Simulations of continuous/cluster-pinned recording media

Continuous/cluster-pinned recording media, consisting of cuboid clusters of various sizes exchange-coupled to a continuous hard layer, were modelled to investigate their suitability for high density data storage. The pinning field due to the clusters was determined by modelling domain-wall motion in the continuous layer. Larger clusters, a thinner continuous layer and increased saturation magnetisation of the clusters all increased the pinning field. Simulations of recording demonstrated the feasibility of using domain-wall pinning to control the written bit size in continuous media.     

Influence of the Magnetoelastic Mechanism on the Switching-field Fluctuations of Fe-based Amorphous Microwires

Experimental switching-field fluctuations on Fe-rich amorphous microwires have been studied at different temperatures. Two processes have been identified to be responsible for the temperature dependence of the switching field: Magnetostrictive volume domain-wall pinning and relaxation effects due to structural rearrangements A simple model accounting for two energy contributions to the switching-field has been used to fit experimental data. The switching field distribution and its unusual temperature dependence is solved in terms of a thermoactivated model. The distribution width is found to be proportional to the switching field.     

Dynamics of domain-wall motion in ferroelectric liquid crystals in an electric field

The director reorientation in smectic liquid crystals with ferroelectric properties has been considered in the case where the interaction of liquid-crystal molecules with the surface leads to a partial unwinding of the helical structure of the liquid crystal and the reorientation occurs as a result of the domain-wall motion. The dependences of the velocity of domain-wall motion on the electric field strength, electric field variation frequency, boundary conditions, spontaneous polarization, and viscosity of the liquid crystal have been determined. It has been demonstrated that an increase in the electric field variation frequency or the polar part of the anchoring energy and the spontaneous polarization of the liquid crystal at a constant field frequency results in an increase of the velocity of domain-wall motion. As a consequence, the time of the electro-optic response of the liquid crystal in weak electric fields (from 0.4 to 2.0 V/μm) decreases by a factor of more than three.     

Diffraction in the semiclassical approximation to Feynman's path integral representation of the Green function

We derive the semiclassical approximation to Feynman's path integral representation of the energy Green function of a massless particle in the shadow region of an ideal obstacle in a medium. The wavelength of the particle is assumed to be comparable to or smaller than any relevant length of the problem. Classical paths with extremal length partially creep along the obstacle and their fluctuations are subject to non-holonomic constraints. If the medium is a vacuum, the asymptotic contribution from a single classical path of overall length L to the energy Green function at energy E is that of a non-relativistic particle of mass E/c² moving in the two-dimensional space orthogonal to the classical path for a time τ=L/c. Dirichlet boundary conditions at the surface of the obstacle constrain the motion of the particle to the exterior half-space and result in an effective time-dependent but spatially constant force that is inversely proportional to the radius of curvature of the classical path. We relate the diffractive, classically forbidden motion in the “creeping” case to the classically allowed motion in the “whispering gallery” case by analytic continuation in the curvature of the classical path. The non-holonomic constraint implies that the surface of the obstacle becomes a zero-dimensional caustic of the particle's motion. We solve this problem for extremal rays with piecewise constant curvature and provide uniform asymptotic expressions that are approximately valid in the penumbra as well as in the deep shadow of a sphere.     

Kinetic temperature and charge of a dust grain in weakly ionized gas-discharge plasmas

Analytical relations are given for estimating the energy of the stochastic motion of an individual dust grain heated by electrostatic ion oscillations in a weakly ionized gas-discharge plasma. Dust grain charging processes are analyzed, and an empirical approximation is obtained for the ion current to the grain surface. The processes are simulated under conditions similar to those of laboratory experiments on dusty plasmas. It is found that the kinetic temperature of a dust grain heated by electrostatic ion oscillations in a gas-discharge plasma can exceed the background gas temperature.     

Spin drift velocity, polarization, and current-driven domain-wall motion in (Ga,Mn)(As,P)

Current-driven domain-wall motion is studied in (Ga,Mn)(As,P) ferromagnetic semiconducting tracks with perpendicular anisotropy. A linear steady state flow regime is observed over a large temperature range of the ferromagnetic phase (0.1T(c)相似文献   

Asymmetry in the dynamics of domain walls in thin exchange-coupled ferromagnetic films

The kinetics of remagnetization after magnetic field switching was studied by the magneto-optic visualization technique in the bilayer hybrid structure composed of exchange-coupled FeNi and FeMn films. It was observed that not only the static but the dynamic characteristics of remagnetization as well depend on the polarity of a field applied along the direction of easy magnetization. The rate of the process was found to be exponentially dependent on the field strength in both directions, but the rate varied by factors of 10 upon inversion of the field. It was shown that this difference is the consequence of variation of both the time of domains nucleation and the velocity of domain walls motion.     

Quantum and classical spin clusters: disappearance of quantum numbers and Hamiltonian chaos

We present a direct link between manifestations of classical Hamiltonian chaos and quantum nonintegrability effects as they occur in quantum invariants. In integrable classical Hamiltonian systems, analytic invariants (integrals of the motion) can be constructed numerically by means of time averages of dynamical variables over phase-space trajectories, whereas in near-integrable models such time averages yield nonanalytic invariants with qualitatively different properties. Translated into quantum mechanics, the invariants obtained from time averages of dynamical variables in energy eigenstates provide a topographical map of the plane of quantized actions (quantum numbers) with properties which again depend sensitively on whether or not the classical integrability condition is satisfied. The most conspicuous indicator of quantum chaos is the disappearance of quantum numbers, a phenomenon directly related to the breakdown of invariant tori in the classical phase flow. All results are for a system consisting of two exchange-coupled spins with biaxial exchange and single-site anisotropy, a system with a nontrivial integrability condition.     

Conditions for infinite reversible susceptibility as magnetic systems reach irreversible switching

The reversible susceptibility approaching a magnetization phase transition corresponding to irreversible switching is examined both analytically and numerically. It is shown that the reversible susceptibility becomes infinite at switching if the reversible rotation just prior to irreversible motion has a component in the switching direction. For the numerical examples of non-uniform processes, as well as analytic analysis of two interacting grains, infinite susceptibility is seen only if the prior reversible rotation is virtually entirely along the subsequent reversal direction.     

Domain-wall dynamics and spin-wave excitations with spin-transfer torques

A generalization of spin-transfer torques in ferromagnetic structures is proposed. For a spatially nonuniform magnetization, the spin torque has a form nearly identical to that in magnetic multilayers. We show that the domain-wall motion driven by the current has many unique features that do not exist in the conventional domain-wall motion driven by a magnetic field. We also demonstrate that the spin torque can generate bulk and surface spin excitations that have been seen in point-contact experiments.     

Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings

Magnetization reversals through the formation of a vortex state and the rotation of an onion state are two processes with comparable probabilities for symmetric magnetic nanorings with a radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and cannot be altered after fabrication. In this work, we report a novel type of nanorings--asymmetric nanorings. By tuning the asymmetry, we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain-wall energy on the local cross-section area for which we have provided theoretical calculations.     

Sampling rare switching events in biochemical networks

Bistable biochemical switches are widely found in gene regulatory networks and signal transduction pathways. Their switching dynamics are difficult to study, however, because switching events are rare, and the systems are out of equilibrium. We present a simulation method for predicting the rate and mechanism of the flipping of these switches. We apply it to a genetic switch and find that it is highly efficient. The path ensembles for the forward and reverse processes do not coincide. The method is widely applicable to rare events and nonequilibrium processes.     

Polarization switching as a result of cascading of two simultaneously phase-matched quadratic processes

A method of intensity-dependent polarization switching is proposed. The effect is based on simultaneous action of two phase-matched second-order processes in a quadratic medium. Using analytical and numerical techniques, we demonstrate that a single linearly polarized fundamental wave, when it is propagating in such a medium, can efficiently generate a new fundamental wave of orthogonal polarization. The polarization switching is explained by an effective four-wave-mixing process that is performed through second-order cascading.     

Velocity of domain-wall motion induced by electrical current in the ferromagnetic semiconductor (Ga,Mn)As

Current-induced domain-wall motion with velocity spanning over 5 orders of magnitude up to 22 m/s has been observed by the magneto-optical Kerr effect in (Ga,Mn)As with perpendicular magnetic anisotropy. The data are employed to verify theories of spin transfer by the Slonczewski-like mechanism as well as by the torque resulting from spin-flip transitions in the domain-wall region. Evidence for domain-wall creep at low currents is found.     

Static investigation of the small clusters on the Cu(111) and Au(111) surfaces

The diffusion properties of small clusters Ag_n, Cu_n, and Au_n on the Cu(111) and Au(111) surfaces were studied using the molecular statics (MS) in order to understand the atomistic processes underlying the motion. In this work, the atomic interaction potential is modeled by a semi-empirical Embedded Atom Method (EAM), while the drag method is used to determine the static activation energy for each diffusion path. The presented results indicate that the dimer can diffuse on the (111) surface via the zig-zag and concerted motion mechanisms. The trimer diffuses according to the concerted motion mechanism. For the tetramer diffusion, the mechanism that consumes the least amount of energy is the zig-zag motion, in which only two atoms are needed for the diffusion process at one time. This allows finding a static activation energy smaller compared to the trimer diffusion.     

Video-rate electron-holographic interference microscopy using a liquid-crystal panel

This paper describes interference microscopy using a liquid-crystal panel as a rewritable medium. This system makes it possible to observe the dynamic behavior of a magnetic field. Experimental results obtained while observing dynamic domain-wall motion in a thin permalloy film are shown.     

Time-resolved x-ray microscopy of spin-torque-induced magnetic vortex gyration

Time-resolved x-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque-induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.     

Generation and characterization of 0.6-THz polarization domain-wall trains in an ultralow-birefringence spun fiber

Polarization domain-wall (PDW) trains have been generated at a repetition rate of 0.6 THz in an ultralow-birefringence spun optical fiber and measured by use of an adapted frequency-resolved optical gating technique. Characterization of the intensity and the phase of the PDW train shows complete switching between adjacent domains of counterrotating circular polarizations and directly confirms predictions based on numerical simulations of the incoherently coupled nonlinear Schr?dinger equations.     

A study of the induced anisotropy in a ferromagnetic grain from an exchange coupled antiferromagnetic grain with uniaxial anisotropy

Approximate analytic solutions for the energy of an antiferromagnetic (AF) grain, which experiences an external exchange torque from a ferromagnetic grain have been obtained for a wide range of AF thickness. The accuracy of the analytic expression is within 2.0% of the exact solution, which do not have a closed analytical form. The model predicts that there are two critical AF grain thickness for each particular exchange energy strength. Below the first critical thickness, the induced anisotropy energy is well approximated by an uniaxial anisotropy term. Above the second critical thickness the induced anisotropy is unidirectional. In the intermediate range the induced anisotropy can not be expressed simply as uniaxial or unidirectional. The exchange bias and coercivity in NiFe/IrMn films have been studied as a function of the IrMn thickness, and the results are consistent with the proposed theory.     

Generation of Intense THz Pulsed Lasers Pumped Strongly by CO2 Pulsed Lasers

A theoretical method dealing with two intense laser fields interacting with a three-level molecular system is proposed. A discussion is presented on the properties of the solutions for time-independent and time-dependent absorption coefficients and gain coefficient on resonance for strong laser fields, based on analytic evaluation of the rate equations for a homogeneously broadened, three-level molecular system. The pump intensity range can be estimated according to the analytic expression of pump saturation intensity. The effects of pulse width, gas pressure and path length on the energy absorbed from pump light are studied theoretically. The results can be applied to the analysis of pulsed, optically pumped terahertz lasers.