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Inductive sensors (ISs) are extremely sensitive sensing platforms that offer high resolution and currently serve as a relatively popular sensing platform for the detection of metals, nonmetals, and semiconductors. However, in spite of wonderful usability, there is a startling lack of IS implementation as a chemical sensor. Similarly, nanofluids (NFs) have gained more traction in the past decade since the physical properties of base fluids become heightened with the addition of nanoparticles (NPs). Regardless of these advantages, both areas lack studies regarding the behavior of NFs under a magnetic field (B-field). We show how a novel technique using a high-resolution, non-invasive inductance-to-digital converter (LDC) sensor is used to detect different NFs of varying physical properties. The LDC proves highly capable of not only serving as an extremely accurate and precise chemical sensor but also allowing us to determine how exposing several NFs to an inductor's B-field affects particle behavior in solution with extremely low signal and concentration detection limits. The four NF systems contain diamond, rutile, magnetite, and gold NPs where the sensor demonstrated superior sensitivity to gold-enhanced NFs. This exciting finding followed expected theoretical trends based on Faraday's laws of magnetism, and the experimental results were validated with finite element simulations within less than 1.0% error. [ABSTRACT FROM AUTHOR]

Radio, millimeter, and sub-millimeter astronomy experiments as well as remote sensing applications often require castable absorbers with well known electromagnetic properties to design and realize calibration targets. In this context, we fabricated and characterized two samples using different ratios of two easily commercially available materials: epoxy (Stycast 2850FT) and magnetite (F e 3 O 4 ) powder. We performed transmission and reflection measurements from 7 GHz up to 170 GHz with a vector network analyzer equipped with a series of standard horn antennas. Using an empirical model, we analyzed the data to extract complex permittivity and permeability from transmission data; then, we used reflection data to validate the results. In this paper, we present the sample fabrication procedure, analysis method, parameter extraction pipeline, and results for two samples with different epoxy-powder mass ratios. [ABSTRACT FROM AUTHOR]

The magnetic and transport properties of magnetite (Fe3O4) films grown on isostructural substrates (MgAl2O4, MgGa2O4, and CoGa2O4), with varying degrees of lattice mismatches (3.8%, −1.4%, and −0.8%, respectively), have been investigated. A significant reduction in the density of antiphase boundary defects is observed for the Fe3O4 films grown under optimal process conditions on smaller lattice mismatch substrates (MgGa2O4 and CoGa2O4) as compared to MgAl2O4. Correspondingly, films on these substrates show much improved magnetic properties and sharper transition in the resistance values at the Verwey transition. Room temperature magnetic hysteresis and ferromagnetic resonance measurement studies indicate the presence of uniaxial magnetic anisotropy induced by substrate-induced strain in the film. Temperature-dependent transport measurements confirm that film thickness has a larger effect on the sharpness of the Verwey transition temperature than the degree of lattice mismatch with substrates. [ABSTRACT FROM AUTHOR]

Biocompatible nanoparticles, with magnetic cores and optically active shells, acting as multifunctional materials with a core size of 6 nm encapsulated in silver shells of varying thickness were synthesized through a novel single phase microemulsion method. Incorporation of silver shells onto the magnetite core enhances the intensity of the highest luminescence peak observed for magnetite by a significant luminescence enhancement factor. A blue photoluminescence peak observed in the visible region of spectrum brightens further with the increase in the thickness of silver shell. The magnetic properties of these core-shell nanomaterials show superparamagnetic behavior at room temperature, which is a mandatory criterion for MRI contrast enhancement agents. The hyperthermic response of synthesized magnetite nanoparticles elevates its temperature to 43 °C in a sharp span of time, which is above the desired temperature for the therapeutic application of these multifunctional nanomaterials. These excellent optical and magnetic properties, of the material having a size range suitable for cellular uptake, make it a potential candidate for both diagnostic and therapeutic uses in biomedical applications. [ABSTRACT FROM AUTHOR]

In magnetic hyperthermia, the spatial distribution of the magnetic nanoparticles (MNPs) inside a malignant tissue, after ferrofluid injection influences significantly the therapeutic temperature field. The inclusion of any intra-tumoral pressure as a result of ferrofluid injection modifies considerably the MNP distribution within the tumor volume. We show that this effect has crucial importance in the magnetic hyperthermia analysis. The temperature field can be controlled by the infusion parameters such as: (i) the ferrofluid infusion rate and (ii) the needle gauge size of the syringe. This paper describes the temperature field within a malignant tissue as a function of the infusion parameters. The local displacements of the tissue during the ferrofluid infusion process were considered. The therapeutic temperature field developed by the magnetite particles when an external magnetic field was applied depends strongly on the elastic parameters of the malignant tissue. The analytical model developed in this paper provides the optimum dosage of the magnetite which has to be injected within a soft malignant tissue with a large size, to achieve the hyperthermic temperature. An analysis of the spatial MNP distributions and their heating in the linear relaxation theory was done for different ferrofluid infusion parameters. [ABSTRACT FROM AUTHOR]

In magnetic hyperthermia, the temperature control within the malignant tissues is an important step to increase the efficiency of the therapy. A temperature analysis is a good method to improve the heating process of the magnetic particles injected within tissues. This paper analyzes the thermal effects induced within malignant tissues by the magnetic systems like: magnetite and Fe-Cr-Nb-B when an external time-dependent magnetic field is applied. The heat generation by Néel and Brown relaxations was modeled using the thermal and magnetic properties of the Fe-Cr-Nb-B particles experimentally determined. A lognormal particle size distribution was considered for these magnetic systems with dimensions from 5 nm to 30 nm. After their injection at the center of the tumor, according to the solution of the transient convection-diffusion equation in a porous medium, the mass concentration of the particles within ferrofluid has a spatial and temporal distribution. The ferrofluid injection process was modeled using the Brinkman equations. The ferrofluid injection rate during the injection process influences significantly the spatial distribution of the particle concentration and temperature field within tumor. Higher values of the ferrofluid flow rate determine a strong convection of the particles to the tumor center. As a consequence, the temperature gradients within tumor are smaller. The performance in Magnetic Hyperthermia of Fe-Cr-Nb-B magnetic systems is discussed. [ABSTRACT FROM AUTHOR]

X-ray magnetic circular dichroism (XMCD) measurements of iron nano-islands grown on graphene and covered with a Au film for passivation reveal that the oxidation through defects in the Au film spontaneously leads to the formation of magnetite nano-particles (i.e., Fe3O4). The Fe nano-islands (20 and 75 monolayers; MLs) are grown on epitaxial graphene formed by thermally annealing 6H-SiC(0001) and subsequently covered, in the growth chamber, with nominal 20 layers of Au. Our X-ray absorption spectroscopy and XMCD measurements at applied magnetic fields show that the thin film (20 ML) is totally converted to magnetite, whereas the thicker film (75 ML) exhibits properties of magnetite but also those of pure metallic iron. Temperature dependence of the XMCD signal (of both samples) shows a clear transition at TV ≈ 120K consistent with the Verwey transition of bulk magnetite. These results have implications on the synthesis of magnetite nano-crystals and also on their regular arrangements on functional substrates such as graphene. [ABSTRACT FROM AUTHOR]

Magnetite (Fe3O4) was doped with radioactive 111mCd and 111In ions as impurities, and their residence sites and thermal stability were investigated by means of time-differential perturbed angular correlation (TDPAC) spectroscopy. Well-defined TDPAC spectra unequivocally show their sites: Cd ions are stably located only in the tetrahedral A site in all the temperature range of the present observation (77 K-873 K); In ions also specifically occupy the A site at low temperature, but at high temperature part of them select the B site in a reversible fashion. The energy difference between the A and B sites for the accommodation of In was experimentally determined to be 0.119 (9) eV by assuming a Boltzmann distribution for the populations of the 111In probe in the respective sites. The element-dependent thermal stabilities observed for Cd and In were well corroborated by density functional theory calculations. The successful observation of thermally activated site-tosite displacement of impurity In ions in Fe3O4 is reported. [ABSTRACT FROM AUTHOR]

Magnetite (Fe3O4) was doped with radioactive 111mCd and 111In ions as impurities, and their residence sites and thermal stability were investigated by means of time-differential perturbed angular correlation (TDPAC) spectroscopy. Well-defined TDPAC spectra unequivocally show their sites: Cd ions are stably located only in the tetrahedral A site in all the temperature range of the present observation (77 K-873 K); In ions also specifically occupy the A site at low temperature, but at high temperature part of them select the B site in a reversible fashion. The energy difference between the A and B sites for the accommodation of In was experimentally determined to be 0.119 (9) eV by assuming a Boltzmann distribution for the populations of the 111In probe in the respective sites. The element-dependent thermal stabilities observed for Cd and In were well corroborated by density functional theory calculations. The successful observation of thermally activated site-tosite displacement of impurity In ions in Fe3O4 is reported. [ABSTRACT FROM AUTHOR]

We report the successful synthesis of ferrimagnetic magnetite powders made using an arc-discharge method in a partial oxygen atmosphere. X-ray and electron diffraction measurements show that the powders also contain some antiferromagnetic hematite and a small amount of FeO and Fe that has not oxidized. The Raman data show that there is a small fraction of ferrimagnetic maghemite that cannot be seen in the x-ray diffraction data. There is a wide particle size distribution where there are nanoparticles as small as 7 nm, larger faceted nanoparticles, and particles that are up to 25 lm in diameter. The saturation magnetization at high magnetic fields is ∼74% of that found in the bulk magnetite, where the lower value is due to the presence of some antiferromagnetic hematite. The temperature dependence of the saturation magnetization changes at the Verwey transition temperature, and it has a power low dependence with an exponent of 3/2 at low temperatures and 2.23 at high temperatures above the Verwey transition temperature. Electronic transport measurements were made on a cold-pressed pellet and the electrical resistance had an exponential dependence on temperature that may be due to electrostatic charging during tunneling between small nanoparticles. A large magnetoresistance from spin-dependent tunneling between the magnetite particles was observed that reached -9.5% at 120K and 8 T. [ABSTRACT FROM AUTHOR]

The pressure dependence of electrical resistance of single-crystal magnetite (Fe3O4) was measured under quasi-hydrostatic conditions to 100 GPa using low-temperature, megabar diamond-anvil cell techniques in order to gain insight into the anomalous behavior of this material that has been reported over the years in different high-pressure experiments. The measurements under nearly hydrostatic pressure conditions allowed us to detect the clear Verwey transition and the highpressure structural phase. The appearance of a metallic ground state after the suppression of the Verwey transition around 20 GPa and the concomitant enhancement of the electrical resistance caused by the structural transformation to the high-pressure phase form reentrant semiconductingmetallic- semiconducting behavior, although the appearance of the metallic phase is highly sensitive to stress conditions and details of the measurement technique. [ABSTRACT FROM AUTHOR]

In this report, we present the synthesis of heterojunction magnetite nanowires in alumina template and describe magnetic and electrical properties from a single nanowire device for spintronics applications. Heterojunction Au-Fe-Au nanowire arrays were electrodeposited in porous aluminum oxide templates, and an extensive and controlled heat treatment process converted Fe segment to nanocrystalline cubic magnetite phase with well-defined Au-Fe3O4 interfaces as confirmed by the transmission electron microscopy. Magnetic measurements revealed Verwey transition shoulder around 120K and a room temperature coercive field of 90 Oe. Current-voltage (I-V) characteristics of a single Au-Fe3O4-Au nanowire have exhibited Ohmic behavior. Anomalous positive magnetoresistance of about 0.5% is observed on a single nanowire, which is attributed to the high spin polarization in nanowire device with pure Fe3O4 phase and nanocontact barrier. This work demonstrates the ability to preserve the pristine Fe3O4 and well defined electrode contact metal (Au)-magnetite interface, which helps in attaining high spin polarized current. [ABSTRACT FROM AUTHOR]

Magnetic drug delivery system (MDDS) is a technique to effectively accumulate drugs, which are combined with ferromagnetic particles, into the affected area using magnetic force control. This study intends to apply MDDS for immunotherapy by enhancing immune responses by a surface treatment of a ferromagnetic particle. The objective of this study is to give the adjuvant effect to a ferromagnetic particle by the surface treatment with alum, which is known as one of the common adjuvants that activates inflammasome pathway. First, magnetite was prepared as a ferromagnetic particle and coated with alum. Alum-coated magnetite increased the expression of caspase-1, which is an activated indicator of inflammasome, in the culture of human monocyte cell (THP-1 cell). To evaluate the potential of the surface coated particles, the particles were subcutaneously injected to mice with a peptide vaccine. As a result, the antibody titer was increased by the surface coated particles as assessed by ELISA. Although a magnetic force has not yet applied in this study, the administration experiment to mice using magnetic force control is our next step. In conclusion, we modified the immune response to magnetite by coating the surface with alum. This can lead to a clinical application for vaccine therapy in future. [ABSTRACT FROM AUTHOR]

Particle size is normally thought to be a major factor to evaluate MRI performance of contrast agents in biological systems. In this regard, three size-relevant regimes, including motional averaging regime, static dephasing regime, and echo-limited regime, have been well developed. In this study, we find the dispersant, which is often used as the subordinate additive in MRI measurements, is another key factor that determines the application of these three regimes in real systems. Our results show that the identically sized particle systems can separately exhibit static dephasing and echo-limited behaviors merely by altering the dispersants in aqueous solution. [ABSTRACT FROM AUTHOR]

We produced a high-quality hard magnetic Fe81Co2Nb1Nd10B6 alloy by melt spinning without additional treatment. The as-spun ribbons produced at a wheel speed of ~25 m/s had the best hard magnetic properties: a remanence Br, coercive force Hc, and maximum energy product (BH)max of 0.97 T, 676 kA/m, and 140 kJ/m3, respectively. The Nd2Fe14B/α-Fe nanocomposite phases had grain sizes of ~10-30 nm. We investigated how the magnetic properties changed with wheel speed, finding that they depended on the as-spun structure and magnetic structure, which changed upon over-and under-quenching from the melt. [ABSTRACT FROM AUTHOR]

The interface between a ferroelectric Fe3O4 thin film and a semiconducting 0.05 wt. % Nb:SrTiO3 substrate was investigated by ferroelectric, pyroelectric, capacitance, transport, and nonlinear dielectric microscopy measurements. Ferroelectric polarization measurements showed that the magnetite films are ferroelectric with an onset temperature that coincides with the Verwey transition at 120 K and that the ferroelectric domains can be reversed by applying an electric field, but only if the films are grown on nondoped SrTiO3 substrates. Pyroelectric measurements and scanning nonlinear dielectric microscopy showed that a polar state is also present in Pd/Fe3O4/Nb:SrTiO3 heterostructures but the polarization could not be switched by applying an electric field. It is shown that the inability to switch the polarity of magnetite films grown on semiconducting Nb:SrTiO3 substrates is caused by the presence of a Schottky barrier that forms at low temperature at the Fe3O4/Nb:SrTiO3 interface. Systematic capacitance measurements were used to extract the film and interface layer capacitances and construct a quantitatively accurate equivalent circuit model for the Fe3O4/Nb:SrTiO3 heterostructures. [ABSTRACT FROM AUTHOR]

Composite core-shell nanoparticles may have morpho-structural, magnetic, and optical (photoluminescence (PL)) properties different from each of the components considered separately. The properties of Fe3O4-TiO2 nanoparticles can be controlled by adjusting the titania amount (shell thinness). Core-shell nanoparticles were prepared by seed mediated growth of semiconductor (TiO2) through a modified sol-gel process onto preformed magnetite (Fe3O4) cores resulted from the co-precipitation method. The structure and morphology of samples were characterized by X-ray diffraction, transmission electron microscopy (TEM), and high resolution-TEM respectively. X-ray photoelectron spectroscopy was correlated with ICP-AES. Magnetic measurements, optical absorption spectra, as well as PL spectroscopy indicate the presence of a charge/spin transfer from the conduction band of magnetite into the band gap of titania nanocrystals. The process modifies both Fe3O4 and TiO2 magnetic and optical properties, respectively. [ABSTRACT FROM AUTHOR]

We examine the magnetic easy-axis directions of stoichiometric magnetite films grown on SrTiO3:Nb by infrared pulsed-laser deposition. Spin-polarized low-energy electron microscopy reveals that the individual magnetic domains are magnetized along the in-plane <100> film directions. Magneto-optical Kerr effect measurements show that the maxima of the remanence and coercivity are also along in-plane <100> film directions. This easy-axis orientation differs from bulk magnetite and films prepared by other techniques, establishing that the magnetic anisotropy can be tuned by film growth. [ABSTRACT FROM AUTHOR]

Highly conductive (∼105 μΩ cm) Mn doped epitaxial Fe3O4 films were fabricated by reactive sputtering. The larger size of magnetic domains compared to grain size with the increasing Mn content indicates that the partial antiferromagnetic coupling across the antiphase boundaries has been weakened, which was further demonstrated by the smaller exchange bias, faster saturated magnetization, and decreasing exchange interaction JAF. The decrease of antiferromagnetic strength originates from the larger Mn-O bond length than that of Fe-O bond. The first-principle calculation shows that the half-metallic feature (100% spin polarization) of Fe3O4 was unchanged with the incorporation of Mn atoms. [ABSTRACT FROM AUTHOR]

Bimagnetic materials were fabricated by decorating the external surface of rattle-type hollow silica microspheres (which themselves contain metallic cobalt nanoparticles) with magnetite nanoparticles; thus, each magnetic substance was spatially isolated by the silica shell. The amount of magnetite decoration on the co-occluded hollow silica was varied from 1 to 17 mass %. Magnetic and electronic properties of the resulting bimagnetic materials were characterized by superconducting quantum interference device measurements and X-ray absorption spectroscopy, respectively. The ferrous iron in the bimagnetic sample was slightly more oxidized than in the magnetite reference, probably from some charge-transfer because of the SiO2 surface contact, although the overall oxidation state of the samples is very similar to that of magnetite. The temperature dependence of the sample magnetization recorded with Zero Field Cooling and Field Cooling resulted in blocking temperatures for the bimagnetic materials that were close to that of magnetite nanoparticles (176 K) and were lower than that for the bare Co-occluded hollow silica (which was above room temperature). Values of coercive force and exchange bias at 300 K became quite small after decoration with only minimal amounts of magnetite nanoparticles (1-3 mass %) and were lower than those of magnetite. This is the first example of enhancing superparamagnetism by spatial separation of both Co and magnetite magnetic nanoparticles using a thin wall of diamagnetic silica. [ABSTRACT FROM AUTHOR]

Magnetite ultrathin films were grown using different deposition rates and substrate temperatures. The structure of these films was studied using (grazing incidence) x-ray diffraction, while their surface structure was characterized by low energy electron diffraction. In addition to that, we performed x-ray photoelectron spectroscopy and magneto optic Kerr effect measurements to probe the stoichiometry of the films as well as their magnetic properties. The diffraction peaks of the inverse spinel structure, which originate exclusively from Fe ions on tetrahedral sites are strongly affected by the preparation conditions, while the octahedral sites remain almost unchanged. With both decreasing deposition rate as well as decreasing substrate temperature, the integrated intensity of the diffraction peaks originating exclusively from Fe on tetrahedral sites is decreasing. We propose that the ions usually occupying tetrahedral sites in magnetite are relocated to octahedral vacancies. Ferrimagnetic behaviour is only observed for well ordered magnetite films. [ABSTRACT FROM AUTHOR]

Here, we report the synthesis of variable sized magnetite (Fe3O4) nano-hollow spheres in one step template free solvothermal method and their size dependent magnetic and electrical properties. Size of the hollow spheres is varied from 100 nm to 725 nm by changing the concentration of capping agent. Trace of Verway transition is found for all sets of spheres and the Verway transition temperature (TV) increases with increasing size of the spheres. The domain structure of these spheres changes from pseudo single domain to multi domain state as the size increases from 100 nm to 725 nm as evident from Day plots. This change in domain structure also changes the magnetic and electric properties of these spheres. Temperature dependent of high field magnetization of the hollow spheres can be well explained by Bloch's power law with higher than the bulk value of Bloch constant. The Bloch exponent varies from 1.94 to 1.69 with increasing size of the spheres. Frequency dependence of electrical conductivity (σ) shows Jonscher's power law type behaviour and bigger spheres are found to be more conductive than smaller ones due to their multidomain configuration. [ABSTRACT FROM AUTHOR]

We investigate the effect of Co2+ ion doping in magnetite (Fe3O4) on its crystal structure, magnetic properties, and phase stability during air and vacuum annealing. The nanoparticles are prepared by co-precipitation method and the particles are characterized by XRD, small angle x-ray scattering (SAXS), themogravimetric and differential scanning calorimetry (DSC), and vibrating sample magnetometer. The SAXS analysis on the doped samples show the most probable size, shape, and the polydispersity of particles, synthesized with different fractions (0-0.6) of Co2+ ion doping remains almost the same. On increasing cobalt content ferrimagnetic to the antiferromagnetic hematite (α-Fe2O3) phase transformation temperature is found to increase dramatically. For 0.1 fraction of Co2+ metal ion doping, an enhancement of 100 °C in the γ-Fe2O3 to α-Fe2O3 phase transition temperature is observed in the air annealed samples, whereas magnetic nature remains stable up to 1000 °C in vacuum annealed samples. On increasing the cobalt fractions beyond 0.2, air annealed samples show no change in the phase transition temperature. The observed enhancement in the phase transition temperature is attributed to the increased activation energy for phase transformation in presence of Co2+. Further, the DSC results corroborate the finding of an increase in the maghemite to hematite phase transition temperature with increase in cobalt fraction (x). The decrease in enthalpy from 89.86 to 17.62 J g-1 with an increase in cobalt content indicates that the degree of conversion of maghemite to hematite decreases with the cobalt content, which is in good agreement with the Rietveld analysis. The decrease in the Ms value in air annealed sample is attributed to the re-distribution of cations in the tetrahedral and octahedral sites, as the Fe3+A-Fe3+B super-exchange interaction is different from the Co2+A-Fe3+B interaction. These results suggest that a very small percentage of Co2+ metal ion doping can dramatically enhance the thermal stability of magnetic nanoparticles, which will have important consequences on the phase stability of ferrite nanocrystals. [ABSTRACT FROM AUTHOR]

We have studied the oxidation of magnetite to Fe2O3 in an electrolytic cell in which the anode is magnetite and the cathode is platinum. We report cyclic voltammagram data consistent with the hypothesis that magnetite, without oxygen gas production but with hydrogen gas production at the cathode, is occurring. The reaction occurs at a potential at the anode of about 0.3 V vs SCE in 1 M NaOH electrolyte, consistent with colloid experiments which also estimated the equilibrium potential of the hypothesized reaction. Electrode characterization results using BET, XEDS, and macroscopic volume and mass measurements are reported, as well as the measurements of the amount of hydrogen gas generated per unit current. The quantity of gas generated is also consistent with our hypothesis concerning the electrode chemistry. Some samples exhibit evidence of two oxidation reactions occurring at the anode and a possible interpretation of these is also discussed. These results suggest the use of magnetite as an anode in a cell electrolysing water to produce hydrogen gas and Fe2O3. In such an electrolyser, the electrical energy cost of producing hydrogen gas could be significantly lower than the cost in a standard electrolyser. The measured steady state currents, equivalent to about 400 mA/g of magnetite, are too low to make a practical electrolyser. We briefly discuss several ways in which the currents might be increased to the levels required. [ABSTRACT FROM AUTHOR]

Magnetite nanoring with vortex domain structure may form stable magnetic colloid for biomedical applications due to its weak magnetic interaction without superparamagnetic (SPM) limitation. In the present study, we perform three-dimensional (3 D) Landau-Liftshitz-Gilbert (LLG) micromagnetics simulation for magnetite nanorings. The ground state phase diagram and stable vortex area (SVA) as a function of outer diameter (Dout), thickness (T), and inner to outer diameter ratios (β) within 100 nm are obtained. The influence of notch, eccentricity, and crystallographic orientation are taken carefully into consideration. In the SVA, the vortex state is not only the ground state but also the remanence state after in-plane is fully magnetized. In particular, the results suggest that a 20 nm inter-rings distance for a typical magnetite nanoring (Dout = 70 nm, T = 50 nm, and β = 0.6) can achieve the stable colloid based on vortex domain structure. Furthermore, these simulation results have been confirmed experimentally and demonstrated by using phosphorylated-mPEG modified magnetite nanorings. The optimization of magnetite nanorings from both simulation and experiments in this work pave the way to achieve such novel and stable vortex domain based magnetic suspension for various biomedical applications. [ABSTRACT FROM AUTHOR]

Aligned nanocomposite electrospun fibers are investigated for electrical (dc, ac, and dielectric) and thermal properties (10-40 wt.%). This nanocomposite consists of poly(methylmethacrylate) (PMMA) grafted core-shell magnetite (CSM) nanoparticles and PMMA. Electrical properties as a function of tunnel gap (in between the CSM nanoparticles) are studied as the polymer shell stays intact and the 'cores' do not touch each other well above the percolation threshold. The depleted improvement in dc conductivity (σdcwt%) with increasing wt.% (the improvement percentage: σdc10 to σdc20 ≈ 100%; σdc20 to σdc30 ≈ 40% and σdc30 to σdc40 ≈ 1.2%) affirms the non-touching 'cores' of CSM. Interestingly, the observed ac conductive behavior (0.1-13 MHz) in the high end of the frequency range is in clear contrast to that of a typical percolating system, in fact it does not explicitly follow the universal power law. Some of the obtained critical exponents are not accommodated by the universal theory and significantly different from the present theoretical/experimental predictions. Additionally, an improvement in thermal stability of ∼30 °C and an overall increase in glass transition temperature are reported. [ABSTRACT FROM AUTHOR]

We report a study on the mechanical properties of magnetorheological (MR) fluids prepared with porous iron particles with rough surfaces. These particles were obtained by reducing a magnetite precursor in a H2 atmosphere at 400 °C. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Results were compared with those obtained for MR fluids prepared with conventional solid carbonyl iron particles of comparable size. We found significant differences between the rheology of both types of suspensions, and, more importantly, we found that simple available models can predict quantitatively those differences as long as the average density of the particles is known and is used to calculate their effective volume magnetization and the real volume fraction of the MR fluids prepared with them. By doing so, we obtained for both the porous iron suspensions and the solid iron suspensions a single master curve of the dimensionless storage modulus at saturation [G'sat/(μoMs2)] as a function of volume fraction (φ), and a good collapse of the viscosity versus Mason number curves as well as of the yield stress versus applied field curves. Particle porosity is thus an important factor in MR fluids and should be considered in their design and modeling. Finally, the porous iron suspensions also exhibited an atypical thickening behavior that was not observed in the solid iron analogues and that we tentatively ascribe to the rougher surface of the porous particles. [ABSTRACT FROM AUTHOR]

Homogeneous and stable magnetic nanofluids containing iron oxide nanoparticles, α-Fe2O3 (hematite) and Fe3O4 (magnetite) in ethylene glycol, were prepared at concentrations up to 25% in mass fraction. Commercial Hexagonal Scalenohedral-shaped α-Fe2O3 nanoparticles were selected while Fe3O4 nanoparticles were synthesized using a coprecipitation method. The products were characterized by transmission and scanning electron microscopy and x-ray diffraction. The thermal conductivity of both nanofluids was measured as a function of volume fraction and temperature. The results illustrate that the enhanced thermal conductivity of the nanofluids increases with volume fraction but is temperature independent. The experimental results show that both types of nanoparticles in this base fluid present no significant aggregation. These experimental values were also compared with theoretical models. Moreover, the density of these nanofluids was measured as a function of volume fraction, temperature, and pressure. The volumetric behavior of nanofluids containing hematite is closer to the ideal behavior than those using magnetite. [ABSTRACT FROM AUTHOR]

Magnetic nanofluids possess some unique properties that can significantly affect their thermal conductivity. We synthesize monodispersed magnetite (Fe3O4) nanofluids in toluene with the particle size from 4 to 12 nm and obtain aqueous nanofluids by a simple 'one-step' phase transfer. Even without the effect of external field, the magnetic-interaction-induced self-assembled aggregation can still be significant in magnetite nanofluids. Investigation of the microstructures of self-assembled aggregation is carried out by the dynamic light scattering, which unveils the variation of aggregated configurations with particle concentration and time. Based on the calculation from the existing models, the aggregates decrease the thermal conductivity of both themselves and the entire system, mainly due to the less solid contents and weaker mobility compared with the single particles as well as the increase in interfacial thermal resistance. As the manifestation of the aggregation-structure variation, the measured thermal conductivity is of a wavelike shape as a function of particle concentration. The particle coating layers are also of importance in cluster formation so that nanofluid thermal conductivity can be manipulated for some nanofluids by changing the stabilizer used and thus controlling the particle aggregated structures. Due to the effects of temperature, viscosity and coating layers, the thermal conductivity for aqueous system varies in a different way as that for the toluene system. [ABSTRACT FROM AUTHOR]

Chemically synthesized magnetite and poly(thiophene)-coated magnetite nanoparticles and the correlations between their magnetic, structural, and microstructural properties are investigated. A typical superparamagnetic behavior was observed for faceted nanoparticle agglomerates of magnetite and nanocomposite. In nanocomposites, the polymer layer causes a sharp decrease in the spin disorder, which reduces the anisotropy constant significantly. This happens because the intimate contact between magnetite and poly(thiophene) leads to charge transfer from the polymer to the core via polaron interactions, causing a structural rearrangement of the nanoparticles and suppression of the spin movement at the surface. As this dynamic interaction can tune the core dimensions, the magnetic properties of nanocomposites can be tuned by controlling the core size through polymer coating. These characteristics can be exploited to design high-performance magnetically tunable nanodevices and applied in many areas of biomedicine (DNA separation, drug targeting, immune detection, and magnetic nanoparticle hyperthermia in cancer treatment). [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles (NP) of magnetite (Fe3O4) coated with oleic acid (OA) and dodecanoic acid (DA) were synthesized and investigated through transmission electron microscopy (TEM), magnetization M, and ac magnetic susceptibility measurements. The OA coated samples were produced with different magnetic concentrations (78%, 76%, and 65%) and the DA sample with 63% of Fe3O4. Images from TEM indicate that the NP have a nearly spherical geometry and mean diameter ∼5.5 nm. Magnetization measurements, performed in zero-field cooled (ZFC) and field cooled processes under different external magnetic fields H, exhibited a maximum at a given temperature TB in the ZFC curves, which depends on the NP coating (OA or DA), magnetite concentration, and H. The temperature TB decreases monotonically with increasing H and, for a given H, the increase in the magnetite concentration results in an increase in TB. The observed behavior is related to the dipolar interaction between NP, which seems to be an important mechanism in all samples studied. This is supported by the results of the ac magnetic susceptibility χac measurements, where the temperature in which χ′ peaks for different frequencies follows the Vogel–Fulcher model, a feature commonly found in systems with dipolar interactions. Curves of H versus TB/TB(H=0) for samples with different coatings and magnetite concentrations collapse into a universal curve, indicating that the qualitative magnetic behavior of the samples may be described by the NP themselves, instead of the coating or the strength of the dipolar interaction. Below TB, M versus H curves show a coercive field (HC) that increases monotonically with decreasing temperature. The saturation magnetization (MS) follows the Bloch’s law and values of MS at room temperature as high as 78 emu/g were estimated, a result corresponding to ∼80% of the bulk value. The overlap of M/MS versus H/T curves for a given sample and the low HC at high temperatures suggest superparamagnetic behavior in all samples studied. The overlap of M/MS versus H curves at constant temperature for different samples indicates that the NP magnetization behavior is preserved, independently of the coating and magnetite concentration. [ABSTRACT FROM AUTHOR]

A thermal reduction method has been developed to prepare magnetite/hematite nanocomposites and pure magnetite nanoparticlestargeted for specific applications. The relative content of hematiteα-Fe2O3 and magnetiteFe3O4 nanoparticles in the product was ensuredby maintaining proper conditions in the thermal reduction ofα-Fe2O3 powder in the presence of a highboiling point solvent. The structural, electronic, and magneticproperties of the nanocomposites were investigated by57Fe-Mössbauer spectroscopy, x-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at every step of the chemical and thermal treatment. It is established that not all iron ions in the octahedral B-sites of magnetite nanoparticles participate in the electron hopping Fe2+<＝>Fe3+ above the Verwey temperature TV, and that the charge distribution can be expressed as (Fe3+)tet[Fe1.852.5+Fe0.153+]octO4. It is shown that surface effects, influencing the electronic states of iron ions, dominate the vacancy effect, and thus govern the observed specific features of the Verwey transition and magnetic properties. The sharp increase in coercivity observed in magnetite nanoparticles below TV is much stronger than for bulk magnetite. [ABSTRACT FROM AUTHOR]

The time-resolved x-ray diffraction method was applied in situ to investigate the carbothermal reduction of magnetite by carbon black in a 2.45 GHz microwave field. The kinetics of the phase transformation sequence is analyzed within the Kolmogorov–Johnson–Mehl–Avrami formalism. The first reduction stage is the solid-state transition of magnetite to wüstite, followed by the partial conversion of nearly stoichiometric wüstite to iron. The kinetics of the initial reduction of Fe3O4 to primary wüstite is phase boundary controlled. Behind the reaction front, primary wüstite rapidly evolves toward stoichiometric FeO. Cation vacancies and the dynamic clustering of structural defects influence the kinetics of the reduction of the nearly stoichiometric secondary wüstite to porous iron. [ABSTRACT FROM AUTHOR]

In this study we address the role of surface anisotropy on the hysteretic properties of magnetite Fe3O4 nanoparticles and the circumstances yielding both horizontal and vertical shifts in the hysteresis loops. Our analysis involves temperature dependence and particle size effects. Different particle sizes ranging from 2 up to 7 nm were considered. Our theoretical framework is based on a three-dimensional classical Heisenberg model with nearest magnetic neighbor interactions involving tetrahedral (A) and octahedral (B) irons. Cubic magnetocrystalline anisotropy for core spins, single-ion site anisotropy for surface spins, and interaction with a uniform external magnetic field were considered. Our results revealed the onset of low temperature exchange bias field, which can be positive or negative at high enough values of the surface anisotropy constant (KS). Susceptibility data, computed separately for the core and the surface, suggest differences in the hard-soft magnetic character at the core-surface interface. Such differences are KS-driven and depend on the system size. Such a hard-soft interplay, via the surface anisotropy, is the proposed mechanism for explaining the observed exchange bias phenomenology. Our results indicate also that the strongly pinned spins at high enough surface anisotropy values are responsible for both the horizontal and vertical shifts in the hysteresis loops. The dependences of the switching and exchange bias fields with the surface anisotropy and temperature are finally discussed. [ABSTRACT FROM AUTHOR]

Surface spin canting has been studied for high quality magnetite nanoparticles in terms of size and shape uniformity. Particles were prepared by thermal decomposition of organic precursors in organic media and in the presence of oleic acid. Results are compared to spin canting effect for magnetic iron oxide nanoparticles of similar size prepared by coprecipitation and subsequently coated with silica. Magnetic characterization and Mössbauer spectroscopy at low temperature and in the presence of a magnetic field have been used in this study. Transmission electron microscopy images and x-ray diffractograms show that iron oxide nanoparticles synthesized by thermal decomposition are more uniform than those prepared by coprecipitation, and they have higher crystal order. Magnetic measurements show superparamagnetic behavior for both samples at room temperature but particles synthesized by thermal decomposition shows higher saturation magnetization and lower coercivity at low temperature. The imaginary part of the ac susceptibility has been used to support the presence of mainly magnetite instead of maghemite in these iron oxide nanoparticles. Mössbauer measurements with and without field demonstrate surface spin canting, only in the octahedral positions for the coprecipitation particles. However, high synthesis temperature and the presence of oleic acid molecules covalently bonded at the particle surface, accounting for the lack of spin canting in particles prepared by thermal decomposition, which justifies the high saturation magnetization and low coercivity at low temperature. [ABSTRACT FROM AUTHOR]

Bimagnetic FePt/Fe3O4 nanoparticles with core/shell or heterodimer structure have been prepared using a sequential synthetic method. The dimension of both FePt and Fe3O4 was tuned by varying the synthesis parameters. The as-synthesized bimagnetic nanoparticles were superparamagnetic at room temperature. After being annealed in a reducing atmosphere, the FePt/Fe3O4 bimagnetic nanoparticles were converted to a hard magnetic nanocomposite with enhanced energy products due to the exchange coupling between the hard and soft magnetic phases. It was found that the exchange coupling in nanocomposites made from the core/shell nanoparticles is stronger than that from the heterodimer nanoparticles. By tuning the dimensions of the FePt and Fe3O4 phases, the energy product up to 17.8 MGOe was achieved in the annealed nanocomposites, which is 36% higher than the isotropic single-phase FePt counterpart. [ABSTRACT FROM AUTHOR]

A model based on Monte Carlo technique is applied to investigating the superparamagnetic magnetite (Fe3O4) colloidal nanocrystal clusters (CNCs) proposed by Ge et al. [Nano Lett. 7, 3203 (2007)]. In other words, the model investigates the following three aspects of CNCS: the morphology of magnetic particles, the formation of field-induced chainlike patterns, and the induced evolution of the magnetization processes. It is shown that the parameters such as diameter, surfactant molecules per unit, and volume concentration of the magnetic fluid are significant factors that enable one to efficiently manipulate the morphology and magnetization process, which eventually leads to the efficient control of the fabrication and multiple applications. The experiment results also evidenced the presence of this self-assembled chain structures. [ABSTRACT FROM AUTHOR]

We report a strong variation of the magnetic properties and of the antiphase boundaries (APBs) density of Fe3O4(001) epitaxial thin films grown on thin Fe(001) and Cr(001) buffer layers on a MgO(001) substrate in comparison with similar magnetite films grown directly on MgO(001). The magnetization of the Fe/Fe3O4 and Cr/Fe3O4 bilayers presents a more squared hysteresis loop and reach saturation at much lower magnetic field (∼10–15 kOe) than the Fe3O4 thin films of same thickness, which hardly saturate at 70 kOe. The magnetization of the magnetite layers has been estimated on both the Fe- and Cr-based samples and a value close to that of the bulk Fe3O4 has been obtained, which points to the magnetic saturation of the Fe3O4 layer induced by the metallic buffers. Transmission electron microscopy has been used to investigate the possible structural origin of this phenomenon studying the APB network of Fe3O4 in the three types of samples. The calculation of the average domain size reveals that the APB density in the bilayers is substantially reduced in comparison with their counterparts without buffer. The domain sizes obtained in Fe and Cr samples are similar, which points to a structural origin probably due to the strain caused by the lattice misfit between the buffers and the magnetite. A lower APB density statistically reduces the influence of the antiferromagnetic interactions existing across some of the boundaries, achieving the saturation of the Fe3O4 layer at much lower magnetic fields. [ABSTRACT FROM AUTHOR]

We report a strong variation of the magnetic properties and of the antiphase boundaries (APBs) density of Fe3O4(001) epitaxial thin films grown on thin Fe(001) and Cr(001) buffer layers on a MgO(001) substrate in comparison with similar magnetite films grown directly on MgO(001). The magnetization of the Fe/Fe3O4 and Cr/Fe3O4 bilayers presents a more squared hysteresis loop and reach saturation at much lower magnetic field (∼10–15 kOe) than the Fe3O4 thin films of same thickness, which hardly saturate at 70 kOe. The magnetization of the magnetite layers has been estimated on both the Fe- and Cr-based samples and a value close to that of the bulk Fe3O4 has been obtained, which points to the magnetic saturation of the Fe3O4 layer induced by the metallic buffers. Transmission electron microscopy has been used to investigate the possible structural origin of this phenomenon studying the APB network of Fe3O4 in the three types of samples. The calculation of the average domain size reveals that the APB density in the bilayers is substantially reduced in comparison with their counterparts without buffer. The domain sizes obtained in Fe and Cr samples are similar, which points to a structural origin probably due to the strain caused by the lattice misfit between the buffers and the magnetite. A lower APB density statistically reduces the influence of the antiferromagnetic interactions existing across some of the boundaries, achieving the saturation of the Fe3O4 layer at much lower magnetic fields. [ABSTRACT FROM AUTHOR]

In this study, we analyze the effect of surface anisotropy on the magnetic properties of magnetite Fe3O4 nanoparticles on the basis of a core-shell model. Magnetization, magnetic susceptibility, and specific heat are computed over a wide range of temperatures. In our model, we stress on magnetite nanoparticles of 5 nm in diameter which consist of 6335 ions. Our theoretical framework is based on a three-dimensional classical Heisenberg Hamiltonian with the nearest magnetic neighbor interactions between iron ions involving tetrahedral (A) and octahedral (B) sites. Terms dealing with cubic magnetocrystalline anisotropy for core ions, a single-ion site surface anisotropy for those Fe ions belonging to the shell, and the interaction with a uniform external magnetic field are considered. To compute the equilibrium averages, a single-spin movement Monte Carlo–Metropolis dynamics was used. Results reveal the occurrence of low-temperature spin configurations different from those expected for a collinear single-domain ferrimagnetic state, depending on the magnitude and sign of the surface anisotropy constant. A transition to a spike state, with magnetization close to zero, is obtained beyond a certain critical positive surface anisotropy value. Such a transition is not observed for negative values. Moreover, a two-pole magnetic state is developed at sufficiently high negative values. Such differences are explained in terms of the interplay between the superexchange couplings and the easy directions imposed by the surface anisotropy vectors. Our results are summarized in a proposal of phase diagram for the different spin structures as a function of the surface-to-core anisotropy ratio. Lastly, hysteretic behavior is evaluated. Nanoparticles become magnetically harder as the surface anisotropy increases in magnitude, and the way in wich the coercive field changes with this quantity is explicitly shown. [ABSTRACT FROM AUTHOR]

Recently, the application of nanosized magnetite particles became an area of growing interest for their potential practical applications. Nanosized magnetite samples of 36 and 9 nm sizes were synthesized. Special care was taken on the right stoichiometry of the magnetite particles. Mössbauer spectroscopy measurements were made in 4.2–300 K temperature range. The temperature dependence of the intensities of the spectral components indicated size dependent transition taking place in a broad temperature range. For nanosized samples, the hyperfine interaction values and their relative intensities changed above the Verwey transition temperature value of bulk megnetite. The continuous transition indicated the formation of dendritelike granular assemblies formed during the preparation of the samples. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of three kinds of Fe oxide films—Fe3O4, a berthollide type of Fe oxide, and a composite of Fe3O4 and FeO—grown on MgO(001) substrates epitaxially at 423 K by pulsed laser deposition were investigated by magnetoresistance measurement and grazing incidence x-ray diffraction. The Fe3O4 film had the largest negative magnetoresistance and the roughest film-substrate interface. The very close lattice matching of single-phase Fe3O4 and MgO facilitates the formation of antiphase boundaries due to natural growth defects and of a rough interface probably due to cation interdiffusion. [ABSTRACT FROM AUTHOR]

Highly oriented magnetite (Fe3O4) thin films have been produced by reactive sputtering in a mixture of hydrogen and argon. While different phases can be achieved by varying the ratio between hydrogen and argon, single phase magnetite films can be achieved with hydrogen concentration γ=0.75%–1%. For the sample grown at γ=1.0%, a Verwey transition at about 111 K can be seen from the temperature dependence of the resistivity, which is confirmed in the magnetization measurements. Maximum magnetoresistance (MR) of about 13.8% is observed just about the Verwey transition at T=115 K. MR results also suggest strong coupling among Fe3O4 nanoparticles originated from the Ruderman-Kittel-Kasuya-Yosida exchange interaction and dipolar interaction, which requires high order terms of (M/Ms)2 to explain the MR behaviors. However, with the fields applied perpendicular to the plane, MR exhibits a distinct behavior. The MR values under the condition of low fields seem to show a linear relationship with |M/MS|. [ABSTRACT FROM AUTHOR]

We have investigated the magnetization and relaxation dynamics of diluted, freeze-dried superparamagnetic magnetite (Fe3O4) nanoparticle samples with organic shells using a differential fluxgate magnetorelaxometry system. The experimental data were analyzed within the framework of the moment superposition model (MSM), providing information on size and size distribution of particle cores as well as on magnetic properties such as saturation magnetization and anisotropy constant. The MSM was refined by introducing an expression for the Néel time constant depending on magnetic field, anisotropy energy, saturation magnetization, and orientation of the magnetic moment of an individual magnetic nanoparticle (MNP) with respect to the external magnetic field. It is shown that especially the dependence of the magnetization and relaxation curves on magnetizing field and magnetization time provides valuable information for an unambiguous and comprehensive determination of MNP core parameters. All experimental findings could be explained with the refined MSM; however, so far, no self-consistent parameter set was obtained quantitatively explaining all measured data. Deviations from the assumed lognormal distribution of core sizes and yet too simplified expressions for the Néel time constant are discussed as potential reasons for the observed discrepancies. [ABSTRACT FROM AUTHOR]

This paper describes the results of investigations on giant negative magnetoresistance (GMR) in nanocomposite aligned fibers prepared using electrospinning. The nanocomposite contains polymethylmethacrylate (PMMA) matrix and the nanoparticles of polymer (PMMA) grafted magnetite, Fe3O4 (PGM). Even for the low magnetic field (1 T) a GMR of about 50% was observed for low loading (5 wt %) of PGM at room temperature. The fibers were characterized using scanning electron microscope and high resolution transmission electron microscope. X-ray diffraction (XRD) patterns were recorded for clean PMMA fibers, PMMA granules, and PGM nanoparticles. The dc conductivity was calculated from the I-V characteristics for the fibers at room temperature. [ABSTRACT FROM AUTHOR]

Strain relaxation studies in epitaxial magnetite (Fe3O4) thin films grown on MgO (100) substrates using high-resolution x-ray diffraction and cross-sectional transmission electron microscopy reveal that the films remain fully coherent up to a thickness of 700 nm. This thickness is much greater than the critical thickness tc for strain relaxation estimated from mismatch strain. Anomalous strain relaxation behavior of Fe3O4/MgO heteroepitaxy is attributed to the reduction in the effective stress experienced by the film due to the presence of antiphase boundaries (APBs) that enable the film to maintain coherency with the substrate at large thickness. However, the stress accommodation in the film depends upon the nature and density of the APBs. [ABSTRACT FROM AUTHOR]

We have studied the magnetic behavior of dextran-coated magnetite (Fe3O4) nanoparticles with median particle size =8 nm. Magnetization curves and in-field Mössbauer spectroscopy measurements showed that the magnetic moment MS of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or nonsaturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy Keff from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulklike ferromagnetic structure with ordered A and B sublattices, but nearly compensated the magnetic moments. The dependence of the blocking temperature with frequency and applied fields, TB(H,ω), suggests that the observed nonmonotonic behavior is governed by the strength of interparticle interactions. [ABSTRACT FROM AUTHOR]

The structure and magnetic properties of magnetite (Fe3O4) nanoparticles synthesized by a solvothermal processing route are investigated. The nanoparticles are grown from the single organometallic precursor Fe(III) acetylacetonate in trioctylamine (TOA) solvent at 260 °C, with and without the addition of heptanoic acid (HA) as a stabilizing agent. From the temporal particle size distributions, x-ray-diffraction patterns, high-resolution transmission electron microscope tilt series experiments, and superconducting quantum interference device magnetometry, we demonstrate that HA, a strong Lewis acid stabilizing agent, slows growth processes during ripening thus reducing the formation of interfacial defects, which we observe in the TOA-only synthesis. Nanoparticles grown with HA remain single crystalline for long growth times (up to 24 h), show a focused particle size distribution for intermediate growth times (3 h), and possess a higher magnetic anisotropy (15.8×104 J/m3) than particles grown without the additional stabilizing agent. The reduced magnetic anisotropy value for the magnetite nanoparticles grown in TOA only (1.29×104 J/m3) is attributed to polycrystallinity induced by the uncontrolled ripening process. This work may have significance for contrast enhancement in magnetic resonance imaging. [ABSTRACT FROM AUTHOR]

Point contacts of magnetite exhibit large, reversible increases of conductance by a factor of up to 5 in the presence of a small magnetic field (<10 mT), provided the conductance is less than the quantum of conductance G0 (12 900 Ω)-1. Smaller effects are found in (La0.7Sr0.3)MnO3 and Co2Cr0.6Fe0.4Al. Comparable effects observed in magnetite nanoconstrictions milled using a focused-ion beam in thin films deposited on oxide substrates are only reversible on a time scale of hours. High-resistance nanogaps show evidence of field emission beyond a voltage threshold, which is itself field dependent. The results are discussed in terms of spin-polarized transport, magnetostriction, magnetic dipole strain, and Fowler–Nordheim tunneling. [ABSTRACT FROM AUTHOR]

Ferromagnetic resonance (FMR) and ac conductivity have been applied to study a polymer composite containing as filler a binary mixture of magnetite (Fe3O4) and cementite (Fe3C) nanoparticles (30–50 nm) dispersed in a diamagnetic carbon matrix, which was synthesized by the carburization of nanocrystalline iron. Ac conductivity measurements showed thermally activated behavior involving a range of activation energies and power law frequency dependence at high frequencies similar to conducting polymer composites randomly filled with metal particles. Ferromagnetic resonance measurements revealed a relatively narrow FMR line at high temperatures indicating the presence of ferromagnetic nanoparticles, where thermal fluctuations and interparticle interactions determine the FMR temperature variation. An abrupt change of the FMR spectra was observed at T<81 K (ΔT<=1 K) coinciding with a sharp anomaly resolved in the temperature derivative of the ac conductivity. This behavior is attributed to the Verwey transition of Fe3O4 nanoparticles, where the concurrent skin depth variation unveils the FMR of large magnetite conglomerates and thus allows discriminating their contribution from relatively isolated nanoparticles. [ABSTRACT FROM AUTHOR]