Constant Magnetic Field Strength Research Articles

Magnetically Actuated Artificial Microswimmers as Mobile Microparticle Manipulators

AbstractMicroscale swimming robots have been envisaged for many biomedical applications such as targeted drug delivery, where the microrobot will be expected to navigate in a fluid environment while carrying a payload. We show that such a payload does not have to be physically bound to the swimmer, but may be instead manipulated by the microrobot through hydrodynamic interaction. We consider a magnetically actuated artificial microswimmer, whose locomotion induces a disturbance velocity field in the fluid, which moves a cargo particle in its vicinity. The problem investigated in this paper is therefore one of coupled locomotion-manipulation of two bodies in a fluid. The swimmer is actuated by a uniform, rotating magnetic field of constant strength leading to net motion in the direction perpendicular to the plane of rotation if the frequency associated with the periodic magnetic field is above a critical frequency. Below this critical frequency, the swimmer tumbles in place without net locomotion. Controlled motion of the particle and swimmer is achieved by switching the planes of rotation of the magnetic field and the frequency of the magnetic field above and below the critical frequency. The results of this paper show that microswimmers can be utilized as mobile manipulators of microparticles in a fluid.

Effect of static magnetic field on bone and its molecular mechanism

As a non-invasive physical factor, static magnetic field (SMF) with a constant magnetic field strength and direction, has a long history of basic and clinical research in bone biology. Animal experiments and clinical studies demonstrate that local or systemic exposure to SMF can prevent osteoporosis, increase bone mass and bone density. Fracture nonunion is a challenge for surgeons. SMF with certain magnetic field intensity can increase fracture healing rate and shorten fracture healing time. Bone defect is a shortage of bone due to trauma or surgery. Studies have shown that exposure to a SMF near the implant can effectively promote bone defect repair. In dental defects, direct magnetic implants can better promote bone formation around the implant, enhance the combination of the implant and bone, and increase the stability of the implant. In addition, the development and pain of osteoarthritis can be suppressed and relieved by SMFs. The effect of SMF on bone diseases may be associated with its regulation on the proliferation and differentiation of bone cells. In vitro , although different studies display that SMFs have different effects on the proliferation of bone marrow mesenchymal stem cells and osteoblasts, the same conclusions are shown that osteogenesis can be induced by SMFs. In contrast, the osteoclastic differentiation and bone resorption activity are inhibited by SMFs. Furthermore, SMF is also possible by affecting bone cells, cartilage cells and endothelial cells involved in remodeling and repair of bone tissue. For the mechanism of SMF affecting bone metabolism, existing studies have revealed that SMFs promote the expression and secretion of cytokines, regulate the expression of cell signal molecules, cause rearrangement of cytoskeleton, change the physical characteristics of cell membranes, affect the concentration and distribution of intracellular calcium ions, and regulate cellular iron metabolism. There are still many challenges in the application of SMF in bone biology. First, more basic experiments need to be performed to further determine the biological effects of SMF on bone. Second, it is necessary to explore the mechanism of the effect of SMF on bone from a physical and chemical level. Third, more clinical experimental data are needed to support the therapeutic effect of SMF on bone diseases. Finally, we should strengthen cooperation between basic research scientists and enterprises that manufacture magnetic therapy products to promote the application of SMF in the field of orthopedics.

Nonlinear vibrations of single- and double-walled carbon nanotubes resting on two-parameter foundation in a magneto-thermal environment

The excellent mechanical, electrical, structural and thermal properties coupled with high strength to weight ratio of carbon nanotubes have tremendously expanded their applications in various industrial, engineering, physical and natural sciences processes. In this work, nonlocal elasticity theory is used to analyze nonlinear vibrations of single and double-walled carbon nanotubes resting on two-parameter foundation in a thermal and magnetic environment. With the aid of Galerkin decomposition method, the systems of nonlinear partial differential equations are transformed into systems of nonlinear ordinary differential equations which are solved using homotopy perturbation method. The developed analytical solutions are used to investigate the influences of elastic foundations, magnetic field, temperature rise, interlayer forces, small scale parameter and boundary conditions on the frequency ratio. From the results, it is observed that the frequency ratio for all boundary conditions decreases as the number of walls increases from single to double. Also, it is established that the frequency ratio is highest for clamped–simple supported and lowest for clamped–clamped supported. Additionally, the results revealed that the frequency ratio decreases with increase in the value of spring constant (k1) temperature and magnetic field strength. This work will enhance the applications of carbon nanotubes in structural, electrical, mechanical and biological applications especially in a thermal and magnetic environment.

Chaos, diffusivity, and spreading of entanglement in magnetic branes, and the strengthening of the internal interaction

We use holographic methods to study several chaotic properties of a super Yang-Mills theory at temperature T in the presence of a background magnetic field of constant strength B. The field theory we work on has a renormalization flow between a fixed point in the ultraviolet and another in the infrared, occurring in such a way that the energy at which the crossover takes place is a monotonically increasing function of the dimensionless ratio ℬ/T2. By considering shock waves in the bulk of the dual gravitational theory, and varying ℬ/T2, we study how several chaos-related properties of the system behave while the theory they live in follows the renormalization flow. In particular, we show that the entanglement and butterfly velocities generically increase in the infrared theory, violating the previously suggested upper bounds but never surpassing the speed of light. We also investigate the recent proposal relating the butterfly velocity with diffusion coefficients. We find that electric diffusion constants respect the lower bound proposed by Blake. All our results seem to consistently indicate that the global effect of the magnetic field is to strengthen the internal interaction of the system.

Graphene oxide activation with a constant magnetic field

The effect of constant magnetic field strength on activation of sensors modified with graphene oxide monolayers was investigated. The use of constant magnetic field resulted in improved electroanalytical properties of the sensors. It was proven that level of GO activation is clearly related to constant magnetic field strength. Moreover, it was demonstrated that observed phenomenon is stable in time.

Seesaw-structured triboelectric nanogenerator for scavenging electrical energy from rotational motion of mechanical systems

This study reports a seesaw-structured triboelectric nanogenerator (S-TENG) for efficiently harvesting electrical energy from rotational motion of mechanical systems. The designed S-TENG with two arms is comprised four contact-pair units with distinctly different triboelectric polarity materials. The generated electric power from the unique structure based on the seesaw system was double at every machine rotation cycle. A magnetically coupled contact mechanism was also utilized to reduce the wear of the polymer material. It was experimentally confirmed that the proposed design significantly improved the reliability and long-term stability in comparison with currently available TENG systems based on the sliding mechanism. In addition, micro-pyramid patterns were formed on the polymer surface to enhance the surface charge density. Through systematic experiments with a variety of operational conditions, an instantaneous maximum output power density of 13.86W/m2 was achieved at a rotation speed of 200rpm under a constant magnetic field strength of 0.2T. The S-TENG has been demonstrated as a direct power source to drive small electronic devices such as commercial LED arrays and to charge an energy storage unit. This study further expanded the potential applications of the S-TENG to realize the self-powered wireless sensor nodes such as structural health and condition monitoring system.

The Cosmic Large-Scale Structure in X-rays (CLASSIX) Cluster Survey

To search for a signature of an intracluster magnetic field, we compare measurements of Faraday rotation of polarised extragalactic radio sources in the line of sight of galaxy clusters with those outside. To this end, we correlated a catalogue of 1383 rotation measures of extragalactic polarised radio sources with galaxy clusters from the CLASSIX survey (combining REFLEX II and NORAS II) detected by their X-ray emission in the ROSAT All-Sky Survey. The survey covers 8.25 ster of the sky at | bII | ≥ 20°. We compared the rotation measures in the line of sight of clusters within their projected radii of r500 with those outside and found a significant excess of the dispersion of the rotation measures in the cluster regions. Since the observed rotation measure is the result of Faraday rotation in several presumably uncorrelated magnetised cells of the intracluster medium, the observations correspond to quantities averaged over several magnetic field directions and strengths. Therefore the interesting quantity is the dispersion or standard deviation of the rotation measure for an ensemble of clusters. In the analysis of the observations we found a standard deviation of the rotation measure inside r500 of about 120 (± 21) rad m-2. This compares to about 56 (± 8) rad m-2 outside. Correcting for the effect of the Galaxy with the mean rotation measure in a region of 10 deg radius in the outskirts of the clusters does not change the outcome quoted above. We show that the most X-ray luminous and thus most massive clusters contribute most to the observed excess rotation measure. Modelling the electron density distribution in the intracluster medium with a self-similar model based on the REXCESS Survey, we found that the dispersion of the rotation measure increases with the column density, and we deduce a magnetic field value of about 2−6 (l/ 10 kpc)− 1/2μG assuming a constant magnetic field strength, where l is the size of the coherently magnetised intracluster medium cells. This magnetic field energy density amounts to a few percent of the average thermal energy density in clusters. On the other hand, when we allowed the magnetic field to vary such that the magnetic energy density is a constant fraction of the thermal energy density, we deduced a slightly lower value for this fraction of 3−10 (l/ 10 kpc)− 1/2 per mille. Compared to the situation in the Milky Way, where the ratio of the magnetic to thermal energy density is about unity, this ratio is much lower in galaxy clusters. The reason for this is most probably the different generation mechanism for the magnetic field, which is mostly powered by supernovae in the Galaxy and by turbulence from cluster mergers in galaxy clusters. The latter process sets a natural upper limit on the growth of the magnetic field.

On eigenvalue accumulation for non-self-adjoint magnetic operators

In this work, we use regularized determinants to study the discrete spectrum generated by relatively compact non-self-adjoint perturbations of the magnetic Schrödinger operator (−i∇−A)2−b in R3, with constant magnetic field of strength b>0. The distribution of the above discrete spectrum near the Landau levels 2bq, q∈N, is more interesting since they play the role of thresholds of the spectrum of the free operator. First, we obtain sharp upper bounds on the number of complex eigenvalues near the Landau levels. Under appropriate hypothesis, we then prove the presence of an infinite number of complex eigenvalues near each Landau level 2bq, q∈N, and the existence of sectors free of complex eigenvalues. We also prove that the eigenvalues are localized in certain sectors adjoining the Landau levels. In particular, we provide an adequate answer to the open problem from [34] about the existence of complex eigenvalues accumulating near the Landau levels. Furthermore, we prove that the Landau levels are the only possible accumulation points of the complex eigenvalues.

Angular Dependence of Tunneling Magnetoresistance in Hybrid Fe/GaAlAs/GaMnAs Magnetic Tunnel Junctions

Tunneling magnetoresistance (TMR) phenomena in hybrid Fe/GaAlAs/GaMnAs magnetic tunnel junctions (MTJs) were investigated by rotating a magnetic field of constant strength in the film plane. When a strong field (e.g., 4000 G) is used, the magnetization in GaMnAs and Fe coherently rotates in both layers, resulting in a smooth angular dependence of TMR. In contrast, abrupt transition steps and plateaus are observed in TMR, when a weak field (below 100 G) is rotated. The behavior observed in strong fields is ascribed to tunneling anisotropic magnetoresistance, an effect that occurs when magnetizations in both magnetic layers in the MTJ are aligned parallel to each other. The tunneling behavior observed in weak fields, on the other hand, is caused by differences in relative magnetization alignments in the two layers that arise from differences in their magnetocrystalline anisotropies. The latter behavior provided the anisotropic TMR that involved with parallel and antiparallel alignments at specific crystallographic directions.

Particle-in-cell simulations of collisionless magnetic reconnection with a non-uniform guide field

Results are presented of a first study of collisionless magnetic reconnection starting from a recently found exact nonlinear force-free Vlasov–Maxwell equilibrium. The initial state has a Harris sheet magnetic field profile in one direction and a non-uniform guide field in a second direction, resulting in a spatially constant magnetic field strength as well as a constant initial plasma density and plasma pressure. It is found that the reconnection process initially resembles guide field reconnection, but that a gradual transition to anti-parallel reconnection happens as the system evolves. The time evolution of a number of plasma parameters is investigated, and the results are compared with simulations starting from a Harris sheet equilibrium and a Harris sheet plus constant guide field equilibrium.

Tunnel magnetoresistance of thin‐film nanogranular metal–dielectric composites (x)Ni–(1−x)PNBZT

Thin‐film nanogranular metal–dielectric composites (x)Ni–(1−x)[Pb0.81Sr0.04(Na0.5Bi0.5)0.15][(Zr0.575Ti0.425)]O3 (denoted as (x)Ni–(1−x)PNBZT) were prepared by the ion‐beam sputtering method. The X‐ray diffraction analysis has revealed the presence of two phases in the composites: a crystalline Ni phase and an amorphous PNBZT phase. Composites (x)Ni–(1−x)PNBZT with compositions below the percolation threshold manifest a negative magnetoresistance, whose highest values of 0.13% at 287 K and 0.78% at 77 K are observed near the percolation threshold at хс = 35 at.% for a composite sample of 0.31Ni–0.69PNBZT placed in a constant magnetic field with the strength of 8000 Oe. The magnetoresistance of the composites (x)Ni–(1−x)PNBZT is increased with the growth of the constant magnetic field strength, and the dependence remains the same as a polarity of the magnetic field is varied. The charge transport in the composites (x)Ni–(1−x)PNBZT over a temperature interval of 188–287 K is shown to be attributed to the inelastic resonant tunneling of electrons through energy barriers in a dielectric layer of PNBZT between crystalline ferromagnetic nanogranules of Ni. Therefore, to explain the experimentally established regularities of the negative magnetoresistance, the tunnel magnetoresistance model has been used.

Direct magnetoelectric effect in three-layer Fe0.45Co0.45Zr0.1-PbZr0.53Ti0.47O3-Fe0.45Co0.45Zr0.1 composites

Samples of three-layer amorphous ferromagnet-piezoelectric-amorphous ferromagnet composites with different thicknesses of the Fe0.45Co0.45Zr0.1 layers have been fabricated by ion-beam sputtering of the Fe0.45Co0.45Zr0.1 cast alloy on largest surfaces of the PbZr0.53Ti0.47O3 piezoelectric plates prepared by ceramic technology. The influence of the alternating magnetic field frequency, thickness of ferromagnetic layers, constant magnetic field strength, and temperature on the specific features of the manifestation of the direct magnetoelectric effect has been investigated for the fabricated composites.

Gamma-ray observations of blazars and the intergalactic magnetic field spectrum

Very-high energy observations of blazars can be used to constrain the strength of the intergalactic magnetic field. A simplifying assumption which is often made is that of a magnetic field of constant strength composed by randomly oriented and identical cells. In this paper, we demonstrate that a more realistic description of the structure of the intergalactic magnetic field is indeed needed. If such a description is adopted, the observational bounds on the field strength are significantly affected in the limit of short field correlation lengths: in particular, they acquire a dependence on the magnetic field power spectrum. In the case of intergalactic magnetic fields which are generated causally, for which the magnetic field large scale spectral index is $n_B\geq 2$ and even, the observational lower bound becomes more constraining by about a factor 3. If instead $-3<n_B<-2$, the lower bound is significantly relaxed. Such magnetic fields with very red spectra can in principle be produced during inflation, but remain up to now speculative.

Modeling the heat transfer in magneto-sensitive shape-memory polymer nanocomposites with dynamically changing surface area to volume ratios

Magneto-sensitive shape-memory polymer nanocomposites (SMPNCs) enable non-contact actuation of a shape-memory effect (SME) by inductive heating in an alternating magnetic field (AMF). Hereby, the achievable temperature (Tmax) at fixed magnetic field strength (H) and frequency is depending on the amount and type of incorporated magnetic fillers as well as on surface area to volume (S/V) ratio of the test specimen.Here we present a heat transfer model for predicting Tmax of SMPNCs samples with different S/V ratios when exposed to an AMF. The obtained temperature difference between sample and surrounding in an AMF of constant magnetic field strength decreases at uni-axial deformation with the square root of the stretching ratio. The model was validated with magnetically heating experiments of two different SMPNC systems (comprising crystallizable or amorphous switching segments) containing the same magnetic nanoparticles, while H was varied from 7 to 27 kA m−1 at a fixed frequency of 258 kHz. The experimentally achieved temperatures at deformations up to 50% could be predicted with a divergence below 6%. Finally the model was applied in a principle design study of a device consisting of a rolled SMPNC stripe, which was stepwise opened by increasing H. The modeling approach might be helpful to predict the temperature profiles of SMPNCs which were heated by other mechanisms, e.g., radiofrequency or near IR.

Unsteady MHD Flow and Heat Transfer over a Stretching Permeable Surface with Suction or Injection

A theoretical investigation is presented to describe a two-dimensional unsteady flow of a viscous incompressible electrically conducting fluid over a stretching permeable surface taking into account a transverse magnetic field of constant strength. The governing boundary layer equations for the problem are formulated and converted to nonlinear ordinary differential equations and then solved numerically using fourth order Runge-Kutta method with shooting technique. Influences of many parameters, like, suction or injection parameter, unsteadiness parameter, magnetic parameter, Prandtl number and Eckert number on velocity and temperature distributions are plotted on graphs while skin-friction coefficient and Nusselt number are shown numerically. Computational results are compared with previously presented results for non-magnetic case.

A survey of quiet auroral arc orientation and the effects of the interplanetary magnetic field

Using data from the THEMIS All‐Sky Imager array, we have carried out an extensive study of the orientation of quiet auroral arcs relative to the magnetic east‐west direction. We used over 7500 images of quiet auroral arcs that were collected during extended solar minimum and at various geomagnetic latitudes and longitudes. For each arc, we determined its “tilt” (the angle the arc makes with the local magnetic east‐west direction) and its “multiplicity” (whether or not the arc was part of a multiple‐arc system). We have found that at more equatorward latitudes, arc tilts are within σSD = ±7.7∘. We determined that both single‐ and multiple‐arc systems tend to tilt a few degrees to the south‐east prior to 23 magnetic local time (MLT) and to the north‐east afterward. This tilt appears to be more prominent at higher latitudes. We compared the auroral arc orientations to the mapping of equatorial contours of constant magnetic field strength into the ionosphere, where we used the T87 and T89 magnetic field models for quiet (Kp = 1,3) conditions for the mappings and to determine the constant equatorial magnetic field strength contours. We found that the MLT trends of the tilts are such that arc alignment appears to follow the constant magnetic field strength contours as projected into the ionosphere. We assert that the systematic dependencies of the orientation of auroral arcs indicate that arc morphology is governed by the large‐scale structure of the magnetosphere as opposed to localized processes within the ionosphere. In addition, we studied the effects of the interplanetary magnetic field (IMF) on the location in MLT of the reversal of the arc tilts. We found that negative IMF Bxand Byconditions cause the reversal location to shift duskward of 23 MLT. Alternately, a positive IMF Bx, coupled with a negative By, results in a shift in reversal location toward magnetic midnight. This behavior is consistent with that found in studies of the MLT distribution of substorm onsets.

Lieb–Thirring type inequalities for non-self-adjoint perturbations of magnetic Schrödinger operators

Let H:=H0+V and H⊥:=H0,⊥+V be respectively perturbations of the unperturbed Schrödinger operators H0 on L2(R3) and H0,⊥ on L2(R2) with constant magnetic field of strength b>0, and V a complex relatively compact perturbation. We prove Lieb–Thirring type inequalities on the discrete spectrum of H and H⊥. In particular, these estimates give a priori information on the distribution of eigenvalues around the Landau levels of the operator, and describe how fast sequences of eigenvalues converge.

532 MCF(磁気混合流体)のクラスターを利用した応用開発研究(流体工学IV)

As one of intelligent fluid that is responsible to a magnetic field, we have developed MCF (magnetic compound fluid) in 2001. It involves magnetic clusters aggregated by particles inner the fluid. As controlling the behavior of the clusters by the application of the magnetic field, we can develop many applications: self-assembly, damper, nano-finishing and rubber sensor. As for the self-assembly, we can make the scale of the clusters memorizing by the application of a constant magnetic field strength. As for the damper, MCF damper has more damping effect than ordinary dampers, for example, magnetic fluid and magneto-rheological fluid dampers. As for polishing, we can obtain nano-finishing like mirror surface at the varied polishing fields for intricate shape, wall inner capillary, and so on. As for sensor, haptic sensor combined MCF with a rubber has high sensitivity to 0.01 N order pressure. The present study clarified these effective results of the engineering applications.

Air-Aided Shear on a Thin Film Subjected to a Transverse Magnetic Field of Constant Strength: Stability and Dynamics

The effect of air shear on the hydromagnetic instability is studied through (i) linear stability, (ii) weakly nonlinear theory, (iii) sideband stability of the filtered wave, and (iv) numerical integration of the nonlinear equation. Additionally, a discussion on the equilibria of a truncated bimodal dynamical system is performed. While the linear and weakly nonlinear analyses demonstrate the stabilizing (destabilizing) tendency of the uphill (downhill) shear, the numerics confirm the stability predictions. They show that (a) the downhill shear destabilizes the flow, (b) the time taken for the amplitudes corresponding to the uphill shear to be dominated by the one corresponding to the zero shear increases with magnetic fields strength, and (c) among the uphill shear-induced flows, it takes a long time for the wave amplitude corresponding to small shear values to become smaller than the one corresponding to large shear values when the magnetic field intensity increases. Simulations show that the streamwise and transverse velocities increase when the downhill shear acts in favor of inertial force to destabilize the flow mechanism. However, the uphill shear acts oppositely. It supports the hydrostatic pressure and magnetic field in enhancing films stability. Consequently, reduced constant flow rates and uniform velocities are observed.

High-frequency properties of Fe/Cr superlattices with thin Cr layers in the millimeter-wavelength range

Microwave properties of Fe/Cr multilayer nanostructures with thin chromium layers (with thickness tCr < 1 nm) are analyzed. Experiments are performed by the method of penetration of microwaves in the frequency range from 26 to 38 GHz. The dependence of the transmission coefficient for microwaves on the constant magnetic field strength exhibits the microwave magnetoresistive effect and magnetic resonance. The resonance spectrum is reconstructed from measurements at various frequencies. The results of microwave measurements are analyzed together with the results of magnetic and magnetoresistive measurements.