32 research outputs found
Multiscale simulations of topological transformations in magnetic Skyrmions
Magnetic Skyrmions belong to the most interesting spin structures for the
development of future information technology as they have been predicted to be
topologically protected. To quantify their stability, we use an innovative
multiscale approach to simulating spin dynamics based on the
Landau-Lifshitz-Gilbert equation. The multiscale approach overcomes the
micromagnetic limitations that have hindered realistic studies using
conventional techniques. We first demonstrate how the stability of a Skyrmion
is influenced by the refinement of the computational mesh and reveal that
conventionally employed traditional micromagnetic simulations are inadequate
for this task. Furthermore, we determine the stability quantitatively using our
multiscale approach. As a key operation for devices, the process of
annihilating a Skyrmion by exciting it with a spin polarized current pulse is
analyzed, showing that Skyrmions can be reliably deleted by designing the pulse
shape
Skyrmion Lattice Phases in Thin Film Multilayer
Phases of matter are ubiquitous with everyday examples including solids and
liquids. In reduced dimensions, particular phases, such as the two-dimensional
(2D) hexatic phase and corresponding phase transitions occur. A particularly
exciting example of 2D ordered systems are skyrmion lattices, where in contrast
to previously studied 2D colloid systems, the skyrmion size and density can be
tuned by temperature and magnetic field. This allows us to drive the system
from a liquid phase to a hexatic phase as deduced from the analysis of the
hexagonal order. Using coarse-grained molecular dynamics simulations of soft
disks, we determine the skyrmion interaction potentials and we find that the
simulations are able to reproduce the full two-dimensional phase behavior. This
shows that not only the static behavior of skyrmions is qualitatively well
described in terms of a simple two-dimensional model system but skyrmion
lattices are versatile and tunable two-dimensional model systems that allow for
studying phases and phase transitions in reduced dimensions.Comment: Corrected Acknowledgement
Direct observation of propagating spin waves in the 2D van der Waals ferromagnet Fe5GeTe2
Magnetism in reduced dimensionalities is of great fundamental interest while also providing perspectives for applications of materials with novel functionalities. In particular, spin dynamics in two dimensions (2D) have become a focus of recent research. Here, we report the observation of coherent propagating spin-wave dynamics in a âŒ30 nm thick flake of 2D van der Waals ferromagnet Fe5GeTe2 using X-ray microscopy. Both phase and amplitude information were obtained by direct imaging below TC for frequencies from 2.77 to 3.84 GHz, and the corresponding spin-wave wavelengths were measured to be between 1.5 and 0.5 ÎŒm. Thus, parts of the magnonic dispersion relation were determined despite a relatively high magnetic damping of the material. Numerically solving an analytic multilayer model allowed us to corroborate the experimental dispersion relation and predict the influence of changes in the saturation magnetization or interlayer coupling, which could be exploited in future applications by temperature control or stacking of 2D-heterostructures
Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering
While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction
(DMI) are candidates as novel information carriers, their dynamics on the fs-ps
timescale is little known. Since with the bulk Heisenberg exchange and the
interfacial DMI two distinct exchange mechanisms are at play, the ultra-fast
dynamics of the chiral order needs to be ascertained and compared to the
dynamics of the conventional collinear order. Using an XUV free-electron laser
we determine the fs-ps temporal evolution of the chiral order in domain walls
in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe
experiment. Upon demagnetisation we observe that the dichroic (CL-CR) signal
connected with the chiral order correlator in the domain walls
recovers significantly faster than the (CL+CR) sum signal representing the
average collinear domain magnetisation . We explore possible
explanations based on spin structure dynamics and reduced transversal
magnetisation fluctuations inside the domain walls and find that the latter can
explain the experimental data leading to different dynamics for collinear
magnetic order and chiral magnetic order.Comment: 28 pages, 14 figure
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Coherent correlation imaging for resolving fluctuating states of matter
Fluctuations and stochastic transitions are ubiquitous in nanometre-scale systems, especially in the presence of disorder. However, their direct observation has so far been impeded by a seemingly fundamental, signal-limited compromise between spatial and temporal resolution. Here we develop coherent correlation imaging (CCI) to overcome this dilemma. Our method begins by classifying recorded camera frames in Fourier space. Contrast and spatial resolution emerge by averaging selectively over same-state frames. Temporal resolution down to the acquisition time of a single frame arises independently from an exceptionally low misclassification rate, which we achieve by combining a correlation-based similarity metric1,2 with a modified, iterative hierarchical clustering algorithm3,4. We apply CCI to study previously inaccessible magnetic fluctuations in a highly degenerate magnetic stripe domain state with nanometre-scale resolution. We uncover an intricate network of transitions between more than 30 discrete states. Our spatiotemporal data enable us to reconstruct the pinning energy landscape and to thereby explain the dynamics observed on a microscopic level. CCI massively expands the potential of emerging high-coherence X-ray sources and paves the way for addressing large fundamental questions such as the contribution of pinning5â8 and topology9â12 in phase transitions and the role of spin and charge order fluctuations in high-temperature superconductivity13,14
Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy
Magnetic skyrmions are highly promising candidates for future spintronic
applications such as skyrmion racetrack memories and logic devices. They
exhibit exotic and complex dynamics governed by topology and are less
influenced by defects, such as edge roughness, than conventionally used domain
walls. In particular, their finite topological charge leads to a predicted
"skyrmion Hall effect", in which current-driven skyrmions acquire a transverse
velocity component analogous to charged particles in the conventional Hall
effect. Here, we present nanoscale pump-probe imaging that for the first time
reveals the real-time dynamics of skyrmions driven by current-induced spin
orbit torque (SOT). We find that skyrmions move at a well-defined angle
{\Theta}_{SH} that can exceed 30{\deg} with respect to the current flow, but in
contrast to theoretical expectations, {\Theta}_{SH} increases linearly with
velocity up to at least 100 m/s. We explain our observation based on internal
mode excitations in combination with a field-like SOT, showing that one must go
beyond the usual rigid skyrmion description to unravel the dynamics.Comment: pdf document arxiv_v1.1. 24 pages (incl. 9 figures and supplementary
information
Breathing mode dynamics of coupled three-dimensional chiral bobbers
Recently, three-dimensional (3D) magnetic textures have moved into the focus of spintronics as both technologically relevant and physically intriguing on a fundamental level. A rich variety of 3D textures is currently being investigated; however, their unambiguous experimental detection and detailed study remains challenging. In this work, a new type of chiral 3D spin-texture, consisting of two antiferromagnetically coupled NĂ©el bobbers, is explored. The static properties of this structure depend on the chirality of the individual bobbers. Different chirality combinations are studied with regard to their phase stability regions by micromagnetic simulations and compared to antiferromagnetically coupled skyrmion tubes. Furthermore, the coupled internal breathing modes are investigated by application of a periodically alternating external magnetic field. The breathing modes of each studied system possess a unique fingerprint, which might allow for the identification of the resonating spin textures via their dispersion curves