Short Field Pulses Research Articles

Switching behavior of a Stoner particle beyond the relaxation time limit

Results of the switching properties of Stoner-like magnetic particles, subject to short magnetic field pulses, obtained by numerical investigations, are reported. The switching properties are discussed as a function of the external field pulse strength and direction, pulse length, and the pulse shape. For field pulses long compared to the precession time, the switching behavior is governed by the magnetic damping term. In the limit of short field pulses, switching properties are dominated by the details of the magnetic precession. In the latter case, the magnetic damping term is of minor importance and ultrafast switching can be achieved by choosing the right field pulse parameters. It is also possible to choose pulse parameters in order to provide switching over a wide range of applied field directions.

Successful suppression of magnetization precession after short field pulses

For the next generation of high data rate magnetic recording above 1 Gbit/s, a better understanding of the switching processes for both recording heads and media will be required, In order to maximize the switching speed for such devices, the magnetization precession after the magnetic field pulse termination needs to be suppressed to a maximum degree. It is demonstrated experimentally for ferrite films that the appropriate adjustment of the field pulse parameters and/or the static applied field may lead to a full suppression of the magnetization precession immediately upon termination of the field pulse. The suppression is explained by taking into account the actual direction of the magnetization with respect to the static field direction at the pulse termination.

Nonlinear excitation of AgBr nanocrystals in the field of short light pulses

Processes of the electron-hole pair generation and excited-state relaxation at earlier stages of the latent image formation in the AgBr nanocrystals are studied theoretically. The experimental dependence of the photographic blackening D in the field of the intense laser light on the light pulse energy I is analyzed in the range of pulse durations τi from 10−13 to 10−8 s. The values of I needed to obtain a fixed blackening D at τi∼10−10 s exceeded those at τi∼10−8 s by three orders of magnitude. However, as τi further decreased to hundreds of fs, these values decreased by two orders of magnitude. It is shown that experimental nonmonotonic dependences can be explained by saturation of the one-photon interband generation of electron-hole pairs accompanied by the development of the two-photon generation. For shortest pulses and, correspondingly, greatest light intensities, the pair generation rate varies more strongly than upon usual two-photon absorption, which is caused by the resonance optical Stark effect.

Time-domain near-field analysis of short-pulse antennas .II. Reactive energy and the antenna Q

For pt. I see ibid., vol.47, no.2, p.271-79 (1999). The time-domain (TD) multipole expansion, developed in the first part of this two-part sequence, is extended here to analyze the power-flow and energy balance in the vicinity of a pulsed antenna. Using the spherical transmission line formulation, we derive expressions for the pulsed power-flow and energy and identify the radiative and the reactive constituents. For time-harmonic fields, the reactive concepts are well understood in terms of the stored energy, but this interpretation is not applicable for short-pulse fields where there is no stored energy. By considering the TD energy balance, we clarify the transition of the near-zone pulsed reactive energy to the radiation power and show that the pulsed reactive energy discharges back to the source once the pulse has been radiated. We thus introduce a TD Q factor that quantifies the radiation efficiency. In particular, we show that super resolution using short-pulse fields involves large TD reactive energies and Q and is, therefore, not feasible. The general TD concepts discussed are demonstrated through a numerical example of radiation from a circular disk carrying a pulsed current distribution.

Analysis and characterization of ultrawide-band scalar volume sources and the fields they radiate. II. Square pulse excitation

For pt.I see ibid., vol.45, no.7, p.198-1106, 1997. In pt.I we studied transient radiation from scalar collimated volume source distributions subjected to impulsive excitation. In this paper, we extend our analysis and results to the case of nonimpulsive excitation, paying special attention to the parameterization of the radiation pattern of three-dimensional (3-D) ellipsoid source distributions driven by square pulses of finite duration. We study the role both of the source's space distribution and of the square pulse duration on the generation of well-collimated short-pulse fields. In particular, we explore the source's angular and range resolutions as functions of the transverse and longitudinal dimensions of the source (for a fixed source volume) and of the pulse duration.

Light-induced phase transitions in stratifying liquid mixtures

It is observed experimentally for the first time that light-induced barodiffusion is an important mechanism for local phase separation in a liquid stratifying mixture in the field of short laser pulses. This effect is applied for determination of the binodal location of the mixture, whereas inducing the laser cavitation in the mixture can be used to determine the spinodal location.

Asymmetry of the scattering amplitude of atoms in the field of short counterpropagating light pulses

Rectification in bipotential scattering of a beam of atoms in the field of short pulses of traveling and standing waves is studied: As a result of the coherence induced by the traveling-wave pulse, the momentum transferred to the atomic beam during scattering by the standing wave is nonzero. The magnitude and sign of the asymmetry in the scattering amplitude are oscillatory functions of the duration of the traveling-wave pulse and the detuning of the frequency of the field from atomic resonance.

Enhancement of short-pulse recombination-pumped gain by soft-x-ray photoionization of the ground state

The gain achieved in lasing to the ground state following short-pulse field ionization by a pump laser is highly transient. It will usually persist for only tens of picoseconds because of the rapid filling and negligible emptying of the ground state. Employing a detailed atomic model of lasing in hydrogen, we show that the removal of ground-state population by an appropriate broadband ionizing radiation field can enhance and prolong the gain in such a laser.

Local light-induced phase separation of binary liquid solutions

Light-induced barodiffusion is shown to be the dominant mechanism of local phase separation of liquid solutions in the field of short laser pulses. This effect is utilised in an experimental method for determination of the spinodal and binodal curves of a solution.

Transport measurements in short pulsed fields up to 100 T

Techniques have been developed for transport measurements under strong magnetic fields up to 100 T generated by the single turn coil method. For the first trial, we performed four terminal measurements of DC magnetoconductance of single crystal graphite.

Excitation of a V-antenna by a pulse electromagnetic field

In order to measure short electromagnetic field pulses in EMC problems, transducers in the form of strip line segments are used. Such transducers reproduce undistorted pulses within the time interval equal to a double run of the signal along the line. However, the strip line has a low sensitivity. The V-antenna seems to be reasonable for recording weak signals, since it is more sensitive when compared to a strip antenna. The equations of V-antenna excitation by a pulse electromagnetic field are derived from Maxwell's equations considered in a space-time domain. The role of the magnetic field of the wave as a source of excitation has been clarified. The transient response of a V-antenna is calculated as a function of the material of a dielectric interlayer between the electrodes, the antenna opening angle and the electromagnetic wave incidence angle. Based on a nonsymmetric version of the V-antenna, transducers were fabricated and experimentally investigated. The results of calculations and measurements are in good agreement with each other. It is shown that one can obtain a transient response of an approximate step shape by adding signals from two oppositely directed antennas. The antenna proposed allows nanosecond low-amplitude electromagnetic pulses to be detected.

Remanence coercivity of recording media in the high speed regime

New measurements of the switching characteristics in short pulse fields of CrO/sub 2/ particulate tape and CoNi metal evaporated tape are reported which complete a survey of materials used in flexible media. An unambiguous measure of the results is the remanence coercivity H/sub CR/ which increases as pulse width /spl tau/ decreases. Three mechanisms which could contribute to the increase are thermally assisted switching, damping limited switching and changes in the reversal process. In samples with high viscosity, the results are in excellent agreement over 12 decades with the simple thermal model proposed by Sharrock within the applicable range of /spl tau/>1/f/sub 0/, where f/sub 0/ is the attempt frequency equal to 10/sup 9/ Hz. For /spl tau/<1/f/sub 0/, H/sub CR/ continues to increase; for example, in CrO/sub 2/ at /spl tau/=0.6 ns, it reaches 1.8 times the value for /spl tau//spl sim/1.0 s. In samples with low viscosity, the rapid increase in H/sub CR/ for /spl tau/>1/f/sub 0/ in some materials confirms that other mechanisms are dominant in these materials.

Australian national pulsed magnet laboratory for condensed matter physics research

The National Pulsed Magnet Laboratory (NPML) provides access to high pulsed and steady magnetic fields, with particular emphasis on combining high fields with temperatures down to the millikelvin regime and a range of physical probes for condensed matter studies. NPML provides both short pulse (≈ 10 ms) fields to 60 T and long duration (1 s) fields to 30 T.

On stabilization by high-frequency, short-pulsed fields

We examine the effects of a high-frequency electric field on a on one-dimensional hydrogen atom in the case that the interaction time is shorter than the classical orbital period and the field frequency, Omega , is sufficiently high that one or two photon ionization is possible. In this short-pulse limit we can obtain simple analytic approximations to the ionization probability. For one-photon ionization we find that after an initial rise, at relatively weak fields, the quantal ionization probability reaches a local maximum at that field for which the energy transfer Omega h(cross) becomes classically accessible. These results are consistent with the numerical results obtained by substantially more elaborate calculations. They show that for short-pulsed, high-frequency fields stabilization can be interpreted as a rainbow phenomenon, the low intensity side of the maximum being on the dark side of the rainbow, with oscillations on the bright side. We also briefly consider some two-frequency fields and show that the ionization probability can be sensitive to the phase difference between field components.

Activation of α2-adrenoceptors attenuates the inotropic effect of field stimulation in atria

Guinea-pig left atria were driven to contract at a rate of 0.5 Hz by stimulation with punctate electrodes. For additional field stimulation, a train of one to eight field pulses (30 V; 0.05–0.4 ms duration) was applied during each refractory period. Cholinergic effects were blocked by atropine and thus the resulting increase of the contractile force was caused by noradrenaline released from sympathetic nerve endings. Trains of several short field pulses delivered in the refractory period after each contraction produced a significantly greater inotropic effect than one single pulse of the same total duration delivered in each refractory period. Phentolamine, rauwolscine and idazoxan, by blocking prejunctional α 2-adrenoceptors selectively increased the inotropic effect of single field pulses. Selective blockers of α 1-adrenoceptors (prazosin, corynanthine) were ineffective in this respect. The effect of α 2-adrenoceptor blockers persisted in the presence of noradrenaline uptake blockers (cocaine or desipramine plus corticosterone) or phenylephrine, but was overcome by the α 2-adrenoceptor agonists, clonidine and guanabenz. It is concluded that activation of presynaptic α 2-adrenoceptors limits the release of noradrenaline by long-lasting single field pulses. Autoinhibition of transmitter release was not seen with trains of short field pulses.

Time-dependent resonant-fluorescence spectrum of two-level atoms in strong short-pulsed fields: A nonperturbative treatment.

A study is presented for the time-dependent resonance-fluorescence spectrum of two-level atoms irradiated by hyperbolic-secant pulsed lasers, allowing for an arbitrary spontaneous-emission rate from the excited level to the ground level. Novel features of the temporally transient spectrum have been observed. The spectrum is dominated by a central peak which oscillates in time and is accompanied by multiple side peaks. Asymptotically (i.e., after the pulse has passed) the side peaks disappear, leaving a tail at the line center which displays a Lorentzian shape of width equal to the sum of the spontaneous-emission rate and the width of the spectrometer considered and of magnitude proportional to the instantaneous intensity of the pulse. The exact behavior of the spectrum is dictated by the area of the engaged pulse and by how the atom is prepared initially. In particular, it is found that the spectrum can be asymmetric even when the laser is tuned to exact resonance if the atoms are initially prepared in a certain class of proper coherent states, irrespective of how the field amplitude may vary in time. An analysis of the spectral symmetry without resorting to any explicit evaluation of the spectrum itself is also presented. Furthermore, the associated optical Bloch equations are solved analytically in terms of hypergeometric functions in a similar manner to that done previously for the undamped two-level Schr\"odinger equation. Specific numerical results are presented for pulses of area equal to \ensuremath{\pi}, 2\ensuremath{\pi}, 3\ensuremath{\pi}, 4\ensuremath{\pi}, and 5\ensuremath{\pi} for both symmetric and asymmetric cases.

Parametric resonant frequency conversion in a field of short pulses with noise loading

Parametric resonant frequency conversion in a field of short pulses with noise loading

Quantitative prediction of high field, short pulse behaviour of amorphous semiconductors

Two sets of carriers reveal themselves in amorphous chalcogenide thin films subjected to high electric fields. The application to the samples of short high field pulses, in the range of tens of nanoseconds, shows the same conduction processes present as when applying pulses whose lengths are nearly a million times longer. The coupled carrier equations have been combined with a band model which assigns the carriers respectively to localized hopping or trapping states near the band edge and to extended states near the mobility edge to explain the conduction behaviour. The success of this theory, found earlier with long pulse measurements, continues with short pulse experiments. The theory is substantially free of adjustable parameters and gives numerical and functional agreement in 10 specific cases when applied to 7 experimental phenomena discovered with the short pulse experiments.

Field dependence of the electrophoretic mobility of particles suspended in low-conductivity liquids

An experimental arrangement is described for the determination of the electrophoretic mobility of suspended particles as a function of particle size. While sedimenting between plane vertical electrodes the particles were deflected to and fro by short rectangular field pulses. Results of polystyrene spheres suspended in isodecane with oil-soluble ionic additives as stabilizers are reported. The field dependence of the mobility at higher conductivity (10 −10 … 10 −9 Ω −1 m −1) and medium field strength (5 × 10 4 … 5 × 10 5 V/m) is interpreted as a partial decoupling of the particles from their diffuse electric double layer. At low conductivity and high field strength, when the double layer is completely stripped off, the charge of the particle can be calculated directly by equating the Coulomb and Stokes forces. The ratio between constant low-field (intact double layer) and constant high-field mobility (1:8) is in good agreement with theoretical calculations.

Switching Speeds in Magnetic Tapes

Comparative measurements are reported on the switching speeds of three different magnetic tape materials, as determined by application of short field pulses of well-defined duration and magnitude, A sensitive measure of the change in magnetization is the length of applied pulse required for the peak readback signal to drop from 60 percent to 40 percent of its peak value. This pulse length was is 2.6 ns for a γ-Fe2O3, tape, 4.1 ns for a CrO2, tape, and 1.4 ns for a cobalt-substituted γ-Fe2O3, tape.