant biomedical applications.     

Preparation and rheological studies of uncoated and PVA-coated magnetite nanofluid

Experimental studies of rheological behavior of uncoated magnetite nanoparticles (MNPs)U and polyvinyl alcohol (PVA) coated magnetite nanoparticles (MNPs)C were performed. A Co-precipitation technique under N₂ gas was used to prevent undesirable critical oxidation of Fe²⁺. The results showed that smaller particles can be synthesized in both cases by decreasing the NaOH concentration which in our case this corresponded to 35 nm and 7 nm using 0.9 M NaOH at 750 rpm for (MNPs)U and (MNPs)C. The stable magnetic fluid contained well-dispersed Fe₃O₄/PVA nanocomposites which indicated fast magnetic response. The rheological measurement of magnetic fluid indicated an apparent viscosity range (0.1–1.2) pa s at constant shear rate of 20 s⁻¹ with a minimum value in the case of (MNPs)U at 0 T and a maximum value for (MNPs)C at 0.5 T. Also, as the shear rate increased from 20 s⁻¹ to 150 s⁻¹ at constant magnetic field, the apparent viscosity also decreased correspondingly. The water-based ferrofluid exhibited the non-Newtonian behavior of shear thinning under magnetic field.     

Magnetic lipid nanoparticles loading doxorubicin for intracellular delivery: Preparation and characteristics

Tumor intracellular delivery is an effective route for targeting chemotherapy to enhance the curative effect and minimize the side effect of a drug. In this study, the magnetic lipid nanoparticles with an uptake ability by tumor cells were prepared dispersing ferroso-ferric oxide nanoparticles in aqueous phase using oleic acid (OA) as a dispersant, and following the solvent dispersion of lipid organic solution. The obtained nanoparticles with 200 nm volume average diameter and −30 mV surface zeta potential could be completely removed by external magnetic field from aqueous solution. Using doxorubicin (DOX) as a model drug, the drug-loaded magnetic lipid nanoparticles were investigated in detail, such as the effects of OA, drug and lipid content on volume average diameter, zeta potential, drug encapsulation efficiency, drug loading, and in vitro drug release. The drug loading capacity and encapsulation efficiency were enhanced with increasing drug or lipid content, reduced with increasing OA content. The in vitro drug release could be controlled by changing drug or lipid content. Cellular uptake by MCF-7 cells experiment presented the excellent internalization ability of the prepared magnetic lipid nanoparticles. These results evidenced that the present magnetic lipid nanoparticles have potential for targeting therapy of antitumor drugs.     

Investigation of magnetic active core sizes and hydrodynamic diameters of a magnetically fractionated ferrofluid

In this work we address the question which relates between the size of the magnetically active core of magnetic nanoparticles (MNPs) and the size of the overall particle in the solution (the so-called hydrodynamic diameter d _hyd) exists. For this purpose we use two methods of examination that can deliver conclusions about the properties of MNP which are not accessible with normal microscopy. On the one hand, we use temperature dependent magnetorelaxation (TMRX) method, which enables direct access to the energy barrier distribution and by using additional hysteresis loop measurements can provide details about the size of the magnetically active cores. On the other hand, to determine the size of the overall particle in the solution, we use the magnetooptical relaxation of ferrofluids (MORFF) method, where the stimulation is done magnetically while the reading of the relaxation signal, however, is done optically. As a basis for the examinations in this work we use a ferrofluid that was developed for medicinal purposes and which has been fractioned magnetically to obtain differently sized fractions of MNPs. The two values obtained through these methods for each fraction shows the success in fractioning the original solution. Therefore, one can conclude a direct correlation between the size of the magnetically active core and the size of the complete particle in the solution from the experimental results. To calculate the size of the magnetically active core we found a temperature dependent anisotropy constant which was taken into account for the calculations. Furthermore, we found relaxation signals at 18 K for all fractions in these TMRX measurements, which have their origin in other magnetic effects than the Néel relaxation.     

Magnetodielectric effect in composites of nanodimensional glass and CuO nanoparticles

Nanocomposites comprising CuO particles of average diameter 21 nm coated with 5 nm silica glass containing iron ions were synthesized by a chemical route. An ion exchange reaction at the nanoglass/CuO interface produced iron-doped CuO with copper ion vacancies within the nanoparticles. Room temperature ferromagnetic-like behavior was observed in the nanocomposites. This was ascribed to uncompensated spins contributed by Fe ions with associated copper ion vacancies. A rather high value of magnetodielectric parameter in the range 16–26% depending on the measuring frequency was exhibited by these nanocomposites at a magnetic field of 10 KOe. This was caused by a magnetoresistance of 33% in the iron doped CuO nanoparticles. The experimental results were fitted to the Maxwell–Wagner Capacitor model developed by Catalan. These materials will be suited for magnetic sensor applications.