Nonequilibrium phases and segregation for skyrmions on periodic pinning arrays

Reichhardt, C.; Ray, Dipanjan

doi:10.1103/physrevb.98.134418

Cited by 47 publications

(35 citation statements)

References 75 publications

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“…For clarity, we have normalized V ⊥ to the value of V || at high drives by dividing V ⊥ by α m /α d and multiplying it by −1. We can also characterize the dynamic phases by measuring the skyrmion velocity deviations in the x and y-directions as in previous work 100,101 Fig. 14(b) indicates that the velocity deviations are largest in the plastic flow phase, and diminish to a constant value in the moving crystal phase.…”

Section: B Intermediate and High Magnus Forcementioning

confidence: 87%

Shear banding, intermittency, jamming, and dynamic phases for skyrmions in inhomogeneous pinning arrays

Reichhardt

2020

Phys. Rev. B

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We examine driven skyrmion dynamics in systems with inhomogeneous pinning where a strip of strong pinning coexists with a region containing no pinning. For driving parallel to the strip, we find that the initial skyrmion motion is confined to the unpinned region and the skyrmion Hall angle is zero. At larger drives, a transition occurs to a phase in which motion also appears within the pinned region, creating a shear band in the skyrmion velocity, while the skyrmion Hall angle is still zero. As the drive increases further, the flow becomes disordered and the skyrmion Hall angle increases with drive until saturating at the highest drives when the system transitions into a moving crystal phase. The different dynamic phases are associated with velocity and density gradients across the pinning boundaries. We map out the dynamic phases as a function of pinning strength, skyrmion density, and Magnus force strength, and correlate the phase boundaries with features in the velocity-force curves and changes in the local and global ordering of the skyrmion structure. For large Magnus forces, the shear banding instability is replaced by large scale intermittent flow in the pinned region accompanied by simultaneous motion perpendicular to the direction of the drive, which appears as oscillations in the transport curves. We also examine the case of a drive applied perpendicular to the strip, where we find a jamming effect in which the skyrmion flow is blocked by skyrmion-skyrmion interactions until the drive is large enough to induce plastic flow. arXiv:1911.06719v1 [cond-mat.mes-hall]

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Section: B Intermediate and High Magnus Forcementioning

confidence: 87%

Shear banding, intermittency, jamming, and dynamic phases for skyrmions in inhomogeneous pinning arrays

Reichhardt

2020

Phys. Rev. B

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“…This effect was first found in continuum-based simulations of moving skyrmions in the strong substrate limit, where it was attributed to an attraction between the skyrmions arising from spin wave excitations generated by the fluctuating internal modes of the skyrmions 67 . Recent simulations with periodic pinning arrays show that a strong clustering effect occurs for moving skyrmions when both the Magnus force and the pinning strength are sufficiently large 84 . In these simulations, which contain no spin waves, the effect was attributed to the drive dependence of the skyrmion Hall angle, which causes different regions of the skyrmions to move toward each other due to their differing values of θ sk if a sufficiently strong velocity gradient can be induced by the pinning.…”

Section: Density Phase Separation and Dynamic Phase Separationmentioning

confidence: 99%

Nonlinear transport, dynamic ordering, and clustering for driven skyrmions on random pinning

Reichhardt

2019

Phys. Rev. B

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Using numerical simulations, we examine the nonlinear dynamics of skyrmions driven over random pinning. For weak pinning, the skyrmions depin elastically, retaining sixfold ordering; however, at the onset of motion there is a dip in the magnitude of the structure factor peaks due to a decrease in positional ordering, indicating that the depinning transition can be detected using the structure factor even within the elastic depinning regime. At higher drives the moving skyrmion lattice regains full ordering. For increasing pinning strength, we find a transition from elastic to plastic depinning that is accompanied by a sharp increase in the depinning threshold due to the proliferation of topological defects, similar to the peak effect found at the elastic to plastic depinning transition in superconducting vortex systems. For strong pinning and strong Magnus force, the skyrmions in the moving phase can form a strongly clustered or phase separated state with highly modulated skyrmion density, similar to that recently observed in continuum-based simulations for strong disorder. As the Magnus force is decreased, the density phase separated state crosses over to a dynamically phase separated state with uniform density but with flow localized in bands of motion, while in the strongly damped limit, both types of phase separated states are lost. In the strong pinning limit, we find highly nonlinear velocity-force curves in the transverse and longitudinal directions, along with distinct regions of negative differential conductivity in the plastic flow regime. The negative differential conductivity is absent in the overdamped limit. The Magnus force is responsible for both the negative differential conductivity and the clustering effects, since it causes faster moving skyrmions to partially rotate around slower moving or pinned skyrmions.

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“…Thus, the collective motion of skyrmions will exhibit a threshold current which separates the pinning state and the sliding state. Recently there have been various theoretical studies on the depinning phase transition of skyrmions [13,18,[24][25][26][27], and the experiments have also detected the phase transition in thin films with quench disorder, including that for the individual skyrmion [10,15,20,23,[28][29][30][31][32][33].…”

Section: Introductionmentioning

confidence: 99%

“…The skyrmion dynamics could be tackled in some respects with the particle model based on a modified Thiele equation. For example, there have been various efforts to understand the depinning phase transition of skyrmions [24,26,34,35]. The static and dynamic phases of skyrmions driven by the magnetic force have been numerically simulated [24].…”

Section: Introductionmentioning

confidence: 99%

“…Moreover, an effective method to reduce the threshold current in experiments is very significant in the application of skyrmions. Disorder generally exits in magnetic materials prepared for experiments, and the insensitivity of the threshold current to the non-adiabatic coefficient and the disorder was suggested [13,26]. But such a critical current is usually dependent of the non-adiabatic coefficient in other magnetic structures, such as domain walls and helical states [13,17,43].…”

Section: Introductionmentioning

confidence: 99%

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Anisotropic critical behavior of current-driven skyrmion dynamics in chiral magnets with disorder

et al. 2020

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The dynamic pinning effects are significant in manipulating skymions in chiral magnetic materials with quenched disorder. Through numerical simulations of the non-stationary current-driven dynamics of skyrmions with the Landau-Lifshitz-Gilbert equation, the critical current, static and dynamic critical exponents of the depenning phase transition are accurately determined for both adiabatic and non-adiabatic spin-transfer torques and with different strengths of disorder, based on the dynamic scaling behavior far from stationary. We find that the threshold current is insensitive to a small non-adiabatic coefficient of the spin-transfer torque, but dramatically reduced for a large one. The critical exponents indicate that the critical dynamic behavior is robust for different spin-transfer torques in the perpendicular component of the Hall motion, while exhibits a weak universality class in the direction of the driving current. The anisotropic behavior around the depinning phase transition provides a quantitative analysis of the drive-dependent skyrmion Hall effect in experiments. Further, the theoretical analysis using the Thiele's approach is presented, and the critical current and the static exponents support the simulation results.

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Nonequilibrium phases and segregation for skyrmions on periodic pinning arrays

Cited by 47 publications

References 75 publications

Shear banding, intermittency, jamming, and dynamic phases for skyrmions in inhomogeneous pinning arrays

Shear banding, intermittency, jamming, and dynamic phases for skyrmions in inhomogeneous pinning arrays

Nonlinear transport, dynamic ordering, and clustering for driven skyrmions on random pinning

Anisotropic critical behavior of current-driven skyrmion dynamics in chiral magnets with disorder

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