95 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 States in Disk Geometry
In this work, we explore the stability of magnetic skyrmions confined in a disk geometry by analyzing how to switch a skyrmionic state in a circular disk into a uniformly magnetized state when applying an external magnetic field. The technologically highly relevant energy barrier between the skyrmion state and the uniformly magnetized state is a key parameter needed for lifetime calculations. In an infinite sample, this relates to the out-of-plane rupture field against the skyrmion-core direction, while in confined geometries the topological charge can also be changed by interactions with the sample edges. We find that annihilating a skyrmion with an applied field in the direction of the core magnetizationâwe call this expulsionâthe energy barrier to the uniform state is generally around one order of magnitude lower than the annihilation via the rupture of the core in the disk center, which is observed when the applied field is acting in the direction opposite to the core magnetization. For the latter case a Bloch point (BP) needs to be nucleated to change the topological charge to zero. We find that the former case can be realistically calculated using micromagnetic simulations but that the annihilation via rupture, involving a Bloch point, needs to be calculated with the Heisenberg model because the high magnetization gradients present during the annihilation process cannot be accurately described within the micromagnetic framework
Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires
Recent studies have shown that material structures, which lack structural
inversion symmetry and have high spin-orbit coupling can exhibit chiral
magnetic textures and skyrmions which could be a key component for next
generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that
stabilizes skyrmions is an anti-symmetric exchange interaction favoring
non-collinear orientation of neighboring spins. It has been shown that material
systems with high DMI can lead to very efficient domain wall and skyrmion
motion by spin-orbit torques. To engineer such devices, it is important to
quantify the DMI for a given material system. Here we extract the DMI at the
Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary
measurement schemes namely asymmetric domain wall motion and the magnetic
stripe annihilation. By using the two different measurement schemes, we find
for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and
0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes
skyrmions at room temperature and that there is a strong dependence of the DMI
on the relative composition of the CoFeB alloy. Finally we optimize the layers
and the interfaces using different growth conditions and demonstrate that a
higher deposition rate leads to a more uniform film with reduced pinning and
skyrmions that can be manipulated by Spin-Orbit Torques
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
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
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
Field-free deterministic ultra fast creation of skyrmions by spin orbit torques
Magnetic skyrmions are currently the most promising option to realize
current-driven magnetic shift registers. A variety of concepts to create
skyrmions were proposed and demonstrated. However, none of the reported
experiments show controlled creation of single skyrmions using integrated
designs. Here, we demonstrate that skyrmions can be generated deterministically
on subnanosecond timescales in magnetic racetracks at artificial or natural
defects using spin orbit torque (SOT) pulses. The mechanism is largely similar
to SOT-induced switching of uniformly magnetized elements, but due to the
effect of the Dzyaloshinskii-Moriya interaction (DMI), external fields are not
required. Our observations provide a simple and reliable means for skyrmion
writing that can be readily integrated into racetrack devices
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