Abstract
The dynamics of emergent magnetic quasiparticles, such as vortices, domain walls, and bubbles are studied by scanning transmission x-ray microscopy (STXM), combining magnetic (XMCD) contrast with about 25 nm lateral resolution as well as 70 ps time resolution. Essential progress in the understanding of magnetic vortex dynamics is achieved by vortex core reversal observed by sub-GHz excitation of the vortex gyromode, either by ac magnetic fields or spin transfer torque. The basic switching scheme for this vortex core reversal is the generation of a vortex-antivortex pair. Much faster vortex core reversal is obtained by exciting azimuthal spin wave modes with (multi-GHz) rotating magnetic fields or orthogonal monopolar field pulses in x and y direction, down to 45 ps in duration. In that way unidirectional vortex core reversal to the vortex core 'down' or 'up' state only can be achieved with switching times well below 100 ps. Coupled modes of interacting vortices mimic crystal properties. The individual vortex oscillators determine the properties of the ensemble, where the gyrotropic mode represents the fundamental excitation. By self-organized state formation we investigate distinct vortex core polarization configurations and understand these eigenmodes in an extended Thiele model. Analogies with photonic crystals are drawn. Oersted fields and spin-polarized currents are used to excite the dynamics of domain walls and magnetic bubbles. From the measured phase and amplitude of the displacement of domain walls we deduce the size of the non-adiabatic spin-transfer torque. For sensing applications, the displacement of domain walls is studied and a direct correlation between domain wall velocity and spin structure is found. Finally the synchronous displacement of multiple domain walls using perpendicular field pulses is demonstrated as a possible paradigm shift for magnetic memory and logic applications.
Highlights
The physics of surfaces, interfaces and nanostructures has become one of the major areas of research, due to the trend in science and technology toward miniaturization of physical systems into the nanoscale
In summary we have presented an overview of the dynamics of confined spin structures
From the dynamic imaging using X-ray based techniques, we have deduced the underlying physics of the dynamics that is found to depend on the material, geometry and excitation method
Summary
The physics of surfaces, interfaces and nanostructures has become one of the major areas of research, due to the trend in science and technology toward miniaturization of physical systems into the nanoscale. The magnetization configuration that constitutes the lowest energy state in a small magnetic structure can for instance be set to a multidomain state with vortices or domain walls, since the dipolar interaction (stray field) leads to the magnetization being parallel to the element edges, which results in a very reproducible and controllable spatially inhomogeneous magnetization distribution (domain configuration). To study such geometrically confined magnetization configurations, direct imaging is the method of choice, as it allows one to deduce the spatially resolved spin structure with high resolution. We focus on selected experiments performed primarily by our groups describing the dynamics of some of the basic magnetic spin structures present in soft magnetic materials patterned onto the nanoscale: first the magnetic vortex and arrays of vortices, domain walls, which are compound quasiparticles that can include vortices and other spin structures and bubble skyrmions that comprise two domains delinated by a domain wall
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