External Pulsed Magnetic Field Research Articles

Flux jump-assisted pulsed field magnetisation of high-Jc bulk high-temperature superconductors

Investigating, predicting and optimising practical magnetisation techniques for charging bulk superconductors is a crucial prerequisite to their use as high performance ‘psuedo’ permanent magnets. The leading technique for such magnetisation is the pulsed field magnetisation (PFM) technique, in which a large magnetic field is applied via an external magnetic field pulse of duration of the order of milliseconds. Recently ‘giant field leaps’ have been observed during charging by PFM: this effect greatly aids magnetisation as flux jumps occur in the superconductor leading to magnetic flux suddenly intruding into the centre of the superconductor. This results in a large increase in the measured trapped field at the centre of the top surface of the bulk sample and full magnetisation. Due to the complex nature of the magnetic flux dynamics during the PFM process, simple analytical methods, such as those based on the Bean critical state model, are not applicable. Consequently, in order to successfully model this process, a multi-physical numerical model is required, including both electromagnetic and thermal considerations over short time scales. In this paper, we show that a standard numerical modelling technique, based on a 2D axisymmetric finite-element model implementing the H-formulation, can model this behaviour. In order to reproduce the observed behaviour in our model all that is required is the insertion of a bulk sample of high critical current density, Jc. We further explore the consequences of this observation by examining the applicability of the model to a range of previously reported experimental results. Our key conclusion is that the ‘giant field leaps’ reported by Weinstein et al and others need no new physical explanation in terms of the behaviour of bulk superconductors: it is clear the ‘giant field leap’ or flux jump-assisted magnetisation of bulk superconductors will be a key enabling technology for practical applications.

Magneto-optical extinction trend inversion in ferrofluids

Effects of pulse magnetic field on the optical transmission properties of thin ferrofluid (FF) layers were experimentally investigated. It was observed that, under an influence of an external uniform magnetic field, pulses applied to the samples surfaces in normal direction decrease the optical transmission with further returning it to its original state, even before the end of the field pulse. The dependencies of the observed effects on the magnetic pulse magnitude and the samples thickness were investigated. The experimental results are explained using FF columnar aggregates growth and lateral coalescence under influence of a magnetic field, leading to a light scattering type Rayleigh-to-Mie transition. Further evolution of this process comes to a geometrical optics scale and respective macroscopic observable opaque FF columnar aggregates emergence. These changes of optical transmission are non-monotonic during the magnetic field pulse duration with minimal value in the case of Mie scattering, which is known as a magneto-optical extinction trend inversion. The residual inversion was detected after the external magnetic field pulse falling edge. Using molecular dynamics simulation, we showed that a homogeneous external magnetic field is enough for the formation of columnar aggregates and their fusion. The results clarify the known Li theory (Li et al., 2004, 2007), implying an inhomogeneous field as a required prerequisite for the magneto-optical extinction trend inversion phenomenon.

The effect of an external magnetic field pulse on a magnetically ordered system at low temperatures

A theoretical study of how external magnetic field pulses affect the magnon system of a magnetically-ordered magnet. It is shown that for a magnetic system in which the projection of the total spin moment, parallel to the external magnetic field, is not conserved, the field pulse causes magnetization vibrations, related to pulse duration. The amplitude and frequency of these vibrations is non-linearly dependent on the amplitude of the field pulse and the relaxation parameters. In the case of a short field pulse, the change in magnetization it causes is proportional to the square of its duration.

Manipulation of Superparamagnetic Beads on Patterned Exchange-Bias Layer Systems for Biosensing Applications.

A technology platform based on a remotely controlled and stepwise transport of an array arrangement of superparamagnetic beads (SPB) for efficient molecular uptake, delivery and accumulation in the context of highly specific and sensitive analyte molecule detection for the application in lab-on-a-chip devices is presented. The near-surface transport of SPBs is realized via the dynamic transformation of the SPBs’ magnetic potential energy landscape above a magnetically stripe patterned Exchange-Bias (EB) thin film layer systems due to the application of sub-mT external magnetic field pulses. In this concept, the SPB velocity is dramatically influenced by the magnitude and gradient of the magnetic field landscape (MFL) above the magnetically stripe patterned EB substrate, the SPB to substrate distance, the magnetic properties of both the SPBs and the EB layer system, respectively, as well as by the properties of the external magnetic field pulses and the surrounding fluid. The focus of this review is laid on the specific MFL design in EB layer systems via light-ion bombardment induced magnetic patterning (IBMP). A numerical approach is introduced for the theoretical description of the MFL in comparison to experimental characterization via scanning Hall probe microscopy. The SPB transport mechanism will be outlined in terms of the dynamic interplay between the EB substrate’s MFL and the pulse scheme of the external magnetic field.

Directed Magnetic Particle Transport above Artificial Magnetic Domains Due to Dynamic Magnetic Potential Energy Landscape Transformation.

An approach for a remotely controllable transport of magnetic micro- and/or nanoparticles above a topographically flat exchange-bias (EB) thin film system, magnetically patterned into parallel stripe domains, is presented where the particle manipulation is achieved by sub-mT external magnetic field pulses. Superparamagnetic core-shell particles are moved stepwise by the dynamic transformation of the particles' magnetic potential energy landscape due to the external magnetic field pulses without affecting the magnetic state of the thin film system. The magnetic particle velocity is adjustable in the range of 1-100 μm/s by the design of the substrate's magnetic field landscape (MFL), the particle-substrate distance, and the magnitude of the applied external magnetic field pulses. The agglomeration of magnetic particles is avoided by the intrinsic magnetostatic repulsion of particles due to the parallel alignment of the particles' magnetic moments perpendicular to the transport direction and parallel to the surface normal of the substrate during the particle motion. The transport mechanism is modeled by a quantitative theory based on the precise knowledge of the sample's MFL and the particle-substrate distance.

Magnetic, in situ, mineral characterization of Chelyabinsk meteorite thin section

AbstractMagnetic images of Chelyabinsk meteorite's (fragment F1 removed from Chebarkul lake) thin section have been unraveled by a magnetic scanning system from Youngwood Science and Engineering (YSE) capable of resolving magnetic anomalies down to 10−3 mT range from about 0.3 mm distance between the probe and meteorite surface (resolution about 0.15 mm). Anomalies were produced repeatedly, each time after application of magnetic field pulse of varying amplitude and constant, normal or reversed, direction. This process resulted in both magnetizing and demagnetizing of the meteorite thin section, while keeping the magnetization vector in the plane of the thin section. Analysis of the magnetic data allows determination of coercivity of remanence (Bcr) for the magnetic sources in situ. Value of Bcr is critical for calculating magnetic forces applicable during missions to asteroids where gravity is compromised. Bcr was estimated by two methods. First method measured varying dipole magnetic field strength produced by each anomaly in the direction of magnetic pulses. Second method measured deflections of the dipole direction from the direction of magnetic pulses. Bcr of magnetic sources in Chelyabinsk meteorite ranges between 4 and 7 mT. These magnetic sources enter their saturation states when applying 40 mT external magnetic field pulse.

Controlling the spins angular momentum in ferromagnets with sequences of picosecond acoustic pulses.

Controlling the angular momentum of spins with very short external perturbations is a key issue in modern magnetism. For example it allows manipulating the magnetization for recording purposes or for inducing high frequency spin torque oscillations. Towards that purpose it is essential to modify and control the angular momentum of the magnetization which precesses around the resultant effective magnetic field. That can be achieved with very short external magnetic field pulses or using intrinsically coupled magnetic structures, resulting in a transfer of spin torque. Here we show that using picosecond acoustic pulses is a versatile and efficient way of controlling the spin angular momentum in ferromagnets. Two or three acoustic pulses, generated by femtosecond laser pulses, allow suppressing or enhancing the magnetic precession at any arbitrary time by precisely controlling the delays and amplitudes of the optical pulses. A formal analogy with a two dimensional pendulum allows us explaining the complex trajectory of the magnetic vector perturbed by the acoustic pulses.

The influence of external magnetic field and femtosecond laser pulses on the anomalous Kerr loops in GdFeCo amorphous film

The anomalous loops obtained by applying the traditional Kerr loop technique, in which the sample was pumped by continuous pulses using the method of successive increment scanning external magnetic field, contained memory and accumulation effects originating, respectively, from external field history and multiple pulse excitations. To eliminate the memory effect and study the influence of external magnetic field on the anomalous loops, an initializing field scanning approach was used to replace the method of successive increment scanning external field. And to confirm the existence of accumulation effect and study the influence of pump pulses on the anomalous loops, a controllable pump-pulse-number magneto-optical Kerr technique was used to replace continuous pulses pump Kerr technique. In both ways, measured with different pump fluence and different pump pulses, the results showed clearly the dependence of the hot coercivity and the degree of magnetization reversal on the external field, pump pulse numbers and pump fluence.

Dynamics of acceleration of millimeter-sized pellets in railgun accelerators with external magnetic field

A physical model of the acceleration of small (1–3 mm) solid pellets in a railgun with a plasma piston and an external pulsed magnetic field is proposed. For the typical operating parameters of the railgun, the effect of the erosion of electrodes, drag force, and dimensions of a body on its final velocity has been studied. It is shown that the saturation of velocity of 1- to 2-mm pellets occurs at short distances (15–20 cm) and is mainly governed by the capture of a portion of the erosion mass by the plasma piston.

A compact railgun accelerator for millimeter-sized dielectric solid armatures

Millimeter-sized dielectric solid armatures have been accelerated in a compact railgun system. It is shown that application of an external pulsed magnetic field can solve the problem of catastrophic erosion of electrodes at the initial stage and accelerate small armatures up to a velocity of about 6 km/s.

Theory of ground-state switching in an array of magnetic nanodots by application of a short external magnetic field pulse

A theory of a ground-state switching in an array of axially magnetized cylindrical magnetic dots arranged in a square lattice is developed. An array can be switched into a quasiregular chessboard-antiferromagnetic state by the application of a short pulse of external in-plane magnetic field having a sufficiently long trailing front. The statistical properties of an array magnetization in its final (after switching) state are determined at the linear stage of growth of unstable collective spin-wave modes of the array under the action of a time-dependent magnetic field, and depend critically on the rate of the field decrease: the slower this decrease, the more regular is the final magnetization state. An analytical procedure is presented that allows one to relate the statistical properties of the final demagnetized state of the array and the linewidth of the array's microwave absorption to the parameters of the external switching pulse. The comparison of the developed analytic theory with the results of numerical simulations is presented and demonstrates good agreement between analytical and numerical results.

Diagnostics of underwater electrical wire explosion through a time- and space-resolved hard x-ray source

A time- and space-resolved hard x-ray source was developed as a diagnostic tool for imaging underwater exploding wires. A ~4 ns width pulse of hard x-rays with energies of up to 100 keV was obtained from the discharge in a vacuum diode consisting of point-shaped tungsten electrodes. To improve contrast and image quality, an external pulsed magnetic field produced by Helmholtz coils was used. High resolution x-ray images of an underwater exploding wire were obtained using a sensitive x-ray CCD detector, and were compared to optical fast framing images. Future developments and application of this diagnostic technique are discussed.

Study of the direct current breakage process in an external nonuniform magnetic field

We study experimentally the influence of an external nonuniform magnetic field with transverse and longitudinal components on the electric characteristics, plasma configuration, and cathode spot arrangement of vaccum-arc discharge. It is revealed that for a cylindrical cathode, cathode spots are nonuniformly distributed on the cathode surface, the spot configuration has no axial symmetry, and the arrangement of spots changes in time depending on the induction value of the external pulsed magnetic field. With an increase in induction, spots (on average) are arranged closer to the cathode, i.e., displaced to the region of weaker transverse field. For two cathode geometries, the probabilities of direct current breakage depending on the induction of the external nonuniform magnetic field are experimentally determined. To determine the conditions of current breakage, a three-dimensional mathematical model of ion motion is suggested and the current breakage criterion is formulated. The trajectories of fast cathode ions in an electrode system with a ring cathode have been calculated using the model. Calculations were performed in a nonuniform magnetic field, the radial and axial components of which have been measured experimentally. It is shown that conditions of current breakage determined with this criterion agree with the results of experiment.

Effect of Pulsed Magnetic Field Treatment on the Magnetic Properties of the Fe-Nd-Al-B Alloys Prepared by Suction Casting

The magnetic properties of the (Fe0.53Nd0.37Al0.10)96B4bulk amorphous alloys annealed in the temperature range of 293-623K for 30 min with and without an external pulsed magnetic field of 5T were studied. The results reveal that the remanence and coercivity of the samples are obviously improved with a pulsed magnetic field when annealed below the Curie temperature of the alloy. The increment of remanence reaches 25.9% for the sample annealed at 523K with a pulsed magnetic field, because of the enhancement of exchange coupling roles between the soft and hard magnetic phases. It provides a new way to improve the magnetic properties of the bulk amorphous alloys produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys.

Pulsed Magneto-motive Ultrasound Imaging Using Ultrasmall Magnetic Nanoprobes

Nano-sized particles are widely regarded as a tool to study biologic events at the cellular and molecular levels. However, only some imaging modalities can visualize interaction between nanoparticles and living cells. We present a new technique, pulsed magneto-motive ultrasound imaging, which is capable of in vivo imaging of magnetic nanoparticles in real time and at sufficient depth. In pulsed magneto-motive ultrasound imaging, an external high-strength pulsed magnetic field is applied to induce the motion within the magnetically labeled tissue and ultrasound is used to detect the induced internal tissue motion. Our experiments demonstrated a sufficient contrast between normal and iron-laden cells labeled with ultrasmall magnetic nanoparticles. Therefore, pulsed magneto-motive ultrasound imaging could become an imaging tool capable of detecting magnetic nanoparticles and characterizing the cellular and molecular composition of deep-lying structures.

An approach to noninvasive delivery, biodistribution, and fertility control potential evaluation of the Cuproferrogel iron oxide-copper-styrene maleic anhydride-dimethyl sulphoxide in the female

Under guidance of an external pulsed magnetic field the Cuproferrogel iron oxide-copper-styrene maleic anhydride-dimethyl sulphoxide delivered into the rat/rabbit oviduct resulted in oocytes with granulated cytoplasm, zona enlargement, membrane disintegration, and finally loss of viability in 72 hours. Also, the percentage biodistribution of magnetic and electrically conductive particles observed under safe level advocates the use of Cuproferrogel as a potential female fertility control molecule.

Magneto-optical detection of single flux quantum signals in superconducting quantum interference device

The properties of an YBa2Cu3O7−d superconducting quantum interference device (SQUID) have been evaluated magneto-optically and electrically. It was found that the operating characteristics of the SQUID observed by means of the magneto-optical (MO) method show a good consistency with those measured by the electrical method. Furthermore, we have succeeded in the MO detection of local magnetic signals corresponding to the single flux quantum pulses generated in the SQUID loop applied with an external pulsed magnetic field. These results indicate that the MO probe has the potential to be applied to an optical output interface for superconducting magnetic flux devices.

Switching properties of ferromagnetic nanoparticles driven by a circularly polarizedmagnetic field

We present a comprehensive study of the magnetization switching of a uniaxialnanoparticle driven by a circularly polarized magnetic field rotated in the planeperpendicular to the easy axis. The conditions for the existence of the uniform andnon-uniform precessions of the nanoparticle magnetic moment are derived. In addition, thedifferences between switchings via uniform and non-uniform precession are determined, andthe essential role of field polarization is demonstrated. The dependence of the switchingtime on the field amplitude and frequency are calculated numerically. We show that apermanent magnetic field can reduce the amplitude and frequency of the switchingrotating field, and that the combined action of these fields is characterized by anextremely strong dependence of the switching time on the field parameters. Wealso demonstrate that the transition process caused by an external magneticfield pulse can decrease the switching amplitude in comparison with the valuepredicted from analysis of the stability criterion. We discuss the advantages ofswitching the magnetization by means of the action of a rotating field over themagnetization switching using a steady field applied perpendicular to the easy axis.

Micromagnetic modeling of the magnetization dynamics in a circularly exchange-biased and exchange-coupled ferromagnetic multilayer

The magnetization dynamics of a magnetically coupled multilayer structure have been studied by analytical and numerical methods. The simulated multilayer is disk-shaped and consists of a circularly exchange-biased ferromagnetic permalloy (Py) layer coupled to an unbiased Py layer, each in a magnetic vortex configuration, separated by a thin nonmagnetic spacer. The sign and strength of the interlayer exchange coupling was varied, leading to either parallel or antiparallel vortex chiralities in the two Py layers. The magnetization dynamics after the application of an external magnetic field pulse normal to the plane of the disks were investigated. Both analytical and numerical models show two branches of frequency response of circularly symmetric eigenmodes for both parallel and antiparallel configurations. However, the upper branch mode in the antiparallel configuration is severely damped in the numerical simulations due to coupling with short-wavelength spin waves. The frequency of the modes can be tuned independently with interlayer exchange coupling strength and exchange-bias strength. The good agreement between the mode frequencies obtained from the analytical and numerical models confirms that the main driving forces for the eigenmodes are the magnetostatic field from the radial motion of the magnetization, and also the interlayer exchange coupling field. In addition, themore » vortex cores, which are neglected in the analytical model, are found to play no significant role in the dynamic response.« less

Optical spin manipulation for minimal magnetic logic operations in metallic three-center magnetic clusters

We present a first-principles scenario where a realistic three-magnetic-center metallic cluster acts as a prototypic magnetic-logic element within the frame of a unified optically induced spin manipulation. We find that the spins of the energetically low-lying triplet states of a ${\text{Ni}}_{3}{\text{Na}}_{2}$ cluster are always localized at a single magnetic center and that controlled spin flips and transfers are possible within a hundred femtoseconds with suitable static external magnetic field and laser pulses. The magnetic state or the position of the spins and the static magnetic field can be used as input bits while the output bit is the final state of the magnetic centers, thus the gates AND, OR, XOR (CNOT), and NAND can be built.