Simple metal clusters in magnetic fields.

We examine a time-dependent, three-dimensional rotation of magnetic ellipsoidal particles in a two-dimensional, simple shear flow and a uniform magnetic field. We consider that the particles have paramagnetic and ferromagnetic properties, and we compare their rotational dynamics due to the strengths and directions of the applied uniform magnetic field. We determine the critical magnetic field strength that can pin the particles’ rotations. Above the critical field strength, the particles’ stable steady angles were determined. In a weak magnetic regime (below the critical field strength), a paramagnetic particle’s polar angle will oscillate toward the magnetic field plane while its azimuthal angle will execute periodic rotations. A ferromagnetic particle’s rotation depends on its initial angles and the magnetic field strength and direction. Even when it is exposed to a critical magnetic field strength, its rotational dynamics will either be pinned in or out of the magnetic field plane. In a weak magnetic regime, a ferromagnetic particle will either execute out-of-plane rotations or will oscillate toward the magnetic field plane and perform periodic rotations. For both particles, we analytically determine the peaks and troughs of their oscillations and study their time-dependent rotations through analytical and numerical analyses.

Effects of applying uniform and non-uniform external magnetic fields on the optimal design of microchannel heat sinks

A detection system for magnetic inclusions of large bulk, such as that of a whole human body, is proposed in this paper. The system consists of both a uniform magnetic field generating apparatus capable of the insertion of a whole human body and also of a high-sensitivity magnetic sensor array installed in the strong magnetic field. The system can detect the magnetic inclusion simultaneously through its magnetization, which is advantageous for detecting low-remanence magnetic materials, such as a cluster of nanoparticles. The thin-film magneto-impedance sensor was reported to be capable of tolerating strong magnetic fields of more than 3000 Gauss (0.3 T) in the substrate’s normal direction and can retain its sensitivity even in strong fields. Through a combination of both uniformity of strength and the placement of its directionally aligned, static magnetic field in a particular measurement area and its array of single-dimensional thin-film magneto-impedance sensors, it was reported that it can estimate a magnetic sample’s 3D position by using a simple equation. The aim of the system developed in this study is to nondestructively detect a cluster of magnetic nanoparticles in a human body and also to detect the position and the concentration of the clustered magnetic particles. In this paper, a prototype system consisting of a magnetic field generator with an area of W500 mm × L400 mm × H210 mm and a uniform magnetic field of 370 Gauss (37 mT) is reported. It also reported that the thin-film magneto-impedance sensor installed in the system verified the detection of 2 mm × 1 mm small ellipsoidal magnetic chips at a distance of 27 mm from the sensor element.

The influence of the magnetic field on the convective heat transfer characteristics of Fe3O4/water nanofluids

Magnetoreception

view Abstract Citations (60) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Reflection and trapping of transient Alfven waves propagating in an isothermal atmosphere with constant gravity and uniform magnetic field An, C. -H. ; Musielak, Z. E. ; Moore, R. L. ; Suess, S. T. Abstract A time-dependent linear magnetohydrodynamic numerical model was used to investigate the propagation of Alfven waves in an isothermal and stratified atmosphere with constant gravity and uniform vertical magnetic field. Results show that the Alfven wave transit time for the wave source to infinity is finite and that the wave exhibits continuous partial reflection which becomes total reflection as the front approaches infinity. The total reflection causes the waves to be trapped in the cavity that extends from the wave source to infinity and in which the wave energy is stored. The results suggest that the reflection of Alfven waves (of sufficiently long period) from the outer corona is an intrinsic phenomenon for any stellar atmosphere stratified by gravity and an open magnetic field, and that, therefore, such waves may be trapped in the stellar atmosphere. Publication: The Astrophysical Journal Pub Date: October 1989 DOI: 10.1086/167933 Bibcode: 1989ApJ...345..597A Keywords: Computational Astrophysics; Magnetohydrodynamic Waves; Stellar Atmospheres; Stellar Magnetic Fields; Wave Propagation; Gravitational Effects; Kinetic Energy; Magnetic Field Configurations; Solar Wind; Stellar Coronas; Trapping; Wave Reflection; Astrophysics full text sources ADS |

Context. Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed. Aims. Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated with particularly strong magnetic fields at the inner edge of the penumbra of active region 10930. Methods. We applied classical diagnostic methods and various inversions with different model atmospheres to the observed Stokes profiles in two selected pixels with superstrong magnetic fields, and compared the results with a magnetohydrodynamic simulation of a sunspot whose penumbra contains localized regions with strong fields (nearly 5 kG at τ = 1) associated with supersonic downflows. Results. The different inversions provide different results: while the SPINOR 2D inversions consider a height-dependent single-component model and return B > 7 kG and supersonic positive vLOS (corresponding to a counter-Evershed flow), height-dependent two-component inversions suggest the presence of an umbral component (almost at rest) with field strengths ∼4 − 4.2 kG and a penumbral component with vLOS ∼ 16 − 18 km s−1 and field strengths up to ∼5.8 kG. Likewise, height-independent two-component inversions find a solution for an umbral component and a strongly redshifted (vLOS ∼ 15 − 17 km s−1) penumbral component with B ∼ 4 kG. According to a Bayesian information criterion, the inversions providing a better balance between the quality of the fits and the number of free parameters considered by the models are the height-independent two-component inversions, but they lie only slightly above the SPINOR 2D inversions. Since it is expected that the physical parameters all display considerable gradients with height, as supported by magnetohydrodynamic (MHD) sunspot simulations, the SPINOR 2D inversions are the preferred ones. Conclusions. According to the MHD sunspot simulation analyzed here, the presence of counter-Evershed flows in the photospheric penumbra can lead to the necessary conditions for the observation of ∼5 kG fields at the inner penumbra. Although a definite conclusion about the potential existence of fields in excess of 7 kG cannot be given, their nature could be explained (based on the simulation results) as the consequence of the extreme dynamical effects introduced by highly supersonic counter-Evershed flows (vLOS > 10 km s−1 and up to ∼30 km s−1 according to SPINOR 2D). The latter are much faster and more compressive downflows than those found in the MHD simulations and therefore could lead to field intensification up to considerably stronger fields. Also, a lower gas density would lead to a deeper depression of the τ = 1 surface, making possible the observation of deeper-lying stronger fields. The superstrong magnetic fields are expected to be nearly force-free, meaning that they can attain much larger strengths than expected when considering only balance between magnetic pressure and the local gas pressure.

Moving coil type electromagnetic microactuator using metal/silicon driving springs and parylene connecting beams for pure in-plane large motion in three axes

Recent laboratory and epidemiological results have stimulated interest in the hypothesis that human beings may exhibit biological responses to magnetic and/or electric field transients with frequencies in the range between 100 Hz and 100 kHz. Much can be learned about the response of a system to a transient stimulation by understanding its response to sinusoidal disturbances over the entire frequency range of interest. Thus, the main effort of this paper was to compare the strengths of the electric fields induced in homogeneous ellipsoidal models by uniform 100 Hz through 100 kHz electric and magnetic fields. Over this frequency range, external electric fields of about 25-2000 V/m (depending primarily on the orientation of the body relative to the field) are required to induce electric fields inside models of adults and children that are similar in strength to those induced by an external 1 microT magnetic field. Additional analysis indicates that electric fields induced by uniform external electric and magnetic fields and by the nonuniform electric and magnetic fields produced by idealized point sources will not differ by more than a factor of two until the sources are brought close to the body. Published data on electric and magnetic field transients in residential environments indicate that, for most field orientations, the magnetic component will induce stronger electric fields inside adults and children than the electric component. This conclusion is also true for the currents induced in humans by typical levels of 60 Hz electric and magnetic fields in U.S. residences.

Non-contact manipulation of nonmagnetic materials by using a uniform magnetic field: Experiment and simulation

We deduce the dissipative Alfven wave equation in a medium stratified in one direction, with a transverse magnetic field, in the presence of dissipation by fluid viscosity and electrical resistance; the dissipative Alfven wave equation generalizes earlier results for homogeneous (Cowling, 1960) and inhomogeneous (Campos, 1983a) media, and corrects an error in the literature (Heyvaerts and Priest, 1983). The wave equation is solved exactly in two cases: a uniform magnetic field, and a magnetic field decreasing with height. In both cases the mean state is assumed to be isothermal, with a constant rate of ionization, so that the magnetic diffusivity is constant, but the dynamic viscosity increases with height. There are therefore two regions, a low- (high-) altitude region where electrical resistance dominates fluid viscosity (or vice versa), and an asymptotic regime relevant to the uppermost (lowermost) layers. The two regions are separated by a transition layer, across which the wave field is cont...

Magneto-active elastomers (MAE) are a new branch of smart materials that consist of hard-magnetic particles such as barium ferrite in an elastomer matrix. Under the application of a uniform magnetic field, the MAE material undergoes large deformation as the material bends due to magnetic torques acting on the distribution of hard-magnetic particles. This behavior demonstrates the potential of MAEs to act as remote actuators. MAEs vary from magnetorheological elastomers (MRE) which use soft-magnetic iron particles in place of the hard-magnetic particles and they are driven by magnetic interactions between particles. In this work, MAEs were fabricated using 30% v/v 325 mesh M-type barium ferrite (BaM) particles in Dow Corning HS II silicone elastomers. Prior to curing, the samples were placed in a uniform (∼2 Tesla) magnetic field to align the magnetic particles and produce a magnetization oriented in the direction of the applied magnetic field. The specimens were bonded to a passive poldymethylsiloxane (PDMS) substrate to form a two-segment accordion structure where the MAEs with magnetization, M, were placed in opposing orientations a prescribed distance apart. The application of a uniform magnetic field perpendicular to the magnetization of the undeformed MAEs would result in a bend (on the PDMS) that is dependent upon the orientation of the magnetic particles and the direction of the applied field. This behavior of the composite structure highlights the ability of the MAE material to perform work. Experimental testing of the MAEs used a two-segment accordion structure with fixed boundary-conditions on both ends of the PDMS substrate and a uniform magnetic field was applied to the structure. The resulting deformation roughly represented either a mountain or valley fold (dependent upon the orientation of the applied field). The resulting axial force was observed and compared to computational simulations which utilized numerical techniques to develop approximate solutions. This procedure was repeated with a prescribed displacement on one of the two fixed boundary conditions to induce bending prior to the application of a uniform magnetic field. Results show a decrease in magnetic work potential with increases in the aforementioned prescribed displacement; results also show an increase in magnetic work potential with increases in the applied magnetic field.

Living organisms inspire the design of microrobots, buttheir functionalityis unmatched. Next-generation microrobots aim to leverage the sensingand communication abilities of organisms through magnetic hybridization,attaching magnetic particles to them for external control. However,the protocols used for magnetic hybridization are morphology specificand are not generalizable. We propose an alternative approach thatleverages the principles of negative magnetostatics and magnetophoresisto control nonmagnetic organisms with external magnetic fields. Todo this, we disperse model organisms in dispersions of Fe3O4 nanoparticles and expose them to either uniform orgradient magnetic fields. In uniform magnetic fields, living organismsalign with the field due to external torque, while gradient magneticfields generate a negative magnetophoretic force, pushing objectsaway from external magnets. The magnetic fields enable controllingthe position and orientation of Caenorhabditis elegans larvae and flagellated bacteria through directional interactionsand magnitude. This control is diminished in live spermatozoa andadult C. elegans due to stronger internalbiological activity, i.e., force/torque. Our study presents a methodfor spatiotemporal organization of living organisms without requiringmagnetic hybridization, opening the way for the development of controllableliving microbiorobots.

We analyze the influence of relativistic effects on the minimum evolution\ntime between two orthogonal states of a quantum system. Defining the initial\nstate as an homogeneous superposition between two Hamiltonian eigenstates of an\nelectron in a uniform magnetic field, we obtain a relation between the minimum\nevolution time and the displacement of the mean radial position of the electron\nwavepacket. The quantum speed limit time is calculated for an electron dynamics\ndescribed by Dirac and Schroedinger-Pauli equations considering different\nparameters, such as the strength of magnetic field and the linear momentum of\nthe electron in the axial direction. We highlight that when the electron\nundergoes a region with extremely strong magnetic field the relativistic and\nnon-relativistic dynamics differ substantially, so that the description given\nby Schroedinger-Pauli equation enables the electron traveling faster than c,\nwhich is prohibited by Einstein's theory of relativity. This approach allows a\nconnection between the abstract Hilbert space and the space-time coordinates,\nbesides the identification of the most appropriate quantum dynamics used to\ndescribe the electron motion.\n

: The ground state of an electron gas in a uniform magnetic field is found to be not the customary uniform state, but rather one in which a spin density wave exists, directed along the field. This conclusion is reached through what is essentially a Hartree-Fock calculation with a repulsive interaction, but in which no restrictive assumptions are made about either the strength or the range of the exchange interaction. Thus static screening does not eliminate the spin density wave in the presence of a magnetic field, as it does in the electron gas when no magnetic field is present. The temperature at which the transition to a spin density wave state occurs approaches zero as the field vanishes. The pertinent question is therefore not the nature of the ground state, but whether there is a range of field strengths and electronic densities for which the transition temperature is observably high. It is found that spin density wave formation is most favorable when only a few Landau levels are occupied, corresponding to large field strengths and low electronic densities. A rough calculation indicates that in InSb a transition temperature as high as 10 millidegrees can be realized. (Author)