Search

Low-frequency hysteresis loops and specific absorption rate (SAR) of various assemblies of elongated spheroidal magnetite nanoparticles have been calculated for a range of particle semiaxis ratios a/b = 1.0–3.0. The SAR of a dilute randomly oriented assembly of magnetite nanoparticles in an alternating magnetic field of moderate frequency, f = 300 kHz, and amplitude H0 = 100–200 Oe is shown to decrease significantly with an increase in the aspect ratio of nanoparticles. In addition, there is a narrowing and shift of the intervals of optimal particle diameters towards smaller particle sizes. However, the orientation of a dilute assembly of elongated nanoparticles in a magnetic field leads to an almost twofold increase in SAR at the same frequency and amplitude of the alternating magnetic field, the range of optimal particle diameters remaining unchanged. The effect of the magneto-dipole interaction on the SAR of a dilute assembly of oriented clusters of elongated magnetite nanoparticles has also been investigated depending on the volume fraction of nanoparticles in a cluster. It has been found that the SAR of the assembly of oriented clusters decreases by approximately an order of magnitude with an increase in the volume fraction of nanoparticles in a cluster in the range of 0.04–0.2.

A linear combination of spherically symmetric heat sources is shown to provide optimal stationary thermal distribution in magnetic hyperthermia. Furthermore, such spatial location of heat sources produces suitable temperature distribution in biological medium even for assemblies of magnetic nanoparticles with a moderate value of specific absorption rate (SAR), of the order of 100 - 150 W/g. We also demonstrate the advantage of using assemblies of magnetic nanocapsules consisting of metallic iron nanoparticles covered with non magnetic shells of sufficient thickness in magnetic hyperthermia. Based on numerical simulation we optimize the size and geometric structure of biocompatible capsules in order to minimize the influence of strong magneto-dipole interaction between closely spaced nanoparticles. It is shown that assembly of capsules can provide sufficiently high SAR values of the order of 250 - 400 W/g at moderate amplitudes H = 50 - 100 Oe and frequencies f = 100 - 200 kHz of alternating magnetic field, being appropriate for application in clinics, Comment: 8 pages, 4 figures

The dynamics of magnetic nanoparticles in a viscous liquid in a rotating magnetic field has been studied by means of numerical simulations and analytical calculations. In the magneto-dynamics approximation three different modes of motion of the unit magnetization vector and particle director are distinguished depending on frequency and amplitude of the rotating magnetic field. The specific absorption rate of a dilute assembly of superparamagnetic nanoparticles in rotating magnetic field is calculated by solving the Landau–Lifshitz stochastic equation for the unit magnetization vector and the stochastic equation for the particle director. At elevated frequencies an optimal range of particle diameters is found where the specific absorption rate of an assembly in a rotating magnetic field has a maximum. It is shown that with an optimal choice of the particle sizes sufficiently large SAR values of the order of 400–500 W/g can be obtained in a rotating magnetic field with a frequency f = 400 kHz and a moderate magnetic field amplitude H0 = 100 Oe.

The specific absorption rate of magnetic nanoparticles in a rotating magnetic field has been calculated taking into account both thermal fluctuations of the particle magnetic moments and st...

A new method for magnetic nano- and submicron-scale particle fabrication is proposed. Magnetic particles of Fe73Co27 alloy were produced by a cavitation process directly into three different liquids: distilled water, benzyl alcohol and methyl methacrylate. The dependency between liquid viscosity and average particle size and size distributions are illustrated. The average particle size is found to be 224 nm in benzyl alcohol and 475 nm in methyl methacrylate. Also, assembly with a size of 20–80 nm was successfully separated from the poly-dispersed mixture obtained in distilled water. Fe73Co27 particles obtained by cavitation inherit the perfect crystal structure of the original macro-sample and possess a high value of the saturation magnetization, Ms = 242 emu/g. These particles can be applied for biomedical purposes and as a component of magneto–polymer composites.

Glass-coated Co-rich amorphous microwires are very promising for the development of tiny magnetic sensors that can be used in portable electronic devises. In this study, a substantial decrease in the residual quenching stress in Co-rich microwires is achieved by reducing the thickness of the glass coating by means of precise etching of the wire in a specially designed gel. This effect is confirmed experimentally by means of the small-angle magnetization rotation method as well as by direct measurement of the off-diagonal component of the giant magnetoimpedance (GMI) tensor of wires with different thicknesses of glass coating as a function of the applied magnetic field. A reduction in the thickness of the glass coating to the range of 0.5–2.0 µm resulted in a nearly twofold increase in the steepness of the off-diagonal GMI component of the studied Co-rich microwires. Therefore, this method can be used to improve the sensitivity of miniature magnetic sensors to weak external magnetic fields.

The ferromagnetic resonance frequencies are obtained for a thin ferromagnetic film with surface anisotropy for the cases when the external magnetic field is applied perpendicularly or parallel to the film surface, and for various combinations of boundary conditions on the film surface. It is shown that in the presence of surface anisotropy the ferromagnetic resonance frequency essentially depends both on the film thickness and on the value of the surface anisotropy constant. The results obtained provide a basis for the correct interpretation of experimental data obtained by means of broadband ferromagnetic resonance in thin film structures., 7 pages, 1 figure

The low frequency hysteresis loops and specific absorption rate (SAR) of assemblies of magnetite nanoparticles with cubic anisotropy are calculated in the diameter range of D = 20–60 nm taking into account both thermal fluctuations of the particle magnetic moments and strong magneto–dipole interaction in assemblies of fractal-like clusters of nanoparticles. Similar calculations are also performed for assemblies of slightly elongated magnetite nanoparticles having combined magnetic anisotropy. A substantial dependence of the SAR on the nanoparticle diameter is obtained for all cases investigated. Due to the influence of the magneto–dipole interaction, the SAR of fractal clusters of nanoparticles decreases considerably in comparison with that for weakly interacting nanoparticles. However, the ability of magnetic nanoparticle assemblies to generate heat can be improved if the nanoparticles are covered by nonmagnetic shells of appreciable thickness.

The stochastic Landau-Lifshitz equation is used to investigate the relaxation process and equilibrium magnetization of interacting assembly of superparamagnetic nanoparticles uniformly distributed in a nonmagnetic matrix. For weakly interacting assembly the equilibrium magnetization is shown to deviate significantly from the Langevin law in the range of moderate and large magnetic fields due to the influence of magnetic anisotropy energy. For dense assemblies with noticeable influence of the magneto-dipole interaction a significant dependence of the initial susceptibility on the assembly density is revealed. The difference between the initial susceptibility and the corresponding Langevin susceptibility can serve as an indication of the influence of the magneto-dipole interaction on the assembly properties. A new self-consistent approach is developed to explain the effect of mutual magneto-dipole interaction on the behavior of dense assembly of superparamagnetic nanoparticles. The probability densities of the components of random magnetic field acting on magnetic nanoparticles are calculated at thermodynamic equilibrium. The self-consistent probability densities of these components are found to be close to Gaussian distribution. It is shown that a decrease in the equilibrium assembly magnetization as a function of density can be explained by the disorienting effect of the random magnetic field on the particle magnetic moments., Comment: 16 pages, 10 figures

Nanoparticles, specifically magnetosomes, synthesized in nature by magnetotactic bacteria, are very promising to be usedin magnetic hyperthermia in cancer treatment. In this work, using the solution of the stochastic Landau&ndash, Lifshitz equation, we calculate the specific absorption rate (SAR) in an alternating (AC) magnetic field of assemblies of magnetosome chains depending on the particle size D, the distance between particles in a chain a, and the angle of the applied magnetic field with respect to the chain axis. The dependence of SAR on the a/D ratio is shown to have a bell-shaped form with a pronounced maximum. For a dilute oriented chain assembly with optimally chosen a/Dratio,a strong magneto-dipole interaction between the chain particles leads to an almost rectangular hysteresis loop, and to large SARvaluesin the order of 400&ndash, 450 W/g at moderate frequencies f= 300 kHz and small magnetic field amplitudes H0 = 50&ndash, 100 Oe. The maximum SAR value only weakly depends on the diameter of the nanoparticles and the length of the chain. However, a significant decrease in SAR occurs in a dense chain assembly due to the strong magneto-dipole interaction of nanoparticles of different chains.

The structure of multi-domain micromagnetic states in hcp cobalt nanoparticles of spheroidal shape has been studied using numerical simulation in the range of diameters 20–200 nm. The single-domain diameters of the particles are determined depending on their aspect ratio. The complicated vortex structure of domain walls for two- and three-domain micromagnetic configurations is investigated. It has been shown that three domain states are actually strongly deformed two vortex states. In hcp cobalt particles of sufficiently large sizes two types of three-domain micromagnetic states with close total energies have been obtained. They differ in different magnetization directions of the exchange cores of the vortex domain walls. The remanent magnetization of particles has been calculated for two- and three-domain micromagnetic states. The single-domain diameters of fcc cobalt nanoparticles with cubic type of magnetic anisotropy were also calculated.

Detailed calculations of the specific absorption rate (SAR) of a dilute assembly of iron oxide nanoparticles with effective uniaxial anisotropy dispersed in a liquid are performed depending on the particle diameters, the alternating (ac) magnetic field amplitude and the liquid viscosity. For small and moderate ac magnetic field amplitudes H0 with respect to particle anisotropy field Hk the SAR of the assembly as a function of the particle diameter passes through a characteristic maximum and then reaches a plateau, whereas for sufficiently large amplitudes, H0 ~ Hk, the SAR increases monotonically as a function of particle diameter. This difference is a consequence of realization of viscous and magnetic oscillation modes for particle unit magnetization vector and director for moderate and sufficiently large H0 values, respectively. It is found that the SAR of the assembly changes inversely with the viscosity only in a viscous mode, for nanoparticles of sufficiently large diameters. In the developed magnetic mode the SAR of the assembly is practically independent of the viscosity, since in this case the nanoparticle director only weakly oscillates around the ac magnetic field direction. At moderate amplitudes of the ac magnetic field the SAR values of the assembly in the liquid and in the solid matrix are found to be close, except of the range of large particle diameters and sufficiently low viscosity. However, at large field amplitudes the SAR of randomly oriented assembly of nanoparticles in a solid matrix is approximately two times less than that in a liquid, because a significant fraction of nanoparticles of the assembly in the solid matrix is not optimally oriented with respect to the ac magnetic field direction. The conditions for the validity of the linear response theory have been clarified by comparison with the numerical simulation data., Comment: 15 pages, 6 figures

The specific absorption rate (SAR) of a dilute assembly of spherical iron nanoparticles with cubic anisotropy distributed in a viscous liquid is calculated using the solution of stochastic Landau - Lifshitz equation for unit magnetization vector and stochastic equations for multiple particle directors that specify the spatial orientation of the nanoparticle in a liquid. The viscous and magnetic magnetization reversal modes of particles are revealed at low and sufficiently high amplitudes of alternating magnetic field, respectively. The SAR of iron nanoparticle assembly is shown to exceed significantly that of iron oxide nanoparticles with uniaxial anisotropy at the same amplitudes and frequencies of applied magnetic field. The linear response theory is shown to be valid only at small magnetic field amplitudes, H0 < 50 - 70 Oe., 11 pages, 5 figures

Very weak stray magnetic fields, H ∼ 10− 4 Oe, of an assembly of FeCo particles of mesoscopic dimensions, d = 10–50 μm, are investigated using a sensitive scanning giant magneto-impedance magnetometer. Direct magnetic force microscopy measurement of individual FeCo particles reveals a complicated domain structure. It is proved, however, that the mean square value of the stray magnetic field of a dilute assembly of FeCo particles randomly distributed on the plane is a statistically well-defined quantity. It depends on the average particle diameter, average distance between the particles, and average particle magnetization per unit volume. It is shown that the average particle magnetization can be determined using a measured mean square value of the stray magnetic field and geometrical parameters of the assembly obtained from optical measurement.

The magnetic properties of amorphous glass coated microwires are determined mainly by the distribution of residual quenching stresses over the microwire cross section. In this paper a control of the residual quenching stresses in amorphous ferromagnetic microwires is achieved by changing the microwire glass shell thickness by etching. To this end, a special gel is synthesized for precision glass shell etching of series of Co-rich microwires. The use of the gel provides a possibility of uniform etching of the microwire glass coating at a rate of 0.2–3.2 μm/h depending on the amount of glass already removed, a surface roughness being about 40–100 nm. It is also possible to completely remove the glass shell of the microwire without damage its metallic nucleus. The small angle magnetization rotation method has been used to determine the change in the amplitude of the residual quenching stress with a gradual decrease of the glass shell thickness of microwires of composition Co67Fe4Ni2Mo2B11Si14 and Co71Fe4Si10B15. It is found that the amplitude of the residual quenching stress deceases by 40–50% in average after removing of the layer of 1 μm thickness from the glass coating.

It is shown that surface domain structure arises in magnetic nanotube with uniaxial anisotropy if surface anisotropy constant is negative and sufficiently high in absolute value. The surface magnetic anisotropy affects also the structure and dynamics of a head-to-head domain wall propagating along the nanotube axis in applied magnetic field. The hopping mode is observed for stationary movement of a head-to-head domain wall. The average speed of the domain wall in the hopping mode is found to be several times less than the stationary velocity of the wall in the absence of surface anisotropy. This effect is important for various applications where fast propagation of the domain wall along the sample is essential.

The whole set of geometrical, mechanical and magnetic parameters of Co-rich amorphous ferromagnetic microwires very promising for the development of sensitive stress and Giant Magneto-Impedance sensors have been determined by means of scanning electron microscopy, selective nanoindentation and small angle magnetization rotation method. The magnetostriction constants obtained are sensitive to the true microwire compositions. The experimentally estimated amplitudes of the residual quenching stress are mainly determined by the ratio of the inner and outer wire diameters. They are found to be in good correspondence with the theoretical values calculated based on the theory of viscoelasticity.

The spatial period of magnetization perturbations that occur near the surface of magnetic nanotube or nanowire under the influence of surface magnetic anisotropy is determined by means of numerical simulation as a function of the sample geometry and material parameters. The surface magnetization distribution obtained is then used to estimate the period of the surface magnetic texture in amorphous microwire of several micrometers in diameter by means of appropriate variational procedure. The period of the surface magnetic texture in amorphous microwire is found to be significantly smaller than the wire diameter.

Three different methods of fabrication of FeCo soft magnetic material using ferromagnetic powders are compared: (i) simple sintering of elemental powders of Fe and Co, (ii) sintering of mechanically alloying FeCo powder, and (iii) sintering of chemically synthesized FeCo powder. The microstructure of ferromagnetic powders and bulk sintered alloys is characterized using scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis. The best magnetic properties with high saturation magnetization, M s = 211.3 emu/g, are obtained for bulk FeCo alloy sintered from chemically synthesized powder. It consists of nearly spherical FeCo particles with diameters from 5 to 15 μ m. The mean particle size of chemically synthesized FeCo powder can be controlled by changing the melt composition, temperature, and process duration. The relatively large size of FeCo particles reduces the influence of surface oxidation on the particle magnetic properties. The low-cost chemical technology developed is promising for a large-scale production of small FeCo magnetic components of arbitrary shapes with high-dimensional precision.

The dynamics of magnetic nanoparticle in a viscous liquid in rotating magnetic field has been studied by means of numerical simulation and analytical calculations. In the magneto- dynamics approximation three different modes of motion of the unit magnetization vector and particle director are distinguished depending on the rotating magnetic field frequency and amplitude. The specific absorption rate of a dilute assembly of superparamagnetic nanoparticles in rotating magnetic field is calculated by solving the Landau – Lifshitz stochastic equation for unit magnetization vector and stochastic equation for particle director. At elevated frequencies an optimal range of particle diameters is found where the specific absorption rate of an assembly in rotating magnetic field has a maximum. It is shown that for magnetic hyperthermia in rotating magnetic field it is preferable to use rotating magnetic fields of moderate amplitude, H 0 = 100 Oe, in the frequency range 400-600 kHz.

Assemblies of magnetic nanoparticles show a great potential for application in biomedicine, particularly, magnetic hyperthermia. However, to achieve desired therapeutic effect in magnetic hyperthermia, the assembly of nanoparticles should have a sufficiently high specific absorption rate (SAR) in alternating magnetic field of moderate amplitude and frequency. Using the Landau–Lifshitz stochastic equation, it is shown that dilute assemblies of iron oxide nanoparticles of optimal diameters are capable of providing SAR of the order of 400–600[Formula: see text]W/g in alternating magnetic field with the amplitude [Formula: see text][Formula: see text]Oe in the frequency range f = 300–500[Formula: see text]kHz. Unfortunately, in dense clusters of magnetic nanoparticles, which are often formed in a biological medium, there is a sharp decrease in SAR due to the influence of strong magneto-dipole interaction of closest nanoparticles. To overcome this difficulty, it is suggested covering the nanoparticles with nonmagnetic shells of sufficient thickness or using non-single-domain nanoparticles being in magnetization curling states.

Weak magnetic fields of pitting corrosion currents in AISI 304 stainless steel have been measured by means of giant magneto-impedance magnetometer. Corrosion current density of a single pit has been calculated by magnetic fields mathematical modelling. The current density was also estimated by alloy mass loss in the pit that had been evaluated through optical measurement. The difference between obtained results was 16%. Moreover, pitting repassivation has been detected experimentally.

In this paper highly oriented magneto-polymer composites have been obtained by means of Co and Fe-rich glass-coated amorphous microwires with various metal nucleus and outer diameters. The microwire segments with length L ~ 0.5–0.6 cm were oriented in the magnesium alloy matrix and were poured with a liquid polymer whose composition was a mixture of methyl methacrylate methacrylic acid and methacrylate copolymers in a ratio of 37/63% by mass. The magneto-polymer composites obtained possess high electric resistance, chemical stability and mechanical strength. The high average saturation magnetizations, = 20–30 emu/cm3, have been obtained for Co and Fe based composites with the volume fraction of the magnetic material in the polymer matrix within the range 0.02–0.05. Both Co and Fe based composites show highly anisotropic magnetic properties. The magneto-polymer samples can be cut into pieces of arbitrary size and shape. This facilitates their use in various technical applications.

A scanning giant magneto-impedance (GMI) magnetometer has been used to measure small magnetic fields of corrosion currents in zinc-copper cells. In situ magnetic images of the copper plates with one or two small anode (zinc) inclusions of different sizes located at various distances at the sample surface have been obtained. To interpret the magnetic images of the corrosion currents a numerical simulation program has been developed. A good qualitative agreement between experimental and theoretical results proves that the scanning GMI magnetometry is a promising tool for in situ investigation of some of local corrosion processes.

The design, main characteristics, and specific features of a new high-sensitivity magnetometer based on the giant magneto-impedance (GMI) effect in amorphous ferromagnetic microwires are considered. It is shown that, in addition to measuring homogeneous fields, the device is capable of detecting weak local magnetic fields of conduction currents and ion currents involved in corrosion processes. Results of in situ magnetic measurements of corrosion processes in the model system of copper–zinc in a sulfuric acid solution qualitatively agree with the data of direct corrosion tests employing the standard gravimetric method.

The ultrasonic oscillation of a macroscopic sample in a viscous liquid is used to create magnetic particles of FeCo alloy with an average diameter D ~100 nm. The FeCo particles obtained have a high specific saturation magnetization, Ms = 1960 emu/cm3, close to that of a bulk sample. The hysteresis loop of a magneto-polymer composite prepared from FeCo particles shows low coercive force and a remanent magnetization close to zero. The magnetic saturation of the magnetic composite samples is achieved in a sufficiently strong magnetic field, H = 6.5 kOe. The hysteresis loop features of the magnetic composite have been explained by means of the numerical simulation of the quasistatic hysteresis loops of non-oriented dilute assemblies of spheroidal FeCo particles with various aspect ratios.

Quasistatic hysteresis loops of a single-domain magnetic nanoparticle with uniaxial anisotropy in a rotating magnetic field have been calculated. Magnetic hysteresis is shown to exist only in a limited range of reduced magnetic field amplitudes, 0.5 1 the total energy of the nanoparticle has a single energy minimum, so that there is no magnetic hysteresis. Quasistatic hysteresis loops of a randomly oriented assembly of non interacting nanoparticles in a rotating magnetic field are also obtained.

Specific absorption rate of superparamagnetic nanoparticles with uniaxial magnetic anisotropy has been calculated both for dilute assembly and for assembly of nanoparticle clusters with various filling factors using numerical simulation. The optimal particle diameters at which the specific absorption rate of assembly reaches a maximum have been obtained depending on the value of the uniaxial anisotropy constant. The optimal particle diameters are found to shift to smaller values with an increase in the anisotropy constant. The range of optimal diameters decreases simultaneously. The specific absorption rate decreases also as a function of cluster filling factor, but the optimal particle diameters remain almost unchanged.

The influence of the crystal structure inhomogeneities on the magnetic properties of cobalt nanoparticles with different aspect ratio and spherical nanoparticles of chromium dioxide, cobalt ferrite and magnetite has been studied by means of numerical simulation. The polycrystalline nanoparticles are modeled by means of subdivision of the nanoparticle volume into tightly bound single-crystal granules with randomly distributed directions of the easy anisotropy axes. The probability of appearance of quasi uniform and vortex states in sufficiently large assemblies of polycrystalline nanoparticles of various types have been calculated depending on the nanoparticle diameter. It is shown that the subdivision of a nanoparticle into single-crystal granules with different orientations of the easy anisotropy axes substantially reduces the effective single-domain diameters for particles with uniaxial type of anisotropy of individual granules. However, for particles with cubic type of magnetic anisotropy the influence of the crystal structure inhomogeneities on the equilibrium properties of the particles is not so important even for magnetically hard cobalt ferrite nanoparticles. It is practically absent for magnetically soft magnetite nanoparticles., Comment: 8 pages, 8 figures

The amplitude of the second harmonic of the electro-motive force occurring in the receiving pick-up coil when alternating electrical current is flowing through the microwire is measured as a function of applied external magnetic field, at different mechanical tensile stresses. In addition, an analytical expression for the amplitude of the second harmonic of the electro-motive force is derived. Comparing the experimental and theoretical data the saturation magnetization, the magnetostriction constant and the amplitude of the residual quenching stress have been determined for a family of Co-rich glass-coated microwires.

A set of equations describing the motion of a free magnetic nanoparticle in an external magnetic field in a vacuum, or in a medium with negligibly small friction forces is postulated. The conservation of the total particle momentum, i.e. the sum of the mechanical and the total spin momentum of the nanoparticle is taken into account explicitly. It is shown that for the motion of a nanoparticle in uniform magnetic field there are three different modes of precession of the unit magnetization vector and the director that is parallel the particle easy anisotropy axis. These modes differ significantly in the precession frequency. For the high-frequency mode the director points approximately along the external magnetic field, whereas the frequency and the characteristic relaxation time of the precession of the unit magnetization vector are close to the corresponding values for conventional ferromagnetic resonance. On the other hand, for the low-frequency modes the unit magnetization vector and the director are nearly parallel and rotate in unison around the external magnetic field. The characteristic relaxation time for the low-frequency modes is remarkably long. This means that in a rare assembly of magnetic nanoparticles there is a possibility of additional resonant absorption of the energy of alternating magnetic field at a frequency that is much smaller compared to conventional ferromagnetic resonance frequency. The scattering of a beam of magnetic nanoparticles in a vacuum in a non-uniform external magnetic field is also considered taking into account the precession of the unit magnetization vector and director.

Magnetic vortices existing in soft magnetic nanoparticles with sizes larger than the single-domain diameter can be efficient nano-heaters in biomedical applications. Using micromagnetic numerical simulation we prove that in the optimal range of particle diameters the magnetization reversal of the vortices in spherical iron and magnetite nanoparticles is possible for moderate amplitudes of external alternating magnetic field. In contrast to the case of superparamagnetic nanoparticles, for the vortex configuration the hysteresis loop area increases as a function of frequency. So that high values of the specific absorption rate, on the order of 1 kW/g, can be obtained at a frequency 1.0 MHz. Because the diameter D of a non single-domain particle is several times larger than the diameter d of a superparamagnetic particle, the volume of heat generation for the vortex turns out to be many times larger. This shows the advantage of vortex configurations for heat generation in alternating magnetic field in biomedical applications., 7 pages, 10 figures

A specific absorption rate of a dilute assembly of various random clusters of iron oxide nanoparticles in alternating magnetic field has been calculated using Landau- Lifshitz stochastic equation. This approach simultaneously takes into account both the presence of thermal fluctuations of the nanoparticle magnetic moments, and magneto-dipole interaction between the nanoparticles of the clusters. It is shown that for usual 3D clusters the intensity of magneto- dipole interaction is determined mainly by the cluster packing density eta = Np*V/Vcl, where Np is the average number of the particles in the cluster, V is the nanoparticle volume, and Vcl is the cluster volume. The area of the low frequency hysteresis loop and the assembly specific absorption rate have been found to be considerably reduced when the packing density of the clusters increases in the range of 0.005 < eta < 0.4. The dependence of the specific absorption rate on the mean nanoparticle diameter is retained with increase of eta, but becomes less pronounced. For fractal clusters of nanoparticles, which arise in biological media, in addition to considerable reduction of the absorption rate, the absorption maximum is shifted to smaller particle diameters. It is found also that the specific absorption rate of fractal clusters increases appreciably with increase of the thickness of nonmagnetic shells at the nanoparticle surfaces., The paper is accepted for Nanoscale Res. Lett

The magnetostatic and electrodynamics properties of amorphous Co-rich microwire with a weak helical anisotropy has been studied taking into account the distribution of the residual quenching stresses over the microwire cross section. The components of the Giant Magneto- Impedance tensor have been obtained taking into account the necessary electromagnetic boundary conditions at the wire surface. A weak helical anisotropy of the microwire has been introduced through a small off-diagonal correction to the residual stress tensor. In this way an adequate description of the results of experimental measurements of the diagonal and off-diagonal components of the wire Giant Magneto- Impedance tensor at moderate frequencies has been obtained.

The micromagnetic simulation of the magnetization reversal process in a Co-rich microwire with a weak helical anisotropy under the influence of applied magnetic field and dc bias current has been carried out. It is shown that due to the influence of the magneto–elastic interaction in a wire with helical anisotropy, the change of the sign of the longitudinal magnetization component under the influence of the axial magnetic field leads to a subsequent jump of the circular magnetization component at some critical value of the applied magnetic field. This leads to a non-trivial behavior of the off-diagonal giant magneto-impedance (GMI) component of the wire as a function of applied magnetic field. The effect is confirmed experimentally for two Co-rich microwires, having significantly different amplitudes of the residual quenching stress.

The design and performance of a magnetometer based on the off-diagonal GMI effect in Co-rich glass-coated microwire are presented. The sensing element of the magnetometer is a 10-mm long piece of Co–Fe–Ni–B–Si–Mo microwire with a small pick-up coil of 85 turns wounded around the microwire. The electronics with a feedback circuit is used to register an electromotive force proportional to the external magnetic field applied along the wire axis. In the absence of the feedback current the magnetometer is capable of measuring a narrow range of magnetic fields, ±3.5 μT, in the frequency range of 0–1 kHz, the level of the equivalent magnetic noise being about 10 pT Hz−1/2 at a frequency of 300 Hz. The use of the feedback circuit increases the range of the measured magnetic fields up to ±250 μT. Photo of the giant magnetoimpedance magnetometer based on the off-diagonal GMI effect in Co-rich glass-coated microwire.

A new method of obtaining magnetic porous composite materials is described, which is based on the self-propagating high-temperature synthesis (SHS) in the form of solid-phase combustion. The SHS process involves transformation of the nonmagnetic α-Fe2O3 particles (contained in the initial mixture) into magnetic Fe3O4 particles. The synthesized material comprises a porous carbonaceous matrix with immobilized Fe3O4 particles. The obtained composite has been characterized by electron microscopy, X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The sorption capacity of the porous material has been studied.

The electrodynamic method is applied to determine the specific absorption rate (SAR) of an assembly of superparamagnetic nanoparticles as a function of frequency and magnetic field amplitude. The home made frequency-adjustable electromagnet is used to create a nearly uniform magnetic field in a core gap of a volume 1×3×3 cm3 in the frequency range f=10–150 kHz and for magnetic field amplitudes up to H 0=250 Oe. Two oppositely connected pick-up coils are used to record the electromotive force signal (EMF) generated by magnetic nanoparticles. By integrating the EMF signal one can determine the low-frequency hysteresis loops of the assembly and the assembly SAR. Using this method the measurement of SAR has been carried out for magnetite nanoparticles with an average diameter D=25 nm. The electrodynamic method is shown to be capable of measuring a small amount of magnetic nanoparticles, up to 5×10−5 g, dispersed in a solid matrix. The maximal SAR ∼ 80 W/g has been obtained for the magnetite nanoparticle assembly investigated.

Chemically synthesized iron oxide nanoparticles and magnetosomes produced by magnetotactic bacteria are of great importance for application in biomedicine. In this paper, we discuss the complicated magnetic anisotropy of the nanoparticles, the influence of the magnetostatic interactions, and thermal fluctuations on the behavior of these assemblies. Numerical simulation for dilute assemblies of iron oxide nanoparticles with combined magnetic anisotropy show that the uniaxial shape anisotropy dominates even for small aspect ratios of the particle, L/D≥1.1–1.2. The quasistatic hysteresis loops are calculated for various clusters of bacterial magnetosomes with diameters D=40–60 nm to understand the influence of magnetostatic interactions. The specific absorption rate (SAR) is calculated for assemblies of magnetic nanoparticles dispersed in solid and liquid media. A new electrodynamic method of measurement is used to obtain the SAR of the assembly of bacterial magnetosomes with average diameter D=48 nm.

The electrodynamic method is used to measure the hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D =25 nm in the frequency range f =10–150 kHz and for magnetic field amplitudes H 0 =100–300 Oe. It is found that the specific loss power is determined by a demagnetizing factor of a whole sample. It diminishes approximately 4.5 times when the sample aspect ratio decreases from L / d =11.4 to L / d ≈1, where L and d are the sample length and diameter, respectively. For H 0 ≤300 Oe the maximal specific loss power 120 W/g is obtained for the sample with L / d =11.4 at f =120 kHz. For comparison, the assembly specific absorption rate has been determined also by means of direct measurement of the temperature difference between the inner and outer surfaces of a flat cuvette containing magnetic nanoparticles. For both methods of measurement close values for the specific absorption rate are obtained for samples with similar demagnetizing factors.