Field Pulse Amplitude Research Articles

Ultra-short pulse magnetic fields on effective magnetic hyperthermia for cancer therapy

Alternating magnetic fields can deliver magnetic energy deeper inside the body for magnetic hyperthermia cancer therapy by using magnetic nanoparticles (MNPs). In this study, we proposed a highly effective heat generation method for the MNPs by the application of an ultra-short pulse wave. We numerically evaluated the heating power with a variety of parameters, such as pulse width, field amplitude, and frequency. The hysteresis curve and magnetization dynamics clearly indicate larger energy dissipation. Hysteresis loss and the input energy increase with increasing field strength and duty ratio and there is a large efficiency power condition. To evaluate the effective heat generation and practical temperature increment, a larger imaginary part of magnetic susceptibility (χ″ > 30) and specific loss power (SLP > 105 W/kg) are required. In addition, larger intrinsic loss power (100 nHm2/kg) is achieved. The results indicate that the contribution of magnetic harmonics signals on the ultra-short pulse wave significantly enhances the heat generation of MNPs for cancer therapy.

Properties of Ferromagnetic Josephson Junctions for Memory Applications

In this work we give a characterization of the RF effect of memory switching on Nb-Al/AlOx-(Nb)-Pd$_{0.99}$Fe$_{0.01}$-Nb Josephson junctions as a function of magnetic field pulse amplitude and duration, alongside with an electrodynamical characterization of such junctions, in comparison with standard Nb-Al/AlOx-Nb tunnel junctions. The use of microwaves to tune the switching parameters of magnetic Josephson junctions is a step in the development of novel addressing schemes aimed at improving the performances of superconducting memories.

Inversion of the domain wall propagation in synthetic ferrimagnets

We report on magnetic domain wall velocity measurements in a synthetic ferrimagnet made of two perpendicular ferromagnetic layers antiferromagnetically exchange coupled. In this system, two types of transitions may be observed: one from a parallel alignment to an antiparallel alignment of the magnetization of the two layers and the other between the two possible antiparallel alignments. Those transitions are shown to be dominated by domain wall propagation. The domain wall velocity as a function of the applied magnetic field pulse amplitude has been measured. Two remarkable features are observed: first, a drastic breakdown of the domain wall velocity and then an inversion of the domain propagation direction are observed when the field pulses reach values comparable to the exchange field between the two layers. This unexpected behavior can be understood qualitatively using a simple model taking into account the competition between interlayer exchange coupling and the external driving field.

Anomalous results observed in magnetization of bulk high temperature superconductors—A windfall for applications

Recent experiments on pulsed-zero field cool magnetization of bulk high Jc YBCO (YBa2Cu3O7-δ) have shown unexpected results. For example, reproducible, non-destructive, rapid, giant field leaps (GFLs) to higher penetrated field are observed. The observations are inconsistent with the critical state model (CSM), in several aspects. Additional experiments have been pursued in an attempt to clarify the physics involved in the observed anomalies. Here, we present experimental results for the Jc dependence of the anomalous features. It is found that the sudden field increase in the GFL is a monotonically increasing function of Jc. The ratio of required pulsed field amplitude, BA,max, to obtain maximum trappable field, BT,max, which CSM predicts to be ≥2.0, gradually approaches 1.0 at high Jc. Tests using values of pulsed, applied field BA,max just below the GFL exhibit two additional anomalies: (i) At high Jc, the highest trapped field is up to ∼6 times lower than predicted by CSM, and (ii) the measured Lorentz force as a function of Jc deviates sharply from CSM predictions. The data rule out heating effects and pinning center geometry as possible physical causes of these anomalies. A speculative cause is considered.

Analytical solution of precessional switching in nanomagnets driven by hard-axis field pulses

The precessional switching process of a magnetic nanoparticle subject to external field pulses applied along the hard-axis is considered. The critical field pulse amplitude necessary to realize the switching is determined. Then, the analytical solution of magnetization switching dynamics is derived in the lossless limit by using elliptic functions. Moreover, expressions for the field pulse duration tolerances which guarantee successful switching are also obtained. The theoretical predictions are verified by macrospin numerical simulations of ultra-fast magnetization switching.

Switching of magnetic vortex core in a Pac-man disk using a single field pulse

We report on the switching of the magnetic vortex core in a Pac-man disk using a magnetic field pulse, investigated via micromagnetic simulations. The minimum core switching field is reduced by 72% compared with that of a circular disk with the same diameter and thickness. However, the core switches irregularly with respect to both the field pulse amplitude and duration. This irregularity is induced by magnetization oscillations that arise owing to the excitation of spin waves when the core annihilates. We show that the core switching can be controlled with the assist magnetic field and by changing the waveform.

Pulse Rise Time Dependence of Switching Field of Co/Pt Multilayer Dot

SUMMARYA voltage pulse generator for magnetic field application was built using a coaxial cable as a capacitor. The pulse duration was fixed to 10 ns and the pulse field rise time was controlled from 70 ps to 4 ns using low‐pass filters. Magnetization switching experiments were carried out on a Co/Pt multilayer dot with diameter of 300 nm with a perpendicular pulse field. The required pulse field amplitude for magnetization switching is independent of the pulse rise time, suggesting that the thermally activated process is dominant in the perpendicular field geometry. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 9–14, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11569

Dynamic switching of the spin circulation in tapered magnetic nanodisks

Magnetic vortices are characterized by the sense of in-plane magnetization circulation and by the polarity of the vortex core. With each having two possible states, there are four possible stable magnetization configurations that can be utilized for a multibit memory cell. Dynamic control of vortex core polarity has been demonstrated using both alternating and pulsed magnetic fields and currents. Here, we show controlled dynamic switching of spin circulation in vortices using nanosecond field pulses by imaging the process with full-field soft X-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core, and the reversal is achieved at significantly reduced field amplitudes when compared with static switching. We further show that both the field pulse amplitude and duration required for efficient circulation reversal can be controlled by appropriate selection of the disk geometry.

Stability of a cable in conduit conductor under fast magnetic field variations

Under fast magnetic field variations, ac losses are deposited in a Cable in Conduit Conductor (CICC). The corresponding power losses are transferred to helium thanks to the high wetted perimeter of the conductor. The critical energy of the CICC can be expected to be proportional to the high volumetric heat capacity of helium and to the temperature margin. To confirm the expectations, stability tests under a transversal pulsed magnetic field were performed in the Sultan test facility on a prototype JT-60SA conductor sample. The shape of the magnetic field variation as a function of time is a truncated sinusoid. The experimental results are not totally in agreement with expected behavior on two particular points: - the deposited energy in the conductor as a function of the pulsed field amplitude, measured by calorimetry, deviates from the expected quadratic behavior for coupling losses. The deviation is also increasingly dependent on the transport current. - at a given Sultan background magnetic field, the critical energies at low temperature margins are reduced in comparison with expected values. An explanation based on the saturation of parts of the cable is proposed.

Pulse Rise Time Dependence of Switching Field of Co/Pt Multilayer Dot

SUMMARY A voltage pulse generator for magnetic field application was built using a coaxial cable as a capacitor. The pulse duration was fixed to 10 ns and the pulse field rise time was controlled from 70 ps to 4 ns using low-pass filters. Magnetization switching experiments were carried out on a Co/Pt multilayer dot with diameter of 300 nm with a perpendicular pulse field. The required pulse field amplitude for magnetization switching is independent of the pulse rise time, suggesting that the thermally activated process is dominant in the perpendicular field geometry. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 9–14, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11569

Repetitive transcranial magnetic stimulator with controllable pulse parameters

The characteristics of transcranial magnetic stimulation (TMS) pulses influence the physiological effect of TMS. However, available TMS devices allow very limited adjustment of the pulse parameters. We describe a novel TMS device that uses a circuit topology incorporating two energy storage capacitors and two insulated-gate bipolar transistor (IGBT) modules to generate near-rectangular electric field pulses with adjustable number, polarity, duration, and amplitude of the pulse phases. This controllable pulse parameter TMS (cTMS) device can induce electric field pulses with phase widths of 10–310 µs and positive/negative phase amplitude ratio of 1–56. Compared to conventional monophasic and biphasic TMS, cTMS reduces energy dissipation up to 82% and 57% and decreases coil heating up to 33% and 41%, respectively. We demonstrate repetitive TMS trains of 3000 pulses at frequencies up to 50 Hz with electric field pulse amplitude and width variability less than the measurement resolution (1.7% and 1%, respectively). Offering flexible pulse parameter adjustment and reduced power consumption and coil heating, cTMS enhances existing TMS paradigms, enables novel research applications and could lead to clinical applications with potentially enhanced potency.

Co/Pt multilayer dot switching experiments with sub-nanosecond pulse field

Magnetization switching experiments on a single Co/Pt multilayer dot of 300 nm in diameter have been carried out using pulse fields with the duration τp = 0.6–10.3 ns and the amplitude up to 3.2 kOe perpendicular to the film plane. The switching field increases from 3.4 kOe in a quasistatic field to 4.56 kOe in a pulse field for τp = 0.6 ns. From the analysis of the field duration dependence of the switching field based on the Néel-Arrhenius model, the energy barrier E0 = 214 kBT and the intrinsic switching field H0 = 5.0 kOe were obtained. Those two parameters well reproduce the experimentally observed dependence of switching probability on pulse field amplitude, indicating that the magnetization behavior of the dot can be described by the thermal fluctuation effect for a single barrier over the field duration ranging from subnanoseconds to quasistatic regime.

Complex pulsed field magnetization behavior and Walker breakdown in a NiFe thin-film

The magnetization behavior of a Permalloy thin-film (nominally Ni81Fe19) was investigated as a function of combined quasistatic and pulsed magnetic fields measured using magneto-optic Kerr effect magnetometry. We observed complex field dependent switching behavior that depends on the relative contributions to the total field of the quasistatic and pulsed fields. As the pulsed field amplitude was increased, complex switching behavior occurs for total fields in excess of the coercive field. A simple phenomenological domain wall propagation model suggests a qualitative understanding of this complex behavior based on Walker breakdown of the domain wall motion occurring in the Permalloy thin-film.

Ionization of a multilevel atom by ultrashort laser pulses

Specific features of ionization of single atoms by laser fields of a near-atomic strength are investigated. Calculations are performed for silver atoms interacting with femtosecond laser pulses with wavelengths λ = 800 nm (Ti:Sapphire) and λ = 1.064 μm (Nd:YAG). The dependences of the probability of ionization and of the form of the photoelectron energy spectra on the field of laser pulses for various values of their duration are considered. It is shown that the behavior of the probability of ionization in the range of subatomic laser pulse fields is in good agreement with the Keldysh formula. However, when the field strength attains values close to the atomic field strength, the discrepancies in these dependences manifested in a decrease in the ionization rate (ionization stabilization effect) or in its increase (accelerated ionization) are observed. These discrepancies are associated with the dependence of the population dynamics of excited discrete energy levels of the atom on the laser pulse field amplitude.

Enhancement of trapped field and total trapped flux on GdBaCuO bulk by theMMPSC+IMRA method

The trapped field BTP(z) andthe total trapped flux ΦT(z) asa function of the distance z from the bulk surface have been measured on a GdBaCuO superconducting bulk of 45 mm indiameter, which was magnetized by a sequential pulsed field application (SPA) method, amodified multi-pulse technique with stepwise cooling (MMPSC), and a subsequent iterativemagnetizing operation with gradually reduced pulse field amplitude (IMRA). A trapped field ofBTP = 3.10 T was realized by the MMPSC method, which was higher than that attained by SPA (BTP = 2.35 T).ΦT (0.5 mm), which was 1.64 mWb after the MMPSC method, was enhanced by about53%, to 2.51 mWb, by the subsequent IMRA method due to the additionalflux trapping. The MMPSC method combined with the IMRA process (MMPSC+IMRA) is an effective and promising pulse field magnetizing technique to enhance bothBTP andΦT on any superconducting bulks.

Dynamical reversal of rectangular nanodot studied by micromagnetics

The dynamical reversal of rectangular CoFe nanodot with initial C-state subject to square magnetic field pulse has been studied by using the micromagnetic simulations. It was found that the reversals proceeded through different modes as the pulse amplitude is increased. For the weak magnetic field pulse， the magnetization reversal is realized through the motion of the end domain walls toward the central part of magnet and， subsequently， the formation and motion of a single vortex. As the strength of magnetic field pulse become stronger， the reversal is accomplished through the motion of the end domain walls plus the formation and motion of two vortex structure. For much stronger field pulse the reversal process is the motion of the end domain walls and subsequent formation and annihilation of several vortices. The dependence of reversal time on the field pulse amplitude displays oscillating characteristic due to the transitions of reversal modes with the field strength.

Enhancement of Trapped Field and Total Trapped Flux on High Temperature Bulk Superconductor by a New Pulse-Field Magnetization Method

A higher trapped field BTP and larger total trapped flux ΦT have been achieved on a SmBaCuO bulk superconductor (φ 45 mm) by a modified multipulse technique with stepwise cooling (MMPSC) and a subsequent iterative magnetizing operation with gradually reduced pulse field amplitude (IMRA). BTP=4.33 T has been realized at the center of the bulk surface by the MMPSC method, which is higher than that attained by a single-pulse application (BTP=3.3 T). After the IMRA process, ΦT (5 mm) = 1.55 mWb was achieved 5 mm above the bulk surface, which is about 35% larger than that after the MMPSC process. The MMPSC method combined with the IMRA process (MMPSC-IMRA) is demonstrated to be a universal and promising pulse-field magnetization technique for enhancing both BTP and ΦT on any superconducting bulks.

Thermal Reversal of Magnetic Grains Under Time Varying Pulse Field

Media thermal magnetization switching under an arbitrary time varying field, pulse shape, and duration is an important issue in many recording applications. This paper shows a scale dilemma in using the time dependent energy barrier approach to describe long time thermal reversal of magnetic grains under a time varying pulse field. A new model approach based on an optimal reversal path is proposed to characterize the thermal reversal behavior of magnetic grains under an arbitrary time varying pulse field. Using this new model approach, the effects of pulse field duration, width, and amplitude on magnetic grain thermal switching behavior are studied.

90° pulsed magnetization of ferrite-garnet films with easy-plane anisotropy

The process of pulsed 90° magnetization of ferrite-garnet films was studied. These films, in addition to easy-plane anisotropy, have biaxial anisotropy in the film plane with an effective field HK2 ≅ 40–55 Oe. the pulsed magnetization curve contains two portions separated by a kink observed at a field pulse amplitude Hp=Hp* ≅ 16–18 Oe. An analysis of the magnetization signals showed that the restoring torque, which is mainly caused by biaxial-anisotropy forces, is overcome in fields Hp ≥ Hp* and that magnetization rotation occurs. In fields Hp Hp*, the process of magnetization is accompanied by oscillations of the magnetization vector. In contrast to free magnetization oscillations, these oscillations are nonlinear and the frequency of the first harmonic (≈5 × 108 Hz) is much lower than that for free oscillations, (7–12) × 108 Hz. Oscillations are excited at a pulse rise time of ≈6 ns.

Focus on Photonics and Imaging

Focus on Photonics and Imaging