Detailed dynamics of a moving magnetic skyrmion lattice in MnSi observed using small-angle neutron scattering under an alternating electric current flow

Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory [Parkin et al., Science 320, 190 (2008); Fert et al., Nat. Nanotechnol. 8, 152 (2013)]. Here, we report in detail on the transient state of a moving magnetic skyrmion lattice in bulk single-crystalline MnSi under alternating current (AC) using small-angle neutron scattering. A rotation and concomitant broadening of the spot width in the azimuthal direction of the magnetic skyrmion reflections originating from the plastic deformation of the magnetic skyrmion lattice were found only at low AC frequencies [Okuyama et al., Commun. Phys. 2, 79 (2019)], whereas above the threshold AC frequency ($f_t\sim 0.12\mathrm{Hz}$) the rotation was not observed, and the spot width becomes sharper. The observed complex response of the magnetic skyrmion reflections can be explained by the change in dislocation density in the magnetic skyrmion lattice. At frequencies higher than $f_t$, the magnetic skyrmions oscillate removing the dislocations, indicating that the dislocation density is controlled by the AC frequency.