Topological textures and their bifurcation processes in 2D ferromagnetic thin films

In this paper, by the use of the topological current theory, the topological structures and the dynamic processes in thin-film ferromagnetic systems are investigated directly from the viewpoint of topology. It is found that the topological charge of a thin-film ferromagnetic system can be changed by annihilation or creation processes of opposite polarized vortex–antivortex pairs taking place at space–time singularities of the normalized magnetization vector field of the system, the variation of the topological charge is integer and can further be expressed in terms of the Hopf indices and Brouwer degrees of the magnetization vector field around the singularities. Moreover, the change of the topological charge of the system is crucial to vortex core reversal processes in ferromagnetic thin films. With the help of the topological current theory and implicit function theorem, the processes of vortex merging, splitting as well as vortex core reversal are discussed in detail.     

Investigation of the boundary value problem corresponding to a generalized Skyrme lagrangian

A detailed numerical analysis of the boundary value problem resulting from the most general Skyrme type lagrangian containing up to quartic terms in field gradients is presented. The additional parameters in the lagrangian can be related to pion-pion scattering lengths. It is found that solutions to the boundary value problem does not exist for all values of the parameters and in particular, for the values predicted from pion-pion scattering data. Physical quantities of the nucleon are calculated for the highest possible values of the parameters admitting a solution and are compared with the corresponding values for the Skyrme model and experimental values.     

Electric current control of creation and annihilation of sub-100 nm magnetic bubbles examined by full-field transmission soft X-ray microscopy

The effect of electric current pulses on a sub-100 nm magnetic bubble state in a symmetric Pt/Co multilayer was directly observed using a full-field transmission soft X-ray microscope (MTXM). Field-induced evolution of the magnetic stripe domains into isolated bubbles with their sizes down to 100 nm was imaged under varying external magnetic fields. Electric current pulses were then applied to the created magnetic bubbles, and it was observed that the bubbles could be either created or annihilated by the current pulse depending on the strength of applied magnetic field. The results suggest that the Joule heating plays a critical role in the formation and/or elimination of the bubbles and skyrmions. Finally, the schematic phase diagram for the creation and annihilation of bubbles is presented, suggesting an optimized scheme with the combination of magnetic field and electric current necessary to utilize skyrmions in the practical devices.     

Symmetry theory of the flexomagnetoelectric interaction in the magnetic vortices and skyrmions

Symmetry classification of the magnetic vortices and skyrmions has been suggested. Relation between symmetry based predictions and direct calculation has been shown. It was shown that electric dipole moment of the vortex is located inside the small vortex core. The antivortices and antiskyrmions do not carry the total core electric dipole induced by the flexomagnetoelectric interaction in the hexoctahedral cubic crystal. The volumetric bound electric charge is distributed around the core. Switching of the core electric dipole direction produces the switching of the core magnetization or vortex chirality and vice versa. The vortices and skyrmions with time-invariant enantiomorphism have two degenerative states: clockwise and counterclockwise state.     

Compass-anisotropy-modulated helical states and skyrmion crystals in chiral magnets

The compass-type anisotropy appears naturally in chiral magnets with strong spin-orbit coupling. In this work, we investigate the critical roles of compass anisotropy in modulating various spin textures of chiral magnets, by Monte Carlo simulations. The simulated results reveal a gradual helical reorientation and varying symmetry of skyrmion crystal structures as a function of compass anisotropy. Furthermore, an extended continuum spin model with the lattice discretization anisotropy is proposed to interpret the dependences of helical and skyrmion crystal structures on the compass anisotropy. It is demonstrated that specific helical propagating directions are favored by the high-order lattice anisotropy arising from spin interactions in discretized lattice. Besides that, some threshold values for the helical structures are identified by analytical approach.     

Short-range interacting skyrmion charges in the long-range interacting skyrmion lattice

A topological excitation-skyrmion in p-wave superconductors is studied in the context of Landau-Ginzburg-Wilson (LGW) theory. Interaction between skyrmion charge densities is shown to be short ranged from a derived effective field model. The computed energy per single skyrmion of skyrmion lattice suggests a long ranged lattice interaction.     

Skyrmion burst and multiple quantum walk in thin ferromagnetic films

We propose a new type of quantum walk in thin ferromagnetic films. A giant Skyrmion collapses to a singular point in a thin ferromagnetic film, emitting spin waves, when external magnetic field is increased beyond the critical one. After the collapse the remnant is a quantum walker carrying spin S. We determine its time evolution and show the diffusion process is a continuous-time quantum walk. We also analyze an interference of two quantum walkers after two Skyrmion bursts. The system presents a new type of quantum walk for S>1/2, where a quantum walker breaks into 2S quantum walkers.     

Discrete double core skyrmions in magnetic thin films

We study the Belavin-Polyakov double core skyrmions in two-dimensional isotropic Heisenberg ferromagnets on a discrete square lattice by means of spin dynamic simulations using periodic boundaries conditions. It is also investigated the presence of external in-plane magnetic fields and lattice defects. We have shown that these discrete skyrmions not cylindrically symmetric have a high degree of stability, similar to their continuum counterparts, even if small spatial inhomogeneities are present into the system.     

Low-energy electronic spin excitations between filling factors ν=1 and studied by optically detected nuclear magnetic resonance

We report measurements of the spin relaxation time (T_1n) for nuclei in the potential well confining a high-mobility two-dimensional electron system at a single GaAs–GaAlAs heterojunction. At low temperatures nuclear spin relaxation is dominated by electron–nuclear spin scattering: we find that T_1n displays sharp maxima at incompressible states throughout the hierarchy of the fractional quantum Hall effect. This behaviour is consistent with the existence of low-energy spin excitations only where the electron system is compressible. Our measurements also provide evidence for a gap in the spin excitation spectrum at .