Peak Effect and Order to Disorder Transitions in Skyrmion Lattices

The peak effect is a feature found in superconducting vortex systems with quenched disorder where, due to a softening of the vortex lattice due either to thermal effects or increasing magnetic field, there can be a strong enhancement of the critical depinning force [1,2]. An interesting feature of the peak effect is that in some cases, the critical depinning force can increase with increasing temperature, whereas in most systems an increase of thermal fluctuations leads to increased creep and a reduced depinning force. Here, using a recently proposed model where skyrmion-skyrmion interactions exponentially decrease with increasing temperature [3,4], we show that the skyrmion lattice also exhibits what we call a skyrmion peak effect. In this case, when the skyrmion-skyrmion interactions became weak enough, there is a transition from a skyrmion lattice to a glassy state that is much better pinned by the substrate. At even higher temperatures, there is an increase in the amount of thermal creep that causes the critical depinning force to decrease. This leads to a peak effect in the critical current, crossing effects in the velocity-force curves, and changes in the differential conductivity. We explore how the skyrmion Hall angle changes across this transition and map out the the general field versus temperature phase diagram showing skyrmion crystal, glass, and fluid regimes for varied values of quenched disorder.

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[2] "Peak effect and anomalous flow behavior of a flux-line lattice," S. Bhattacharya and M. J. Higgins, Phys. Rev. B 49, 10005 (1994).

[3] "Statics and dynamics of skyrmions interacting with disorder and nanostructures," C. Reichhardt, C. J. O. Reichhardt, and M. V. Miloševic, Rev. Mod. Phys. 94, 035005 (2022).

[4] "Exponential temperature effects on skyrmion-skyrmion interaction," Yu Wang et al, Phys. Rev. Appl., in press.