Antivortex gyrotropic motion in a nanocontact

A vortex-antivortex pair can form in the free layer of a nanocontact device as a result of the Oersted field produced by the current. In a large-scale free layer having uniform magnetization boundary conditions an in-plane external magnetic field will tend to confine the vortex-antivortex pair, which undergoes gyrotropic motion about the nanocontact after an initial displacement from the static equilibrium position. With the vortex pinned to a defect at the nanocontact the antivortex dynamics can be isolated and gyrotropic precession of the antivortex will be the dominant mode. The frequency of antivortex precession increases as the external magnetic field increases, and the frequency decreases as the nanocontact current increases.     

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.     

Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven nanocontacts

Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.     

Self-consistent nonlinear analysis of overmoded gyrotron oscillators

The self-consistent nonlinear analysis is presented for an overmoded gyrotron, in which the gyrotropic beam interacts with more than one cavity eigenmodes. The nonlinear equations are obtained in the slow time scale for the motion of each individual particle and for the evolution of the amplitude and frequency of each individual cavity eigenmode. A numerically efficient algebraic longtime-step algorithm is developed, which includes magnetostatic field tapering and time transients. Numerical examples are presented for the excitation and competition of the cavity eigenmodes.Work supported by the U.S. Naval Research Laboratory, Plasma Physics Division, under Contract No. N00173-79-C-0200.     

Directed motion of vortices in a current-driven Josephson junction network

Dynamics of directed motion of vortices in a Josephson junction network (JJN) with a ladder structure is studied using a numerical simulation. By applying spatial and temporal modulation of external bias currents, directed motion of vortices occurs in the absence of a ratchet-type asymmetric potential. In the present system, the asymmetry of the directed motion emerges as a dynamical effect due to the modulated bias current. Some dynamical effects such as mode-locking and vortex–antivortex excitation are relevant to the directed dynamics. We clarify the details of the directed motion of vortices in the JJN.     

Theory of magnetodynamics induced by spin torque in perpendicularly magnetized thin films

A nonlinear model of spin-wave excitation using a point contact in a thin ferromagnetic film is introduced. Large-amplitude magnetic solitary waves are computed, which help explain recent spin-torque experiments. Numerical simulations of the fully nonlinear model predict excitation frequencies in excess of 0.2 THz for contact diameters smaller than 6 nm. Simulations also predict a saturation and redshift of the frequency at currents large enough to invert the magnetization under the point contact. The theory is approximated by a cubic complex Ginzburg-Landau type equation. The mode's nonlinear frequency shift is found by use of perturbation techniques, whose results agree with those of direct numerical simulations.     

On gyrotropic chiral sculptured thin films for magneto-optics

The planewave response of a chiral sculptured thin film with gyrotropic properties is examined, under the assumption of axial excitation. Although the reflectances and the transmittances are definitely affected by gyrotropy, the circular Bragg phenomenon is still exhibited. Most importantly, structural chirality and gyrotropy contribute cooperatively to transmission optical rotation.     

Eigenmode scattering relations for plane-stratified gyrotropic media

We consider the 4×4 scattering matrixS for a plane wave incident on a plane-stratified gyrotropic multilayer slab, which is assumed to have a single symmetry axis (the magnetisation vector in a ferrite sheet, or the external magnetic field direction in a plane-stratified magnetoplasma). In the given (original) problem the plane of incidence is at an azimuthal angle ϕ with respect to the magnetic meridian plane (the plane containing the normal to the stratification, and the gyrotropic symmetry axis), and there is a corresponding “conjugate” set of wave fields, in which the plane of incidence is at an azimuthal angle (π−ϕ), with a corresponding conjugate scattering matrixS′. Adjoint wave fields, obtained by reversing the magnetic symmetry vector, are used to yield the eigenmode amplitudes required in the definition of the scattering matrices. At an interface between two adjacent layers the scattering matrices are shown to be uniquely determined by the characteristic wave polarisations, and this is used to prove that the given and conjugate scattering matrices,S andS′, for the overall multilayer system are mutually transposed, i.e. .     

Trajectory and frequency of vortex gyration in a multi-nanocontact geometry

Nonlinear vortex gyrotropic motion in a three-nanocontacts system is investigated by micromagnetic simulations and analytical calculations. Three out-of-plane spin-polarized currents are injected into a nanodisk through a centered nanocontact and two off-centered nanocontacts, respectively. For current combination(ip1, ip0, ip2) =(-1, 1,-1), the trajectory of the vortex core is a peanut-like orbit, but it is an elliptical orbit for(ip1, ip0, ip2) =(1, 1,-1). Moreover, the gyrotropic frequency displays peaks for both current combinations. Analytical calculations based on the Thiele equation show that the changes of frequency can be ascribed mainly to the forces generated by the Oersted field accompanying the currents. We also demonstrate a dependence of eigenfrequency shifts on the direction and distance of the applied currents.     

The Higgs boson in fractal quantum systems with active nanoelements

Stochastic deformation and stress fields within a fractal multilayer nanosystem are investigated theoretically and by numerical modeling. It is shown that the averaged displacement functions of lattice nodes are complex. Their behavior changes from regular to stochastic when the control parameters are altered. A set of ultracold ²³Na atoms in an optical trap is chosen as the active nanoelement. It is demonstrated that certain physical properties (rate and quantization of the flow; hysteresis) of elementary excitations such as a vortex–antivortex pair are associated with the influence of a superfluid Bose–Einstein condensate (where a Higgs boson is the elementary excitation).     

Amplitude-Phase Effects in Acousto-Optical Interaction in Gyrotropic Cubic Crystals

The amplitude-phase features of an intermediate regime of light diffraction on ultrasound in gyrotropic cubic crystals have been investigated. It is established that in the gyrotropic medium excited by ultrasound two coupled phase lattices of photoelasticity appear as a result of the rotation of the polarization planes of interacting waves. These lattices determine the polarization and energy characteristics of a diffracted light. For a gyrotropic cubic crystal of bismuth germanate, good agreement between the theoretical and experimental dependences of the efficiency of diffraction on the ultrasonic intensity in the intermediate regime close to the Bragg regime of diffraction is obtained.     

Generalization of a scattering theorem for plane-stratified gyrotropic media

A previously derived eigenmode scattering theorem for plane-stratified gyrotropic media established that the scattering matricesS andS′, defined, respectively, in terms of a given and a conjugate set of eigenmode amplitudes, are mutually transposed, i.e. , the theorem being valid specifically for the incoming and outgoing eigenmodes ofgyrotropic bounding media. In this paper the theorem is extended to include linearly and circularly polarized base-modes in theisotropic media which may bound the multilayered, gyrotropic structure. Transformation of the eigenmode scattering matrix leads to a generalization of the scattering theorem to include base modes which are not necessarily eigenmodes of the medium. In the important special case in which the ambient magnetic field is parallel to the plane of the stratification, the scattering matrix is shown to have a special symmetry whose form depends on the base modes chosen (eigenmodes, linear modes or circular modes).     

Resonant activation of a synthetic antiferromagnet

The magnetic decay time of a synthetic antiferromagnet comprised of two closely spaced magnetic dipoles is measured in the presence of microwave excitation. The system is known to be highly stable with respect to switching between its two antiparallel ground states under quasistatic magnetic fields. We show that an order of magnitude lower field can switch the pair, provided the field is applied in resonance with the optical eigenmode of the collective spin dynamics in the system. We furthermore show that thermal agitation can play an essential role in spin-flop switching for resonant excitations of near- or subcritical amplitude.     

Competing symmetries and broken bonds in superconducting vortex-antivortex molecular crystals

Hall probe microscopy has been used to image vortex-antivortex molecules induced in superconducting Pb films by the stray fields from square arrays of magnetic dots. We have directly observed spontaneous vortex-antivortex pairs and studied how they interact with added free (anti)fluxons in an applied magnetic field. We observe a variety of phenomena arising from competing symmetries which either drive added antivortices to join antivortex shells around dots or stabilize the translationally symmetric antivortex lattice between the dots. Added vortices annihilate antivortex shells, leading first to a stable "nulling state" with no free fluxons and then, at high densities, to vortex shells around the dots stabilized by the asymmetric antipinning potential. Our experimental findings are in good agreement with Ginzburg-Landau calculations.     

Three-dimensional vectorial analysis of waveguide structures with the method of lines

The method of lines (MoL) a special eigenmode algorithm has been proven as an efficient tool for the analysis of waveguide structures in optics and microwaves. The electric and magnetic fields in the cross-section and their derivatives with respect to the cross-section coordinates are discretized with finite differences (FD) while analytic expressions are used in the direction of propagation. The numerical effort for analyzing three-dimensional structures with a two-dimensional discretization can be very high, particularly if vectorial characteristics have to be taken into account. In this paper we introduce a reduction of the eigenmode system to keep the effort moderate. Only a certain number of eigenmodes is determined with the Arnoldi algorithm. We will show then how the electric field distribution of the eigenmodes can be computed from the magnetic field and vice versa. To match the fields at the interfaces we introduce left eigenvectors which are the inverse of the field distributions. The formulas were applied to the analysis of a polarization converter consisting of a periodical perturbation of a waveguide structure. A rotation angle greater than 80° was determined.     

Vortex–antivortex states in nanostructured superconductor–ferromagnet hybrids

The formation of vortex–antivortex states in a superconducting film with a square array of magnetic dipoles magnetized perpendicularly to the film is investigated in the framework of the time-dependent Ginzburg–Landau equations. It is shown that a possible route to obtain equilibrium states is obtained following an experimentally realizable field-cooling procedure. The states thus obtained demonstrate a rich variety of phases, depending on magnetic moment intensity and dipole array-to-superconducting film distance. For instance, in the region of the phase diagram where each dipoles is able to generate N = 2 vortex–antivortex pairs, the antivortices induced by the negative stray fields of the dipoles undergo two transitions before ultimately merging into doubly-quantized giant antivortices. For N = 4, a state consisting on a three-quanta giant vortex below each dipole and an interstitial vortex–antivortex molecule was observed. Such state is thermodynamically stable and is induced by the fourfold symmetry of the dipole array, similar to symmetry-induced vortex–antivortex molecules found in mesoscopic superconductors.     

A note on debye potentials for spherically gyrotropic media

The Debye potentials are generalized to the case of electromagnetic fields in spherically gyrotropic media. A medium is called spherically gyrotropic if it is locally gyrotropic with the distinguished axis having a radial direction determined by a central point. Expressions for electromagnetic fields in terms of the generalized potentials are presented and the system of differential equations for the potentials is derived. The results are summarized in the form of a theorem. Basic facts about the Debye potentials in isotropic media are recalled.     

Images of a spin-torque-driven magnetic nano-oscillator

We present the first space- and time-resolved images of the spin-torque-induced steady-state oscillation of a magnetic vortex in a spin-valve nanostructure. We find that the vortex structure in a nanopillar is considerably more complicated than the 2D idealized structure often-assumed, which has important implications for the driving efficiency. The sense of the vortex gyration is uniquely determined by the vortex core polarity, confirming that the spin-torque acts as a source of negative damping even in such a strongly nonuniform magnetic system. The orbit radius is ～10 nm, in agreement with micromagnetic simulations.     

Dynamics of vortex–antivortex pair in a superconducting thin strip with narrow slits

In the framework of phenomenological time-dependent Ginzburg-Landau(TDGL) formalism,the dynamical properties of vortex-antivortex(V-Av) pair in a superconductor film with a narrow slit was studied.The slit position and length can have a great impact not only on the vortex dynamical behavior but also the current-voltage(Ⅰ-Ⅴ) characteristics of the sample.Kinematic vortex lines can be predominated by the location of the slit.In the range of relatively low applied currents for a constant weak magnetic field,kinematic vortex line appears at right or left side of the slit by turns periodically.We found such single-side kinematic vortex line cannot lead to a jump in the Ⅰ-Ⅴ curve.At higher applied currents the phase-slip lines can be observed at left and right sides of the slit simultaneously.The competition between the vortex created at the lateral edge of the sample and the V-Av pair in the slit will result in three distinctly different scenarios of vortex dynamics depending on slit length:the lateral vortex penetrates the sample to annihilate the antivortex in the slit;the V-Av pair in the slit are driven off and expelled laterally;both the lateral vortex and the slit antivortex are depinned and driven together to annihilation in the halfway.     

二维激子极化激元凝聚中涡旋叠加态稳态及动力学特性研究

利用分步Crank-Nicolson方案的虚时和实时有限差分方法求解耗散系统的Gross-Pitaevskii(GP)方程,研究了二维激子极化激元凝聚(exciton-polariton condensates)体系中正反涡旋叠加态的稳态结构并直观地验证这种稳态结构在半导体微腔旋转下的稳定性和动力学特性.通过虚时和实时演化相结合的方法求解出几种角动量情况下所对应的稳定涡旋叠加态.然后利用实时演化方法研究在半导体微腔旋转的情况下,正反涡旋叠加态的稳定性及其旋转角速率和半导体微腔旋转角速率之间的定量关系.最后研究了单涡旋态在半导体微腔旋转时形成涡旋阵列的动力学过程,并给出了泵浦光宽度和增益项对涡旋阵列结构的影响.研究表明,系统的泵浦,损耗和增益对稳定性和动力学特性有重要影响.