The influence of magnetic impurities in the vortex core dynamics in magnetic nano-disks

In this work we have used spin dynamics simulations to study the gyrotropic frequency behavior in nano-disks of Permalloy with magnetic impurities. We consider the effect of attractive impurity and repulsive impurity placed near the vortex core gyrotropic trajectory. We observed that the gyrotropic frequency is affected by the presence of impurity. The gyrotropic frequency shift depends on the relative position between the impurity and the vortex core gyrotropic trajectory and if impurity is attractive or repulsive. Our results agree with the analytical model and with experimental behavior for the gyrotropic frequency shown in the literature.     

Imaging excitations in magnetic thin film microstructures

We show how a photoemission electron microscope (PEEM) installed at a synchrotron can be used to image magnetic objects with very high spatial and temporal resolution. Sub-nanosecond magnetic field pulses are used to excite the fundamental magnetic modes of micron sized permalloy particles. The time evolution of the magnetization is imaged using a pump-probe technique where the magnetic contrast is given by X-ray magnetic circular dichroism (XMCD). Depending on the shape and size of the magnetic object we can observe modes related to either the homogenously magnetized domains, to the domain walls or to the vortex. All of these can be analyzed quantitatively yielding their frequencies, amplitudes and damping time constants. For objects with controlled defects we show that the magnetic vortex can be switched between defects using magnetic field pulses.     

Quantum information and triangular optical lattices

The regular structures obtained by optical lattice technology and their behavior are analyzed from the quantum information perspective. Initially, we demonstrate that a triangular optical lattice of two atomic species, bosonic or fermionic, can be employed to generate a variety of novel spin-1/2 models that include effective three-spin interactions. Such interactions can be employed to simulate specific one-or two-dimensional physical systems that are of particular interest for their condensed matter and entanglement properties. In particular, connections between the scaling behavior of entanglement and the entanglement properties of closely spaced spins are drawn. Moreover, three-spin interactions are well suited to support quantum computing without the need to manipulate individual qubits. By employing Raman transitions or the interaction of the atomic electric dipole moment with magnetic field gradients, one can generate Hamiltonians that can be used for the physical implementation of geometric or topological objects. This work serves as a review article that also includes many new results.     

Dynamic origin of vortex core switching in soft magnetic nanodots

The magnetic vortex with in-plane curling magnetization and out-of-plane magnetization at the core is a unique ground state in nanoscale magnetic elements. This kind of magnetic vortex can be used, through its downward or upward core orientation, as a memory unit for information storage, and thus, controllable core switching deserves some special attention. Our analytical and micromagnetic calculations reveal that the origin of vortex core reversal is a gyrotropic field. This field is induced by vortex dynamic motion and is proportional to the velocity of the moving vortex. Our calculations elucidate the physical origin of the vortex core dynamic reversal, and, thereby, offer a key to effective manipulation of the vortex core orientation.     

Vortex properties of the acoustic intensity vector in a shallow sea

We study the vortex properties of the field of the acoustic intensity vector as a function of distance and radiation frequency, determined using a combined acoustic system in Peter the Great Bay in the Sea of Japan. We present probability density histograms of the normalized intensity vector curl components at a frequency of 110 Hz for the distance interval between the source and receiver within the limits of 1200–1725 m. The vortex structure of the acoustic intensity vector was observed in the vertical plane at the full distance (4000 m) from the source to the receiver. The discovered vortex structures are of interest both for physical acoustics and for applied problems in underwater acoustics.     

Response theory of particle-anti-particle plasmas

The physical motivation for our work on particle-anti-particle systems comes primarily from astrophysical objects such as pulsars and white dwarf stars.We deal first with the longitudinal dielectric response of an electron-positron or pair plasma in zero and non-zero magnetic fields. The response function must be renormalized using the standard techniques of quantum electrodynamics. For zero magnetic field, the dispersion relation and damping for plasma oscillations are given together with the screening potential about a test charge. For the case of non-zero magnetic field, the longitudinal dielectric response function is again calculated after renormalization. The response function takes on a different form depending upon whether the longitudinal oscillations are parallel or perpendicular to the direction of the magnetic field. The real and imaginary parts of the response function are then exhibited for both cases.The next topic is concerned with the plasma thought to exist in the deep interior of neutron stars or in the ephemeral plasmas of heavy ion collisions. Use of the Feshbach-Villars formalism for the Klein-Gordon equation allows for a similar approach to that previously used for the fermion-anti-fermion case.An adaption of the formalism developed here to two-dimensional systems is also given.     

Vortex fusion and giant vortex states in confined superconducting condensates

In a direct scanning tunneling spectroscopy experiment we address the problem of the quantum vortex phases in strongly confined superconductors. The strong confinement regime is achieved in in situ grown ultrathin single nanocrystals of Pb by tuning their lateral size to a few coherence lengths. Upon an external magnetic field, the scanning tunneling spectroscopy revealed novel ultradense arrangements of single Abrikosov vortices characterized by an intervortex distance up to 3 times shorter than the bulk critical one. At yet stronger confinement we discovered the giant vortex phase; the spatial evolution of the excitation tunneling spectra in the cores of these unusual quantum objects was explored. We anticipate the giant vortex phase to be a common feature of other confined quantum condensates such as superfluids, Bose-Einstein condensates of cold atoms, etc.     

Thermally assisted switching in FePt single-domain particles with a Gaussian distribution of temperature

Magnetic nanoscale systems,including nanodots,nanofibers,nanowires and nanoparticles,are currently attracting great interest due to their interesting physical and promising applications in various fields,such as magnetic recording,sensors,target drugs and catalysts,as well as others.To achieve ultrahigh recording density,the method of heat assisted magnetic recording(HAMR) has been introduced.In this work,with the help of a Monte Carlo method,the mechanisms of thermally assisted magnetization switching in F...     

Magnetic Vortex Filaments, Universal Scale Invariants, and the Fundamental Constants

An explanation for the observed scale invariants in the universe is presented. Force-free magnetic vortex filaments are proposed to play a crucial role in the formation of superclusters, clusters, galaxies, and stars by initiating gravitational compression. The critical velocities involved in vortex formation are shown to explain the observed constant orbital velocities of clusters, galaxies, and stars. A second scale invariant nr = C where n is particle density and r is average distance between objects, is also noted here and explained by our model. The model predicts a maximum size for magnetic vortices, which is comparable to the dimensions of the observable universe and a density for such vortices which is close to that actually observed, eliminating any theoretical need for missing mass. On this basis, we present an alternative cosmology to that of the "Big Bang," one which provides a much better fit to recent observations of large-scale structure and motion. The model suggests scale invariants between microscopic and cosmological scales, leading to the derivation of a simple analytical expression for the fundamental constants G, mp/me, and e2/hc. We conclude that these expressions indicate the existence of vortex phenomena on the particle level.     

Muons as local probes of three-body correlations in the mixed state of type-II superconductors

The vortex glass state formed by magnetic flux lines in a type-II superconductor is shown to possess nontrivial three-body correlations. While such correlations are usually difficult to measure in glassy systems, the magnetic fields associated with the flux vortices allow us to probe these via muon-spin rotation measurements of the local field distribution. We show via numerical simulations and analytic calculations that these observations provide detailed microscopic insight into the local order of the vortex glass and more generally validate a theoretical framework for correlations in glassy systems.     

Stochastic nature of magnetic processes studied by full-field soft X-ray microscopy

In nanomagnetism, one of the crucial scientific questions is whether magnetic behaviors are deterministic or stochastic on a nanoscale. Apart from the exciting physical issue, this question is also of paramount highest relevance for using magnetic materials in a wealth of technological applications such as magnetic storage and sensor devices. In the past, the research on the stochasticity of a magnetic process has been mainly done by macroscopic measurements, which only offer ensemble-averaged information. To give more accurate answer for the question and to fully understand related underlying physics, the direct observation of statistical behaviors in magnetic structures and magnetic phenomena utilizing advanced characterization techniques is highly required. One of the ideal tools for such study is a full-field soft X-ray microscope since it enables imaging of magnetic structures on the large field of view within a few seconds. Here we review the stochastic behaviors of various magnetic processes including magnetization reversal process in thin films, magnetic domain wall motions in nanowires, and magnetic vortex formations in nanodisks studied by full-field soft X-ray microscopy. The origin triggering the stochastic nature witnessed in each magnetic process and the way to control the intrinsic nature are also discussed.     

Direct observation of geometrical phase transitions in mesoscopic superconductors by scanning tunneling microscopy

Using scanning tunneling microscopy, we mapped the distribution of the local density of states in a single crystal superconductor heterostructure with an array of submicron normal metal islands. We observe the coexistence of strongly interacting multiquanta vortex lattice with interstitial Abrikosov vortices. The newly formed composite magnetic flux structure undergoes a series of phase transitions between different topological configuration states. The vortex configuration states are strongly dependent on the number of flux quanta and the nanoscale confinement architecture of the mesoscopic superconductor. Here, we present images of vortex phase transitions due to confinement effects when the number of magnetic flux quanta in the system changes. The vortex dynamics in these systems could serve as a model for behavior of confined many-body systems when the number of particles changes.     

Chaotic capture of vortices by a moving body. I. The single point vortex case

The study of the dynamical properties of vortex systems is an important and topical research area, and is becoming of ever increasing usefulness to a variety of physical applications. In this paper, we present a study of a model of a rotational singularity which obeys a logarithmic potential interacting with a bluff body in a uniform inviscid laminar flow, e.g., a line vortex interacting with a cylinder in three dimensions or a point vortex with a circular boundary in two dimensions. We show that this system is Hamiltonian and simple enough to be solved analytically for the stagnation points and separatrices of the flow, and a bifurcation diagram for the relevant parameters and classification of the various types of motion is given. We also show that, by introducing a periodic perturbation to the body, chaotic motion of the vortex can be readily generated, and we present analytic criteria for the generation of chaos using the Poincare-Melnikov-Arnold method. This leads to an important dynamical effect for the model, i.e., that the possibility exists for the vortex to be chaotically captured around the body for periods of time which are extremely sensitive to initial conditions. The basic mechanism for this capture is due to the chaotic dynamics and is similar to that of other chaotic scattering phenomena. We show numerically that cases exist where the vortex can be captured around an elliptic point external to (and possibly far from) the body, and the existence of other very complicated motions are also demonstrated. Finally, generalizations of the problem of the vortex-body interaction are indicated, and some possible applications are postulated such as the interaction of line vortices with aircraft wings.     

关于圆盘状物体的几个物理量的计算

在大学物理教学中,关于圆盘状物体的物理量有很多,如电学中带电圆环、带电圆盘,其周围空间产生的电场强度和电势怎样计算？关于铜质圆盘状物体在磁场中转动时产生的电动势的计算问题等;力学中圆盘的转动惯量、椭圆盘的转动惯量,圆盘状物体转动时的摩擦力矩问题、驱动力做功问题等.本文将对这些物理量进行分析,并找出一些共同的规律和特点,将其归纳总结,计算所需要的物理量.     

Vortex chains induced by anisotropic spin-orbit coupling and magnetic field in spin-2 Bose-Einstein condensates

We investigate the anisotropic spin-orbit coupled spin-2 Bose-Einstein condensates with Ioffe-Pritchard magnetic field. With nonzero magnetic field, anisotropic spin-orbit coupling will introduce several vortices and further generate a vortex chain. Inside the vortex chain, the vortices connect to each other, forming a line along the axis. The physical nature of the vortex chain can be explained by the particle current and the momentum distribution. The vortex number inside the vortex chain can be influenced via varying the magnetic field. Through adjusting the anisotropy of the spin-orbit coupling, the direction of the vortex chain is changed, and the vortex lattice can be triggered. Moreover, accompanied by the variation of the atomic interactions, the density and the momentum distribution of the vortex chain are affected. The realization and the detection of the vortex chain are compatible with current experimental techniques.     

具有面内四极磁场的旋转玻色-爱因斯坦凝聚体的基态结构研究

通过虚时演化方法研究了具有面内四极磁场的旋转玻色-爱因斯坦凝聚体的基态结构.结果发现:面内四极磁场和旋转双重作用可导致中央Mermin-Ho涡旋的产生;随着磁场梯度增强,Mermin-Ho涡旋周围环绕的涡旋趋向对称化排布;在四极磁场下,密度相互作用和自旋交换相互作用作为体系的调控参数,可以控制Mermin-Ho涡旋周围的涡旋数目;该体系自旋结构中存在双曲型meron和half-skyrmion两种拓扑结构.     

界面Dzyaloshinskii-Moriya相互作用下辐射状磁涡旋形成机制

辐射状磁涡旋结构是一种稳定的拓扑磁结构,因其具有热稳定性高、驱动电流小等特点,成为当前继斯格明子之后又一新兴的研究热点.本文利用微磁学模拟方法研究了在界面Dzyaloshinskii-Moriya相互作用（IDMI）下辐射状磁涡旋形成机制.结果表明：纳米盘直径越小,能稳定形成辐射状磁涡旋的IDMI强度范围就越大,当圆盘厚度增加一个数量级时,虽然可以稳定形成辐射状磁涡旋,但IDMI强度取值范围会随之变小.通过对不同磁矩初始态下辐射状磁涡旋的形成过程中磁矩、斯格明子数及各项能量变化的研究发现,环形涡旋和单畴均可作为辐射状磁涡旋形成的初始状态,但单畴初始态的形成时间比环形涡旋初始态的形成时间更长,其能量衰减时间比以环形涡旋为初始态的衰减时间更短.这表明形成辐射状磁涡旋极性比形成辐射旋性需要更长时间,且能量变化主要与涡旋核的生成及面内辐射状磁矩有关,而与涡旋核在盘中的位置无关.研究结果揭示了辐射状磁涡旋的形成机制,为基于辐射状磁涡旋的具体应用提供了理论依据.     

Geometry and nonlinear evolution equations

We briefly review the nonlinear dynamics of diverse physical systems which can be described in terms of moving curves and surfaces. The interesting connections that exist between the underlying differential geometry of these systems and the corresponding nonlinear partial differential equations are highlighted by considering classic examples such as the motion of a vortex filament in a fluid and the dynamics of a spin chain. The association of the dynamics of a non-stretching curve with a hierarchy of completely integrable soliton-supporting equations is discussed. The application of the surface embeddability approach is shown to be useful in obtaining such connections as well as exact solutions of some nonlinear systems such as the Belavin-Polyakov equation and the inhomogeneous Heisenberg chain.     

Analysis method of physical processes occurring at the interface between vortex and Meissner regions in high-temperature superconductors

A technique for probing the sample interior using the leading edge of an alternating evanescent magnetic field is suggested. It makes it possible to highlight and study physical processes taking place at the vortex front in superconductors. Successively increasing the field amplitude by equal amounts, one can study the formation and transformation of the magnetic microstructure in response to variation of the sample’s crystal microstructure. For samples with different microstructures, qualitatively new results are obtained. The technique suggested may be used for testing high-temperature superconductor epitaxial films and magnetic materials.     

Directional dependence of vortex core resonance in a square-shaped ferromagnetic dot

The resonant property of the magnetic vortex confined in a square-shaped ferromagnetic dot has been investigated. We showed that the field dependence of the resonant frequency has a unique directional dependence originating from a four-fold rotational symmetry of the square. The resonant frequency is found to be strongly modulated by the magnetic field along the diagonal direction although the magnetic field applied along the side of the square hardly modified the resonant frequency. The modulation ratio of the resonant frequency defined by the ratio between minimum and maximum frequencies for the vortex resonance was found to be tuned by the lateral dimension of the square. These unique frequency tunabilities controlled by the magnitude and the direction of the magnetic field may provide additional functions in the application of the magnetic vortex systems.