1,720,995 research outputs found
Emergence of Transient Domain Wall Skyrmions after Ultrafast Demagnetisation
Get PDFIt is known that ultrafast laser pulses can be used to deterministically switch magnetisation and create skyrmions, however the deterministic creation of a single Néel skyrmion after ultrafast demagnetisation remains an open question. Here we show domain wall skyrmions also emerge in systems with broken inversion symmetry after exposure to an ultrafast laser pulse, carrying an integer topological charge. Whilst domain wall skyrmions do not appear in the relaxed state due to quick thermal decay following an Arrhenius law, they play a key role in controlling the final skyrmion population through annihilations with skyrmions of opposite topological charge, with the resultant skyrmion states following a Poisson distribution. Using single-shot linearly polarised laser pulses, as well as a train of circularly polarised laser pulses, we show that when a high degree of disorder is created, the possibility of nucleating a single Néel skyrmion is accompanied by the possibility of nucleating a skyrmion with domain wall skyrmion pair, which results in a self-annihilation collapse
Effect of inter-layer spin diffusion on skyrmion motion in magnetic multilayers
Get PDFIt is well known that skyrmions can be driven using spin-orbit torques due to the spin-Hall effect. Here we show an additional contribution in multilayered stacks arises from vertical spin currents due to inter- layer diffusion of a spin accumulation generated at a skyrmion. This additional interfacial spin torque is similar in form to the in-plane spin transfer torque, but is significantly enhanced in ultra-thin films and acts in the opposite direction to the electron flow. The combination of this diffusive spin torque and the spin-orbit torque results in skyrmion motion which helps to explain the observation of small
skyrmion Hall angles even with moderate magnetisation damping values. Further, the effect of material imperfections on threshold currents and skyrmion Hall angle is also investigated. Topographical surface roughness, as small as a single monolayer variation, is shown to be an important contributing factor in ultra-thin films, resulting in good agreement with experimental observations
Speeding up explicit numerical evaluation methods for micromagnetic simulations using demagnetizing field polynomial extrapolation
Get PDFThe performance of numerical micromagnetic models is limited by the
demagnetizing field computation, which typically accounts for the majority of
the computation time. For magnetization dynamics simulations explicit
evaluation methods are in common use. Higher order methods call for evaluation
of all effective field terms, including the demagnetizing field, at all
sub-steps. Here a general method of speeding up such explicit evaluation
methods is discussed, by skipping the demagnetizing field computation at
sub-steps, and instead approximating it using polynomial extrapolation based on
stored previous exact computations. This approach is tested for a large number
of explicit evaluation methods, both adaptive and fixed time-step, ranging from
2nd order up to 5th order. The polynomial approximation order should be matched
to the evaluation method order. In this case we show higher order methods with
polynomial extrapolation are more accurate than lower order methods with full
evaluation of the demagnetizing field. Moreover, for higher order methods we
show it is possible to achieve a factor of 2 or more computation speedup with
no decrease in solution accuracy
All-optical magneto-thermo-elastic skyrmion motion
Get PDFIt is predicted magnetic skyrmions can be controllably moved on surfaces using a focused laser beam. Here an absorbed power of the order 1 mW, focused to a spot-size of the order 10 μm, results in a local temperature increase of around 50 K, and a local perpendicular strain of the order 10−3 due to the thermo-elastic effect. For positive magneto-elastic coupling this generates a strong attractive force on skyrmions due to the magneto-elastic effect. The resultant motion is dependent on forces due to i) gradients in the local strain-induced magnetic anisotropy, ii) gradients in the effective anisotropy due to local temperature gradients, and magnetic parameters temperature dependences, and iii) Magnus effect acting on objects with non-zero topological number. Using dynamical magneto-thermo-elastic modelling, it is predicted skyrmions can be moved with significant velocities (up to 80 m/s shown), both for ferromagnetic and antiferromagnetic skyrmions, even in the presence of surface roughness. This mechanism of controllably moving single skyrmions in any direction, as well as addressing multiple skyrmions in a lattice, offers a new approach to constructing and studying skyrmionic devices with all-optical control
Accelerating micromagnetic and atomistic simulations using multiple GPUs
Get PDFIt is shown micromagnetic and atomistic spin dynamics simulations can use multiple GPUs in order to reduce computation time, but also to allow for a larger simulation size than is possible on a single GPU. Whilst interactions which depend on neighbouring spins, such as exchange interactions, may be implemented efficiently by transferring data between GPUs using halo regions, or alternatively using direct memory accesses, implementing the long-range demagnetizing interaction is the main difficulty in achieving good performance scaling, where the data transfer rate between GPUs is a significant bottleneck. A multi-GPU convolution algorithm is developed here, which relies on single-GPU FFTs executed in parallel. It is shown that even for micromagnetic simulations where the demagnetizing interaction computation time dominates, good performance scaling may be achieved, with speedup factors up to 1.8, 2.5, and 3.1, for 2, 3, and 4 GPUs respectively. The code developed here can be used for any number of GPUs in parallel, with performance scaling strongly dependent on inter-GPU data transfer rate and connection topology. This is further improved in micromagnetic simulations which include a spin transport solver, obtaining speedup factors up to 1.96, 2.8, and 3.7, for 2, 3, and 4 GPUs respectively. The best case scenario is obtained for atomistic spin dynamics simulations, where the demagnetizing interaction is implemented with spin-averaged cells. Using a single workstation with 4 GPUs, it is shown atomistic spin dynamics simulations with up to 1 billion spins, and atomistic Monte Carlo simulations with up to 2 billion spins are possible, with a near-ideal performance scaling
Domain wall scattering and current induced magnetic switching in patterned ferromagnetic nanowires
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Accelerating micromagnetic and atomistic simulations using multiple GPUs
Get PDFIt is shown micromagnetic and atomistic spin dynamics simulations can use
multiple GPUs in order to reduce computation time, but also to allow for a
larger simulation size than is possible on a single GPU. Whilst interactions
which depend on neighbouring spins, such as exchange interactions, may be
implemented efficiently by transferring data between GPUs using halo regions,
or alternatively using direct memory accesses, implementing the long-range
demagnetizing interaction is the main difficulty in achieving good performance
scaling, where the data transfer rate between GPUs is a significant bottleneck.
A multi-GPU convolution algorithm is developed here, which relies on single-GPU
FFTs executed in parallel. It is shown that even for micromagnetic simulations
where the demagnetizing interaction computation time dominates, good
performance scaling may be achieved, with speedup factors up to 1.8, 2.5, and
3.1, for 2, 3, and 4 GPUs respectively. The code developed here can be used for
any number of GPUs in parallel, with performance scaling strongly dependent on
inter-GPU data transfer rate and connection topology. This is further improved
in micromagnetic simulations which include a spin transport solver, obtaining
speedup factors up to 1.96, 2.8, and 3.7, for 2, 3, and 4 GPUs respectively.
The best case scenario is obtained for atomistic spin dynamics simulations,
where the demagnetizing interaction is implemented with spin-averaged cells.
Using a single workstation with 4 GPUs, it is shown atomistic spin dynamics
simulations with up to 1 billion spins, and atomistic Monte Carlo simulations
with up to 2 billion spins are possible, with a near-ideal performance scaling
BORIS – Micromagnetic, Spin Transport and Multiscale Atomistic Software for Modelling Magnetic Information Storage
Get PDFA brief review of BORIS is given here, together with a review of recent works using this software, including applications to modelling magnetic hard-disk-drive read heads, ultrafast magnetization processes, computation of thermodynamic equilibrium states using Monte Carlo algorithms, and modelling skyrmions as information carriers. BORIS is a state-of-the-art multi-physics and multi-scale research software designed to solve three-dimensional magnetization dynamics problems, coupled with a self-consistent charge and spin transport solver, heat flow solver with temperature-dependent material parameters, and elastodynamics solver including thermoelastic and magnetoelastic/magnetostriction effects, in arbitrary multi-layered structures and shapes. Both micromagnetic and atomistic models are implemented, also allowing multi-scale modelling where computational spaces may be configured with multiple simultaneous micromagnetic and atomistic discretization regions. The software allows multi-GPU computations on any number of GPUs in parallel, in order to accelerate simulations and allow for larger problem sizes compared to single-GPU computations – this is the first magnetization dynamics software to allow multi-GPU computations, enabling large problems encompassing billions of cells to be simulated with unprecedented performance
Boris computational spintronics—High performance multi-mesh magnetic and spin transport modeling software
Get PDFThis work discusses the design and testing of a new computational spintronics research software. Boris is a comprehensive multi-physics open-source software, combining micromagnetics modeling capabilities with drift-diffusion spin transport modeling and a heat flow solver in multi-material structures. A multi-mesh paradigm is employed, allowing modeling of complex multi-layered structures with independent discretization and arbitrary relative positioning between different computational meshes. Implemented micromagnetics models include not only ferromagnetic materials modeling, but also two-sublattice models, allowing simulations of antiferromagnetic and ferrimagnetic materials, fully integrated into the multi-mesh and multi-material design approach. High computational performance is an important design consideration in Boris, and all computational routines can be executed on graphical processing units (GPUs), in addition to central processing units. In particular, a modified 3D convolution algorithm is used to compute the demagnetizing field on the GPU, termed pipelined convolution, and benchmark comparisons with existing GPU-accelerated software Mumax3 have shown performance improvements up to twice faster
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