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Magnetically induced heterogeneous catalysis has been attracting attention due to its high energy efficiency and flexibility for dynamic reactor control. Iron wool is a commercial, low-cost, and versatile heating agent, which has been used in several magnetic catalysis studies, but its heating properties have never been investigated. Here, the properties of three types of Fe wool were studied using optical and electronic microscopy, x-ray diffraction, and measurements of both heating power and high-frequency hysteresis loops. The effects of strand width, packing, and magnetic field amplitude and frequency were studied. A maximum specific absorption rate (SAR) around 700 W/g under a rms field of 47.4 mT at 93 kHz was measured for the larger width Fe wool. High-frequency hysteresis loops were used to quantify the contribution of hysteresis losses and eddy currents to total heating. Eddy currents contribute 65%–90% to the global heating depending on the strand width. Coating the wool with SiO2 and Ni has negative effects on the SAR but none on hysteresis losses. It is interpreted as originating from the cut-off of inter-wire eddy currents due to the insulating (SiO2, oxidized Ni) nature of the coating. Last, it was found that adding more Fe wool in a given volume mostly decreases the SAR. This effect could be not only due to the absorption and/or screening of the field by surface strands but also due to magnetic interactions. The results described in this work give insights into the magnetic heating of microscale magnetic materials and optimize their use for heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

The heating power of magnetic nanoparticles (MNPs) submitted to high-frequency magnetic fields is generally probed using calorimetric methods, which suppose that the heating power does not evolve with time. Among the several parameters governing MNP heating properties, their organization into chains under the influence of the applied magnetic field is of key importance, though the dynamic of this phenomenon has been rarely studied experimentally. In the present article, time-resolved high-frequency hysteresis loops are used to probe the dynamics of chain formation on a sample of 17.3 ± 2.2 nm FeNi3 MNPs. Chains are formed on a timescale, which strongly depends on the magnetic field amplitude, ranging from several tens of seconds to less than 100 ms, but does not depend on frequency in the range studied here (from 9 to 78 kHz). Both the heating power and hysteresis loop squareness increase with time as chains progressively form. These findings have important methodological consequences when defining protocols or analyzing data issued from calorimetric measurements since, in samples where chains form, the heating power varies on a time scale that can be comparable to typical measurement times. [ABSTRACT FROM AUTHOR]

Magnetic heating, namely, the use of heat released by magnetic nanoparticles (MNPs) excited with a high-frequency magnetic field, has so far been mainly used for biological applications. More recently, it has been shown that this heat can be used to catalyze chemical reactions, some of them occurring at temperatures up to 700 °C. The full exploitation of MNP heating properties requires the knowledge of the temperature dependence of their heating power up to high temperatures. Here, a setup to perform such measurements is described based on the use of a pyrometer for high-temperature measurements and on a protocol based on the acquisition of cooling curves, which allows us to take into account calorimeter losses. We demonstrate that the setup permits to perform measurements under a controlled atmosphere on solid state samples up to 550 °C. It should in principle be able to perform measurements up to 900 °C. The method, uncertainties, and possible artifacts are described and analyzed in detail. The influence on losses of putting under vacuum different parts of the calorimeter is measured. To illustrate the setup possibilities, the temperature dependence of heating power is measured on four samples displaying very different behaviors. Their heating power increases or decreases with temperature, displaying temperature sensibilities ranging from −2.5 to +4.4% K−1. This setup is useful to characterize the MNPs for magnetically heated catalysis applications and to produce data that will be used to test models permitting to predict the temperature dependence of MNP heating power. [ABSTRACT FROM AUTHOR]

To optimize the heating properties of magnetic nanoparticles (MNPs) in magnetic hyperthermia applications, it is necessary to calculate the area of their hysteresis loops in an alternating magnetic field. The separation between 'relaxation losses' and 'hysteresis losses' presented in several articles is artificial and criticized here. The three types of theories suitable for describing hysteresis loops of MNPs are presented and compared to numerical simulations: equilibrium functions, Stoner-Wohlfarth model based theories (SWMBTs), and a linear response theory (LRT) using the Néel-Brown relaxation time. The configuration where the easy axis of the MNPs is aligned with respect to the magnetic field and the configuration of a random orientation of the easy axis are both studied. Suitable formulas to calculate the hysteresis areas of major cycles are deduced from SWMBTs and from numerical simulations; the domain of validity of the analytical formula is explicitly studied. In the case of minor cycles, the hysteresis area calculations are based on the LRT. A perfect agreement between the LRT and numerical simulations of hysteresis loops is obtained. The domain of validity of the LRT is explicitly studied. Formulas are proposed to calculate the hysteresis area at low field that are valid for any anisotropy of the MNP. The magnetic field dependence of the area is studied using numerical simulations: it follows power laws with a large range of exponents. Then analytical expressions derived from the LRT and SWMBTs are used in their domains of validity for a theoretical study of magnetic hyperthermia. It is shown that LRT is only pertinent for MNPs with strong anisotropy and that SWMBTs should be used for weakly anisotropic MNPs. The optimum volume of MNPs for magnetic hyperthermia is derived as a function of material and experimental parameters. Formulas are proposed to allow to the calculation of the optimum volume for any anisotropy. The maximum achievable specific absorption rate (SAR) is calculated as a function of the MNP anisotropy. It is shown that an optimum anisotropy increases the SAR and reduces the detrimental effects of the size distribution of the MNPs. The optimum anisotropy is simple to calculate; it depends only on the magnetic field used in the hyperthermia experiments and the MNP magnetization. The theoretical optimum parameters are compared to those of several magnetic materials. A brief review of experimental results as well as a method to analyze them is proposed. This study helps in the determination of suitable and unsuitable materials for magnetic hyperthermia and provides accurate formulas to analyze experimental data. It is also aimed at providing a better understanding of magnetic hyperthermia to researchers working on this subject. [ABSTRACT FROM AUTHOR]

We report on hyperthermia measurements on a colloidal solution of 14.2±1.5 nm monodisperse FeCo nanoparticles (NPs). Losses as a function of the magnetic field display a sharp increase followed by a plateau, which is what is expected for losses of ferromagnetic single-domain NPs. The frequency dependence of the coercive field is deduced from hyperthermia measurement and is in quantitative agreement with a simple model of noninteracting NPs. The measured losses (1.5 mJ/g) compare to the highest of the literature, although the saturation magnetization of the NPs is well below the bulk one. [ABSTRACT FROM AUTHOR]

Ni particles were prepared by spark erosion in a fixed-gap apparatus, and in the usual “shaker-pot” assembly, in an investigation of the influence of various processing parameters on the particles’ properties. The sizes of the particles were studied as functions of spark energies ranging from 10 μJ to 1 J, and a scaling relation derived from a simple model was verified. Several different static and rotating electrode configurations were compared with respect to their suitability for producing significant yields of small particles. The advantages of stirring the dielectric with the fixed-gap apparatus and of rotating the electrodes were demonstrated. Water, kerosene, and liquid argon and nitrogen were used as dielectric liquids. When compounds were formed, the reaction with the dielectric proceeded inversely with particle size. Spark erosion in kerosene at low spark energies, followed by annealing, proved to be an effective method to produce fine nickel particles. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We fabricated tunnel junctions containing clusters embedded in a thin insulating layer. Low-temperature dc transport measurements reveal typical Coulomb blockade features. We measured the differential capacitance at different dc voltages and frequencies of these samples in the Coulomb blockade regime. The differential capacitance is constant at low dc bias and increases by 35% above a threshold voltage. This behavior can be well understood using a simple model which takes into account the Coulomb blockade effect. Such measurements could be used as a probe of Coulomb blockade properties of clusters assemblies. This effect is also promising for the fabrication of capacitors whose capacitance can be changed by varying the applied voltage. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the magnetic and hyperthermia properties of 5.5 nm in diameter iron nanoparticles synthesized by organometallic chemistry, which display the bulk magnetization. Quantitative analysis of alternative susceptibility measurements allows the determination of the effective anisotropy Keff=1.3×105 J m-3. Hyperthermia measurements are performed at a magnetic field up to 66 mT and at frequencies in the range of 5-300 kHz. Maximum measured specific absorption rate (SAR) is 280 W/g. SAR displays a square dependence with the magnetic field below 30 mT but deviates from this power law at higher value. SAR is linear with the applied frequency for μ0H =19 mT. These results are discussed in the light of linear response theory. [ABSTRACT FROM AUTHOR]

Stable iron nanoparticles have been synthesised by the decomposition of {Fe(N[Si(CH3)3]2)2}2 under dihydrogen pressure. Those conditions lead to a system of monodisperse and metallic nanoparticles which diameter is less than 2 nm and stabilized by HN[Si(CH3)3]2. The magnetization is found to be MS=1.92μB/at., i.e., 10% lower than the bulk value. The Mössbauer spectrum is fitted by two contributions of metallic iron. The magnetic anisotropy energy constant increases up to 5.2×105 J/m3, i.e., ten times the bulk one. [ABSTRACT FROM AUTHOR]

We have investigated, by means of ac susceptibility experiments, the magnetic relaxation of layers of Co nanoclusters (D ≃ 2.6 nm) embedded in Al[SUB2]O[SUB3]. Superparamagnetic blocking takes place at higher temperatures as the number of layers increases. These results are interpreted using a simple model in which dipole--dipole interactions between nearest neighbor particles increase the relaxation time. The influence of interactions is affected by the application of bias magnetic field H. As the magnetic moments of the particles become polarized by H, the blocking temperature approaches the behavior expected for noninteracting particles. [ABSTRACT FROM AUTHOR]

Studies with transplanted tumors in animals and clinical trials have provided the proof-of-concept of magnetic hyperthermia (MH) therapy of cancers using iron oxide nanoparticles. Interestingly, in several studies, the application of an alternating magnetic field (AMF) to tumor cells having internalized and accumulated magnetic nanoparticles (MNPs) into their lysosomes can induce cell death without detectable temperature increase. To explain these results, among other hypotheses, it was proposed that cell death could be due to the high-frequency translational motion of MNPs under the influence of the AMF gradient generated involuntarily by most inductors. Such mechanical actions of MNPs might cause cellular damages and participate in the induction of cell death under MH conditions. To test this hypothesis, we developed a setup maximizing this effect. It is composed of an anti-Helmholtz coil and two permanent magnets, which produce an AMF gradient and a super-imposed static MF. We have measured the MNP heating power and treated tumor cells by a standard AMF and by an AMF gradient, on which was added or not a static magnetic field. We showed that the presence of a static magnetic field prevents MNP heating and cell death in standard MH conditions. The heating power of MNPs in an AMF gradient is weak, position-dependent, and related to the presence of a non-zero AMF. Under an AMF gradient and a static field, no MNP heating and cell death were measured. Consequently, the hypothesis that translational motions could be involved in cell death during MH experiments is ruled out by our experiments. [ABSTRACT FROM AUTHOR]

A setup for measuring the high-frequency hysteresis loops of magnetic samples is described. An alternating magnetic field in the range 6-100 kHz with amplitude up to 80 mT is produced by a Litz wire coil. The latter is air-cooled using a forced-air approach so no water flow is required to run the setup. High-frequency hysteresis loops are measured using a system of pick-up coils and numerical integration of signals. Reproducible measurements are obtained in the frequency range of 6-56 kHz. Measurement examples on ferrite cylinders and on iron oxide nanoparticle ferroflu-ids are shown. Comparison with other measurement methods of the hysteresis loop area (complex susceptibility, quasi-static hysteresis loops, and calorific measurements) is provided and shows the coherency of the results obtained with this setup. This setup is well adapted to the magnetic characterization of colloidal solutions of magnetic nanoparticles for magnetic hyperthermia applications. [ABSTRACT FROM AUTHOR]

We show theoretically that, in an inhomogeneous alternating magnetic field of frequency f, due to the alternating gradient, magnetic nanoparticles oscillate mechanically and generate ultrasound waves. This effect is maximized and better controlled if a static magnetic field is superimposed to an alternating gradient. It makes possible the generation of ultrasounds intracellularly and might also explain recent experiments in magnetic hyperthermia in which cells have been killed without any global temperature increase. Combined to an efficient targeting, it could permit ultrasound therapy with an unprecedented spatial resolution. [ABSTRACT FROM AUTHOR]

We describe a low-cost and simple setup for hyperthermia measurements on colloidal solutions of magnetic nanoparticles (ferrofluids) with a frequency-adjustable magnetic field in the range of 5–500 kHz produced by an electromagnet. By optimizing the general conception and each component (nature of the wires, design of the electromagnet, etc.), a highly efficient setup is obtained. For instance, in a useful gap of 1.1 cm, a magnetic field of 4.8 mT is generated at 100 and 500 kHz with output powers of 3.4 and 75 W, respectively. A maximum magnetic field of 30 mT is obtained at 100 kHz. The temperature of the colloidal solution is measured using optical fiber sensors. To remove contributions due to heating of the electromagnet, a differential measurement is used. In this configuration the sensitivity is better than 1.5 mW at 100 kHz and 19.3 mT. This setup allows one to measure weak heating powers on highly diluted colloidal solutions. The hyperthermia characteristics of a solution of Fe nanoparticles are described, where both the magnetic field and the frequency dependence of heating power have been measured. [ABSTRACT FROM AUTHOR]

The influence of a transverse static magnetic field on the magnetic hyperthermia properties is studied on a system of large-losses ferromagnetic FeCo nanoparticles. The simultaneous measurement of the high-frequency hysteresis loops and of the temperature rise provides an interesting insight into the losses and heating mechanisms. A static magnetic field of only 40 mT is enough to cancel the heating properties of the nanoparticles, a result reproduced using numerical simulations of hysteresis loops. These results cast doubt on the possibility to perform someday magnetic hyperthermia inside a magnetic resonance imaging setup. [ABSTRACT FROM AUTHOR]

By combining optical spin injection techniques with transport spectroscopy tools, we demonstrate a spin-photodetector allowing for the electrical measurement and active filtering of conduction band electron spin at room temperature in a simple nonmagnetic GaAsN semiconductor structure. By switching the polarization of the incident light from linear to circular, we observe a spin dependent photoconductivity change reaching up to 40% without the need of an external magnetic field. The spin dependent photoconductivity change relies on the efficient spin filtering effect of conduction band electrons on N-induced Ga self-interstitial deep paramagnetic centers. [ABSTRACT FROM AUTHOR]

The origin of perpendicular anisotropy in Co/Pd multilayers has been investigated. We examined this question by studying multilayers in which the total equivalent thicknesses of Co and Pd are kept constant, but in which the Co/Pd interface is progressively alloyed using codeposition of the two elements. The multilayers have the structure: [(0.3 nm Co codeposited with y nm Pd)/(2.0-y) nm Pd][sub 15], with y=0, 0.1, 0.3, 0.5, 1, 1.5, 2.0. X-ray diffraction confirms that the total thicknesses of the samples and their overall Co/Pd ratios stay constant. Magnetic properties are almost unaffected by alloying up to y=0.5 nm, suggesting that the interface of the usual Co/Pd multilayer could be modeled by a mix of 0.3 nm of Co with 0.5 nm of Pd embedded between 2 monolayers of Pd. Using this model and reasonable assumptions about the strain and magnetostriction of a Co-Pd alloy, we show that the magnetoelastic effect yields a perpendicular anisotropy between 2.3×10[sup 5] and 3.5×10[sup 5] J/m[sup 3]. The perpendicular anisotropy estimated in our samples (1.5×10[sup 5] J/m[sup 3]) can thus be satisfactorily explained. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the observation of magnetoresistance in a Sr[sub 2]FeMoO[sub 6] (SFMO)-based tunnel junction. This result is obtained by combining a three-step process for the growth of the Sr[sub 2]FeMoO[sub 6] layer by pulsed laser deposition with a technology allowing the definition of nanometer-sized junctions. A clear positive magnetoresistive signal of 50% is obtained at low temperature in a Sr[sub 2]FeMoO[sub 6]/SrTiO[sub 3]/Co junction. Since the SrTiO[sub 3]/Co interface is known to have a negative spin polarization of about 20%, this result yields a negative spin polarization of SFMO, which we find to amount to more than 85% in our film. This confirms the half-metallic character of this compound, predicted by band structure calculations. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The article mentions that all online versions of the article "Simple models for dynamic hysteresis loop calculations of magnetic single-domain nanoparticles: Application to magnetic hyperthermia optimization," which was published online on April 21, 2011 were corrected on April 26, 2011.

We report on the electrical characterization of various types of nanocontacts fabricated by nanoindentation and electrodeposition. Arrays of holes with depths ranging from 0 to 20 nm were produced by nanoindenting at different strengths an Al[sub 2]O[sub 3]-50 Å/NiFe-150 Å//Si bilayer. NiFe was then electrodeposited, which led to the growth of particles in the holes. The resistance of the particles was measured with a conducting tip atomic force microscope. Depending on the strength used during the nanoindentation, the resistance ranges from less than 5 × 10[sup 3] Ω to more than 10[sup 12] Ω. The low-resistance constrictions can be used to study ballistic transport in materials. High-resistance contacts presumably correspond to tunnel nanojunctions. [ABSTRACT FROM AUTHOR]

We show that the electrodeposition of Ni[sub 80]Fe[sub 20] on top of a thin aluminum oxide barrier leads to particle growth occurring on preferential nucleation centers. The particle sites are attributed to local defects in the aluminum oxide barrier. As a function of the thickness of the barrier, different growth modes can occur. For thinner barriers, new nucleation centers are created during electrodeposition. The resistance of the defects, characterized by conducting atomic force microscopy, ranges from less than 10[sup 4] to greater than 10[sup 12] Ω. Various I(V) characteristics were also obtained, depending on the resistance of the defect. These results suggest that this experimental technique could be a very interesting one with which to fabricate nanoconstrictions dedicated to ballistic magnetoresistance studies.© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A correction to the article "Spin-dependent photoconductivity in nonmagnetic semiconductors at room temperature," by F. Zhao and colleagues in the June 10, 2010 issue, is presented.