Vortex Annihilation Research Articles

Optical Rogue Waves in Vortex Turbulence.

We present a spatiotemporal mechanism for producing 2D optical rogue waves in the presence of a turbulent state with creation, interaction, and annihilation of optical vortices. Spatially periodic structures with bound phase lose stability to phase unbound turbulent states in complex Ginzburg-Landau and Swift-Hohenberg models with external driving. When the pumping is high and the external driving is low, synchronized oscillations are unstable and lead to spatiotemporal vortex-mediated turbulence with high excursions in amplitude. Nonlinear amplification leads to rogue waves close to turbulent optical vortices, where the amplitude tends to zero, and to probability density functions (PDFs) with long tails typical of extreme optical events.

Switching the Magnetic Vortex Core in a Single Nanoparticle

Imaging and manipulating the spin structure of nano- and mesoscale magnetic systems is a challenging topic in magnetism, yielding a wide range of spin phenomena such as skyrmions, hedgehog-like spin structures, or vortices. A key example has been provided by the vortex spin texture, which can be addressed in four independent states of magnetization, enabling the development of multibit magnetic storage media. Most of the works devoted to the study of the magnetization reversal mechanisms of the magnetic vortices have been focused on micrometer-size magnetic platelets. Here we report the experimental observation of the vortex state formation and annihilation in individual 25 nm molecular-based magnetic nanoparticles measured by low-temperature variable-field magnetic force microscopy. Interestingly, in these nanoparticles the switching of the vortex core can be induced with very small values of the applied static magnetic field.

Individual vortex nucleation/annihilation in ferromagnetic nanodots with broken symmetry observed by micro-Hall magnetometry

We studied vortex nucleation/annihilation process and its temperature dependence in micromagnetic objects with lowered symmetry using micro-Hall magnetometry. Magnetization reversal curves were obtained for the Pacman-like nanodots placed directly on Hall probes. Lowered symmetry of the object leads to good control of its chirality. Vortex nucleation and annihilation fields strongly depend on the angle of the external in-plane magnetic field with respect on the nanodot symmetry. The micromagnetic simulations support the experimental results - the vortex nucleation fields are controlled by local magnetization configurations present in the object (C-, S-, and double S-states) for field just above vortex nucleation field. The experiments also confirm that the vortex nucleation proceeds via thermal activation over an energy barrier.

Hemodynamics in the Left Atrium and Its Effect on Ventricular Flow Patterns.

In the present study, we investigate the hemodynamics inside left atrium (LA) and understand its impact on the development of ventricular flow patterns. We construct the heart model using dynamic-computed tomographic images and perform simulations using an immersed boundary method based flow solver. We show that the atrial hemodynamics is characterized by a circulatory flow generated by the left pulmonary veins (LPVs) and a direct stream from the right pulmonary veins (RPVs). The complex interaction of the vortex rings formed from each of the PVs leads to vortex breakup and annihilation, thereby producing a regularized flow at the mitral annulus. A comparison of the ventricular flow velocities between the physiological and a simplified pipe-based atrium model shows that the overall differences are limited to about 10% of the peak mitral flow velocity. The implications of this finding on the functional morphology of the left heart as well the computational and experimental modeling of ventricular hemodynamics are discussed.

Domain of a magnetic flux in superconductors with ferromagnetic pinning centers

The processes of magnetization reversal by the transport current self-field of a high-temperature superconductor sample with ferromagnetic particles as defects in the presence of an external magnetic field and current-voltage characteristics of the system have been calculated using the Monte Carlo method. It has been found that an S-shaped feature is observed in the current-voltage characteristics due to a change in the effective interaction of the magnetic moments of impurity particles with a vortex system of the superconductor, which is induced by the magnetization reversal of ferromagnetic impurities under the influence of the local magnetic field of the vortex lattice. It has been demonstrated that, in the vicinity of the instability, there is the motion of a wave of magnetization reversal of magnetic defects (magnetic domain) and waves of annihilation of Abrikosov vortices (vortex density domains).

Propagation properties of partially coherent electromagnetic hyperbolic-sine-Gaussian vortex beams through non-Kolmogorov turbulence.

Propagation properties of partially coherent electromagnetic hyperbolic-sine-Gaussian (PCESHG) vortex beams through non-Kolmogorov atmospheric turbulence, including the spectral degree of polarization and evolution behavior of coherent vortices and average intensity are investigated in detail by using the extended Huygens-Fresnel principle and the spatial power spectrum of the refractive index of non-Kolmogorov turbulence. It is shown that the motion, creation and annihilation of the coherent vortices of PCESHG vortex beams in non-Kolmogorov turbulence may appear with the increasing propagation distance, and the distance for the conservation of the topological charge depends on the turbulence parameters and beam parameters. In additions, the evolution behavior of coherent vortices, average intensity and spectral degree of polarization vary significantly for different values of the generalized exponent parameter and the generalized refractive-index structure parameter of non-Kolmogorov turbulence, and the beam parameters as well as the propagation distance.

Mechanisms for the symmetric and antisymmetric switching of a magnetic vortex core: Differences and common aspects

Three-dimensional micromagnetic simulations of the switching of a magnetic vortex core in a cylindrical nanodisk are performed, for excitations with out-of-plane fields (symmetric switching) or with various types of time-dependent in-plane fields (asymmetric switching). Although the switching mechanisms are different in detail, all switching events must involve the movement of a Bloch point through the disk, because the switching leads to a change of the Skyrmion number which is a topological invariant as long as there is no action of a Bloch point. The momentary magnetization configurations are different in different layers of the disk. Because of the three-dimensionality it is often difficult to decide whether the asymmetric switching is caused by the splitting of the dip close to the vortex core into a vortex-antivortex pair, and the annihilation of the original vortex with the antivortex (whereby a Bloch point moves). It is suggested that there are situations for which such a switching occurs by the formation of a Bloch point in a configuration which is already similar to a vortex-antivortex configuration, but by a movement of this Bloch point before the formation of a complete pair and without the annihilation of the original vortex with an antivortex.

Modeling of Magnetization Distribution Near Shaped Boundary of Garnet Film Core in Fluxgate Magnetometer

Micromagnetic modeling of magnetization in-plane distribution near shaped boundary of garnet film was made to analyze the possibility to prevent chaotic formation of magnetic flux distortions in a fluxgate disc-like core excited by rotating magnetic field. Calculations made in the limit of zero anisotropy for sinusoidal, rectangular, and cogged (with right angle) boundaries show regular macroscopic vortex formation and annihilation in opposite phases of external magnetic field inside every apex of garnet film shaped boundary. Vortices are formed due to magnetostatic conservation of magnetization initial local direction along smooth or straight boundaries of the film during the whole period of rotation. Vortices near smooth curved boundary exhibit a stability range in the external field that exceeds that near rectangular and cogged shapes by an order of magnitude. Transformations of vortex states and low field saturation near broken line boundary are explained by peculiarities of exchange energy caused by mutually perpendicular spin orientations along adjacent straight sides of right angles. Calculation results show the way for control of magnetization distribution distortions in fluxgate garnet disc core by film boundary shape.

Emergence of order from turbulence in an isolated planar superfluid.

We study the relaxation dynamics of an isolated zero temperature quasi-two-dimensional superfluid Bose-Einstein condensate that is imprinted with a spatially random distribution of quantum vortices. Following a period of vortex annihilation the remaining vortices self-organize into two macroscopic coherent "Onsager vortex" clusters that are stable indefinitely--despite the absence of driving or external dissipation in the dynamics. We demonstrate that this occurs due to a novel physical mechanism--the evaporative heating of the vortices--that results in a negative-temperature phase transition in the vortex degrees of freedom. At the end of our simulations the system is trapped in a nonthermal state. Our computational results provide a pathway to observing Onsager vortex states in a superfluid Bose gas.

Magnetization dynamics of coupled Ni80Fe20 dots: Effects of configurational anisotropy and dipolar coupling

The effects of dipolar coupling on the static and dynamic behavior of coupled Ni80Fe20 dots in two lattice geometries have been systematically investigated as a function of film thickness t. For coupled two dots, a significant decrease in vortex nucleation and annihilation fields and a marked increase in ferromagnetic resonance (FMR) frequency were observed when compared with isolated dots of identical geometrical parameters. Interestingly, for t ≥ 50 nm, when three dots are coupled along two orthogonal directions, two distinct FMR absorption peaks and a two-step switching of vortex annihilation were observed due to configurational anisotropy. Our experimental results are in good agreement with both the analytical and micromagnetic simulations.

Temperature dependent magnetization reversal of exchange biased magnetic vortices in IrMn/Fe microcaps

We have investigated the magnetization reversal of vortex structures in Fe as well as in IrMn/Fe magnetic caps at elevated temperatures up to 450 K. The caps were formed by film deposition onto dense arrays of self-assembled silica particles of 900 nm diameter. In the investigated temperature range, the magnetization reversal in Fe caps evolves via nucleation and annihilation of magnetic vortices. However, in exchange coupled IrMn/Fe caps, the magnetic vortex at zero magnetic field vanishes as the temperature approaches the blocking temperature of IrMn accompanied by an increase in coercivity. Further increase in temperature, approaching the Néel temperature of IrMn, results in a reduction of coercivity as well as remanence indicating the re-stabilization of a vortex state.

Magnetic field penetration in MgB2 single crystals: Pinning and Meissner holes

The evolution of flux distribution in MgB2 single crystals during their remagnetization was imaged with magneto-optical technique. Meissner holes, formed as the areas where the annihilation of vortices and antivortices takes place, were found at the boundary between oppositely magnetized parts of the crystal. Gradient of magnetic induction in the vicinity of Meissner holes was found to be enhanced. Finger-like structures of convex shape, formed during the penetration of magnetic flux inside the crystal, were observed and explained as an effect of inhomogeneous pinning and demagnetizing field redistribution in the sample.

Vortex Dynamics in Ferromagnetic Nanoelements Observed by Micro-Hall Probes

In this work we measure the nucleation and annihilation of magnetic vortices in Pacman-like (PL) micromagnets prepared from Permalloy (Ni81Fe19, Py) at 77 K. Lateral dimensions of explored objects are ≤ 1 μm with thickness of about 40 nm. The micromagnets are located directly on the high-sensitive micro-Hall probe based on GaAs/AlGaAs heterostructure by lift-o process. Experiments show good agreement of the magnetization reversal with the micromagnetic simulation. Other shapes of micromagnets are also considered to obtain more precise picture of the vortex dynamics.

Dynamical behaviors of nonlinear dust acoustic waves: From plane waves to dust acoustic wave turbulence

The dust acoustic wave (DAW), associated with longitudinal dust oscillations in dusty plasmas, can be self-excited from the free energy of ion streaming. It is not only a fundamental plasma wave but also a paradigm to understand the generic dynamical behaviors of self-excited nonlinear longitudinal density waves through optically monitoring particle motion and dust density evolutions over a large area. In this paper, the dynamical behaviors of the wave-particle interaction and wave breaking in ordered self-excited DAW with straight wave fronts, and the defect-mediated wave turbulence with fluctuating defects and chaotic low amplitude hole filaments along defect trajectories in the 2+1D space-time space, are briefly reviewed. The first experimental observation of acoustic vortices with helical waveforms in self-excited acoustic-type defect-mediated wave turbulence, and the dynamics of spontaneous pair generation, propagation, and pair annihilation of acoustic vortices, is demonstrated and discussed.

Hysteresis-free switching between vortex and collinear magnetic states

We demonstrate a lossless switching between vortex and collinear magnetic states in circular FePd disks arranged in a square lattice. Above a bifurcation temperature we show that thermal fluctuations are enough to facilitate flipping between the two distinctly different magnetic states. We find that the temperature dependence of the vortex annihilation and nucleation fields can be described by a simple power law relating them to the saturation magnetization.

Breaking the configurational anisotropy in Fe single crystal nanomagnets

In this work, we improve the ability to tailor the switching mechanism in nanomagnets by introducing an additional, highly controlled source of anisotropy: magnetocrystalline anisotropy. We analyze the vortex dynamics in single crystal Fe nanotriangles with different orientations of the crystalline axes. By experimental studies and simulation, we show that the angular dependence of the vortex annihilation field springs from the convolution of the crystalline and configurational anisotropies. In contrast, the remanence and the nucleation field present a much simpler behavior controlled by the existence of a single symmetry axis when shape and crystalline orientation are taken into account.

Strong vortex core pinning and Barkhausen-free magnetization response in thin Permalloy disks induced by implantation of 1 × 104 Ga+ ions

Artificial vortex core pinning sites are induced in thin Permalloy disks by point exposure to as few as 10 000 ions from a focused Ga+ beam. These pinning sites yield a first-order change in the magnetization response of the disk. A single site can keep the vortex core pinned over an applied field range comparable to the vortex annihilation field of the unaltered disk. Several widely separated sites can work together to keep the core pinned in one place, while the Barkhausen effect is eliminated from the magnetization curve over a range approaching the saturation moment of the disk.

Effect of Asymmetric Shape on Vortex Control in Permalloy Rings

A method to control the vortex state (flux-closure state) nucleation and annihilation field in asymmetric permalloy (Py) rings is presented. By introducing an inner circle shifted from the center of outer circle by a shift length, samples of asymmetric permalloy rings with different asymmetric ratios were fabricated by using E-beam lithography, sputtering, lift-off techniques, and measured by focused magneto-optic Kerr effect magnetometry. The vortex nucleation (Hn) and vortex annihilation (Han) fields, analyzed from the simulated and experimental hysteresis loops of 500 nm and 1000 nm outer diameters, have linear relations to the asymmetric ratio. When the lateral size reduces down to nanosize, two switching behaviors for the simulated hysteresis loops were observed. The single switching loop indicates that the switching process is single-domain switching and the all the spins rotate coherently. The double switching loop corresponds to the switching process via the formation of a vortex state. Both analyzed switching fields from the experiment and simulation present a linear relation with the aspect ratio in the small range of variation. This relation indicates that the Hn and Han of asymmetric ring arrays are mainly controlled by the shape anisotropy. It is observed that the switching behavior is in agreement with the experimental results.

Probing the energy barriers in nonuniform magnetization states of circular dots by broadband ferromagnetic resonance

The time evolution of the ferromagnetic resonance output signal in the arrays of permalloy circular dots of submicron sizes was measured near the critical fields of the vortex nucleation and annihilation. Surprisingly short times of the transition from the quasiuniform to the vortex magnetization state (several milliseconds) were detected. The observed effects are explained by overcoming the field dependent energy barriers in the process of vortex core nucleation. The energy barrier values found from the time dependences of the ferromagnetic resonance peak intensities were compared with the ones calculated within the rigid vortex model. The rigid vortex model overestimates the nucleation barriers and a more adequate magnetization reversal model is needed. There is a strong dependence of the stable, metastable energy minima and energy barriers on the magnetic field and dot geometrical parameters.

Efficient Langevin simulation of coupled classical fields and fermions

We introduce an efficient Langevin method to study bilinear Fermionic Hamiltonians interacting with classical fields. Our method is suitable for very large systems and offers high accuracy. To demonstrate the method, we study complex non-coplanar chiral spin textures on the triangular Kondo lattice model. We also explore non-equilibrium mesoscale physics such as chiral domain coarsening and Z2 vortex annihilation.