
**Skyrmion Hall effect in spin-orbit torque driven topologically trivial and
nontrivial structures **
J. Leliaert*
Ghent University, Ghent, Belgium
The dynamics of magnetic skyrmions are governed by the complex interplay
between driving forces, thermal fluctuations and material disorder. This
interplay leads to rich behavior, e.g. a creep motion that persists up to
almost 100 m/s, which needs to be fully understood before skyrmions can be
reliably used in technological applications like the racetrack memory [1].
To assist in this understanding, there exist theoretical approaches which rely
on the assumption that skyrmions are rigid objects. However, especially in the
technologically relevant regime where skyrmions are under the constant
influence of disorder and temperature, they do not behave as rigid objects,
making micromagnetic simulations indispensable to bridge theoretical models
and experimental results. To this end, we developed an algorithm offering a
twentyfold speedup without a loss of accuracy to perform simulations at
nonzero temperatures [2]. After validating this methodology against
theoretical results for skyrmion diffusion [3], we use it in a large-scale
study of the impact of temperature and disorder on the skyrmion motion and
compare the results against experimental data of the velocity and skyrmion
Hall (SkH) angle as function of the driving force [4].
Our results show that the skyrmion velocity as a function of current density
falls on a universal curve when the the temperature dependence of the spin-
orbit torques is accounted for. This allows the skyrmion trajectories in a
device to be engineered, although the problem remains that high velocities are
accompanied by large SkH angles, which could eventually lead to the
annihilation of the skyrmion after a collision with the edge of the racetrack.
Recently, it was suggested that this problem could be mitigated by replacing
skyrmions by topologically trivial structures like two coupled skyrmions in a
synthetic antiferromagnet or a skyrmionium. Both structures have no net
topological charge, which in the case of spin-transfer torque driven motion
also results in a zero SkH angle. However, we show that this notion is
generally false for spin-orbit torque driven objects [5]. Instead, the SkH
angle is directly related to the objects’ helicity and imposes an unexpected
roadblock for developing faster and low-power racetrack memories based on
spin-orbit torques.
References
*All presented results were obtained in collaborations with the authors of refs. [2], [4] and [5].
[1] A. Fert, et. al, Nature Nanotechnology 8, 152156 (2013)
[2] J.Leliaert, et al,. AIP Advances 7, 125010 (2017)
[3] J. Miltat, et al., Physical Review B 97, 214426 (2018)
[4] K. Litzius, et al., Nature Electronics 3, 30-36 (2020)
[5] R. Msiska, et al., arXiv:2110.07063
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ONLINE nanoGUNE: Dr. J. Leliaert, Ghent University, Belgium \- CIC nanoGUNE
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