  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. 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, whereas above
the threshold AC frequency (ft ~ 0.12 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 ft, the magnetic
skyrmions oscillate removing the dislocations, indicating that the dislocation
density is controlled by the AC frequency.
