Mechanism of Current Oscillations in Gallium ArsenidePhotoconductive Semiconductor Switches

Semi-insulating photoconductive semiconductor switch with an electrode gap of 4 mm, triggered by a laser pulse with energy of 0.5md, and applied bias of 2.5kV, the periodicity current oscillation with a cycle of 12ns is obtained. It is indicated that the current oscillation is one mode of transferred electron effect, namely quenched domain mode. This mode of trans-electron oscillator is obtained when the instantaneous bias electric field drops below the sustaining field （the minimum electric field required to support the domain） before the domain reaches the anode, which leads to the domain disappears somewhere in the bulk of the switch and away from the ohmic contacts. We mainly analyse the time-dependent characteristic of the mode, the theoretical analysis results are in excellent agreement with the experiment.     

Magnetic relaxation measurements of exchange biased (Pt/Co) multilayers with perpendicular anisotropy

Magnetic relaxation measurements were carried out by magneto-optical Kerr effect on exchange biased (Pt/Co)5/Pt/FeMn multilayers with perpendicular anisotropy. In these films the coercivity and the exchange bias field vary with Pt spacer thickness, and have a maximum for 0.2 nm. Hysteresis loops do not reveal important differences between the reversal for ascending and descending fields. Relaxation measurements were fitted using Fatuzzo’s model, which assumes that reversal occurs by domain nucleation and domain wall propagation. For 2 nm thick Pt spacer (no exchange bias) the reversal is dominated by domain wall propagation starting from a few nucleation centers. For 0.2 nm Pt spacer (maximum exchange bias) the reversal is strongly dominated by nucleation, and no differences between the behaviour of the ascending and descending branches can be observed. For 0.4 nm Pt spacer (weaker exchange bias) the nucleation density becomes less important, and the measurements reveal a much stronger density of nucleation centers in the descending branch.     

Shape-induced oscillation of exchange bias in the Co nanopillar with lateral oxidation

A modified Monte Carlo Metropolis method is performed to simulate the hysteresis loops of a laterally oxidized Co nanopillar with an excised arc or an inclined circular plane. At low temperature after field cooling, an oscillatory exchange bias is observed with excised length or angle of inclination. However, the behaviors of coercivity exhibit a monotonous decrease with excision but an oscillation with tilt. The phenomena are intriguing and corroborated by means of the microscopic spin configurations and the magnetization reversal mechanisms. The occurrence of oscillatory exchange bias in such a Co/CoO composite nanopillar with direct exchange couplings is mainly because of the shape inducement in the nanopillar, which results in the energy fluctuations of interfacial coupling and the nonuniformity of domain distribution.     

Self-sustained ultrafast pulsation in coupled vertical-cavity surface-emitting lasers

We demonstrate that high-frequency, narrow-band, self-pulsating intensity oscillation can be generated with two coupled vertical-cavity surface-emitting lasers (VCSELs) and that such oscillation is greatly enhanced by collecting one of the two field lobes. The coupled VCSELs provide an ideal source for high-repetition-rate (over 40 GHz), sinusoidallike modulated laser light with Gaussian-like near- and far-field profiles. We also show that the modulation frequency can be tuned by inter-VCSEL separation or by a dc bias level.     

Rapid magnetization reversal in magnetic small particles induced by non-static bias field

Magnetization reversal processes by a switching field in single-domain nano-sized magnetic particles in the presence of a small transverse non-static bias field are studied. Applying an oscillating bias field instead of a static field, the reversal time becomes much shorter when the switching field is slightly stronger than the effective anisotropy field. A pulsed bias field of a suitably chosen duration in the nanosecond scale is found to induce a rapid switching, even when the switching field is smaller than the anisotropy field. The dependence of the reversal time on the frequency of an oscillating bias field and the duration of a pulsed bias field are studied. The present work thus complement the earlier studies on switching in the presence of a static bias field.     

Magnetization reversal in perpendicular exchange-biased multilayers

Co/Pt multilayers with perpendicular magnetic anisotropy exhibit an exchange bias when covered with an IrMn layer. The exchange bias field, which is about 7 mT for 3 Co/Pt bilayer repetitions and a Co layer thickness of 5 Å, can be increased up to 16.5 mT by the insertion of a thin Pt layer at the Co/IrMn interface. The interfacial magnetic anisotropy of the Co/IrMn interface (KSCo/IrMn =-0.09 mJ/m2) favours in-plane magnetization and tends to tilt the Co spins away from the film normal. Dynamical measurements of the magnetization reversal process reveal that both thermally activated spin reversal in the IrMn layer and domain wall nucleation in the Co/Pt multilayer influence the interfacial spin structure and therefore the strength of the perpendicular exchange bias field.     

Exchange bias and asymmetric reversal in nanostructured dot arrays

The size dependence of exchange bias field HE and coercivity Hc was studied by measuring exchange biased Fe-FeF2 dot arrays in comparison with an unstructured exchange biased Fe-FeF2 bilayer. The domain sizes in the ferromagnet (FM) and the antiferromagnet (AFM) play an important role for exchange bias (EB), and thus interesting phenomena may be expected when the size of an EB system becomes comparable to these sizes. We observe drastic changes of HE and Hc in nanostructured Fe-FeF2, which are unexpected because they appear even at a structure size which is too large for matching with AFM or FM domain size to play a role. We propose that under certain conditions the hysteresis loop is affected differently in the two branches of the reversal by shape anisotropy due to patterning. This is possible because the EB induces a reversal asymmetry already in the unpatterned bilayer system.     

Biased magnetization reversal in bi-phase multilayer microwires

We report on the magnetic behaviour of a novel family of two-magnetic-phase multilayer microwires consisting of: (i) a bistable FeSiB glass-coated amorphous microwire as soft nucleus, and (ii) a polycrystalline CoNi outer microtube as harder layer. Such bi-phase microwires are prepared by combined quenching and drawing plus sputtering and electroplating techniques. The stray field produced by the harder outer layer after premagnetizing it to saturation is used to bias the magnetization reversal process of the soft nucleus via dipolar magnetostatic coupling. A detailed analysis of the asymmetric low-field magnetization reversal process of the soft nucleus is presented together with the study of the fluctuating switching field and its asymmetric behaviour. The study of the domain wall characteristics under the presence of a nucleation coil at one end of the microwire allows us to draw conclusions on the role of the bias field generated by the premagnetized hard outer layer.     

GdFeCo磁光薄膜中RE-TM反铁磁耦合与激光感应超快磁化翻转动力学研究

使用飞秒时间分辨抽运-探测磁光克尔光谱技术,研究了激光加热GdFeCo磁光薄膜跨越铁磁补偿温度时稀土-过渡金属(RE-TM)反铁磁交换耦合行为和超快磁化翻转动力学. 实验观察到由于跨越铁磁补偿温度、净磁矩携带者交换而引起的磁化翻转反常克尔磁滞回线以及在同向外磁场下,反常回线上大于和小于矫顽力部分的饱和磁化强度不同,显示出GdFeCo中RE与TM之间的非完全刚性反铁磁耦合. 在含有Al导热底层的GdFeCo薄膜上观测到饱和磁场下激光感应磁化态翻转及再恢复的完整超快动力学过程. 与剩磁态的激光感应超快退磁化过 关键词： 补偿温度 磁化翻转 反铁磁耦合 GdFeCo     

Effect of current sweep reversal on the magnetic field stability for a Bi-2223 superconducting solenoid

The effect of current sweep reversal on the temporal drift in magnetic field intensity for a Bi-2223 solenoid was investigated by experiment and using numerical simulation. Current sweep reversal, by as small as 1% of the peak current, was found to stabilize the drift in magnetic field intensity for a Bi-2223 tape solenoid. The field drift was due to flux creep in the Bi-2223 tape and the current sweep reversal formed a barrier for flux entrance at the upper and lower surface of the conductor, preventing flux creep. With a current reversal of several% of the peak current, the barrier formation extended over half of the solenoid and the magnetic field intensity became constant with time. The current sweep reversal technique should prove useful to stabilize an ultra-high field low/high-temperature superconducting nuclear magnetic resonance magnet operated at frequencies (field intensities) beyond 1 GHz (23.5 T).     

A rigorous time domain analysis of gyrotrons

A self-consistent time domain analysis of gyrotrons is presented which allows studying multi-mode, multi-frequency operation. The electromagnetic field in the gyrotron cavity is expanded with respect to complete sets of eigenfunctions so that space charge effects are included in the analysis. Two improvements of the modal expansion inside gyrotron cavities are suggested which significantly increase the accuracy and the numerical efficiency of this method, namely, the removal of the non-uniform convergence of some field series at the coupling apertures and the estimation of the asymptotic values of some slowly converging series related to the modal analysis by a moderate number of cavity eigenfunctions. Discrete Fourier transform is used to obtain the time dependence of the electromagnetic field. The electron beam is described by a set of relativistic single particles. It is demonstrated that the strong numerical requirements of the suggested method can be overcome by using a vector computer. Two gyrotrons are investigated, namely, a low Q 35 GHz TE₀₁- and a 150 GHz TE₀₃-gyrotron. Both oscillation build-up and steady state operation are investigated including mode competition and window reflections. The simulations show that the assumption of a monofrequent steady state operation of gyrotrons, which is made by the commonly used frequency domain methods, is not always justified.     

Modulation of structure and dynamics of water under alternating electric field and the role of hydrogen bonding

Using Molecular Dynamics simulations, we investigate the effect of alternating (AC) electric field on static and dynamic properties of water. The central question we address is how hydrogen bonds respond to perpetual field-induced dipole reorientations. We assess structural perturbations of water network and changes of hydrogen bond dynamics in a range of alternating electric field strengths and frequencies using a non-polarisable water model, SPC/E, and two distinct polarisable models: SWM4-NDP and BK3. We confirm that AC field causes only moderate structural perturbations. Dynamic properties, including the rates of bond breaking, switching of hydrogen-bonding partners, and diffusion, accelerate with the strength of AC fields. All models reveal a nonmonotonic frequency dependence with fastest dynamics at frequencies around 200?GHz where the period of the field oscillation is commensurate with the average time it takes a typical proton to switch from one acceptor to another. Higher frequencies result in smaller amplitudes of angle oscillations and in reduced probability to complete the switch to another acceptor before the field reversal restores the original configuration. As frequency increases, these effects gradually weaken the influence of the field on the kinetics of hydrogen bonding and the associated rates of translational and rotational diffusion in water.     

Field sweep rate dynamics in magnetic tunnel junctions

The dynamical behavior of magnetic tunnel junctions (MTJs) was investigated by varying the magnetic field sweep rate from 0.01 mT/s to 10 T/s in a magneto optical Kerr effect set-up. The bias fields of the pinned and free ferromagnetic electrodes were found to drastically decrease above a field sweep rate of 1 T/s. This decrease in the bias fields coincides with a change in the magnetization reversal process from domain wall motion at low-field sweep rates to domain nucleation at high-field sweep rates. The nucleation of inverse domains in the ferromagnetic layer changes the interfacial spin structure of the antiferromagnetic layer and therefore the magnitude of the exchange bias effect. Furthermore, the nucleation of domains induces a discontinuous magnetic charge density at the tunnel barrier interfaces and this reduces the interlayer coupling between the two ferromagnetic electrodes of the MTJ.     

Mode-hop-free fine tuning of an external-cavity diode laser with an intracavity liquid crystal cell

A planar nematic liquid crystal (NLC) cell is incorporated into a Littman-type external cavity as the wavelength-tuning device for a semiconductor laser diode. By varying the driving voltage of the NLC cell and laser diode bias current simultaneously, we demonstrate single-mode oscillation and mode-hop-free tuning over 19.2 GHz at 775 nm. The result is in good agreement with the theoretical predictions.     

连续调谐太赫兹回旋管非稳定振荡状态研究

太赫兹回旋管可实现高功率输出,并具有一定的频率调谐范围,是核磁共振波谱系统理想的高功率太赫兹辐射源。设计了263 GHz,TE_5,2基波连续调谐回旋管,通过磁场调节实现频率调谐范围为1.39 GHz,利用时域多模多频自洽非线性理论对设计的连续调谐回旋管非稳定振荡状态进行了研究。结果表明,在低次纵向谐波模式工作磁场范围内,当工作电流大于起振电流时,连续调谐回旋管先进入稳定状态,高次纵向谐波模式被抑制,工作模式TE_5,2的输出功率随时间不变;当电流增大,纵向谐波模式间的竞争引起回旋管由稳定状态进入到非稳定振荡状态,工作模式TE_5,2的输出功率随时间呈振荡变化且互作用效率大大降低;随着电流的进一步增大,回旋管又回到与低电流不同的稳定状态,互作用效率进一步降低。同时发现非稳定振荡状态的起始电流随着磁场增加而增大。本研究对需工作于稳定状态的面向DNP-NMR应用的连续调谐太赫兹回旋管的研制具有一定指导意义。     

All-optical clock frequency divider using Fabry-Perot laser diode based on the dynamical period-one oscillation

We propose and experimentally demonstrate a frequency divider implemented by an optically injected Fabry-Perot laser diode (FP-LD) based on the nonlinear dynamical period-one oscillation. Injected by optical pulses, the FP-LD will oscillate in unstable dynamical period-one (P1) oscillation. Through changing the injected strength, emitting wavelength and bias current of the FP-LD, the oscillating frequencies of the P1 state can be varied. Once one of the harmonic frequencies is adjusted to match the repetition frequency of injected optical clock pulse, the P1 oscillation will be locked, and then a divided clock at the fundamental frequency of the P1 oscillation can be generated. By utilizing this divider, we can achieve the optical clock frequency division of divide-by-two, -three and -four in a wide input frequency range, for instance, of 9.0 to 20.0 GHz for divide-by-two. The influence of injected optical power on the timing jitter of the divided clock is also investigated. It is expected that this frequency divider can be applied to high frequency division exceeding 100 GHz due to its fast P1 oscillation.     

Nonuniform and sequential magnetization reversal via domain structure formation for multilayered system with grain size induced enhanced exchange bias

We report on the magnetization reversal in series of exchange-biased multilayers NiFe(10.0 nm)/[ Ir20Mn80(6.0 nm)/Co80Fe20(3.0 nm)] N studied by specular reflection and off-specular scattering of polarized neutrons. All specimens are sputtered and post-annealed at 530 K (i.e. above the IrMn Néel temperature of 520 K) in Ar atmosphere before cooling to room temperature in the presence of a field of 130 Oe which induces the unidirectional anisotropy. We find HEB is dependent upon the number of bilayers N as it gradually increases from 0.33 kOe for N=1 to a considerably higher value of upto ≈0.9 kOe for N=10. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N. Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the ferromagnetic domain structure. The enhancement of the exchange bias effect is presumably related to the shrinking of the related domain size in the antiferromagnet due to the structural evolution in the multilayers. Polarized neutron reflectometry (PNR) measurements are done at different applied fields sweeping both branches of the hysteresis loop. The spin-flip (SF) cross section of both the N=10 and 3 samples show diffusely scattered intensity appears gradually as the field approaches HEB and is most intense where the net magnetization vanishes. The disappearance of diffuse scattering in saturation indicates that the off-specular intensity is related to the reversal process. The reversal proceeds sequentially starting with the bottom (top) CoFe layer for decreasing (increasing) field and is related to the evolution of the grain size along the stack. The reversal of each CoFe layer is for both field branches due to domain wall motion. Thus as a main result, we observe a sequential and symmetric magnetization reversal in exchange-biased multilayers. The concomitant in-plane magnetization fluctuations revealed by off-specular spin-flip scattering indicate a more complex reversal mechanism than hitherto considered. Moreover, although the grain size decreases from N=3 to 10 by a factor of about four the reversal mechanism remains similar.     

On the possibility of detecting asymmetric magnetization reversal processes in exchange bias systems by low temperature nuclear orientation

Low temperature nuclear orientation has been used for the first time to investigate the magnetization reversal processes in an exchange bias system (Co/Au/CoO), with the advantage of observing both the Co and Au layers at the same time. By monitoring the counting-rate ratio of two γ-ray detectors, the measurements may be used to distinguish between reversal processes dominated by domain wall motion as opposed to rotation of the magnetization.     

Asymmetric reversal modes in ferromagnetic/antiferromagnetic multilayers

Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e.g., by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a nonuniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.     

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.