Asymmetric slow dynamics of the skyrmion lattice in MnSi

Some magnetic systems feature spin textures, real-space patterns in the orientation of spins that can topologically form non-trivial configurations. Among them, a vortex-like spin swirling texture known as a magnetic skyrmion has attracted particular attention. Lattices of skyrmions form in the helimagnet MnSi with a periodicity of 18 nm, which makes them amenable to investigation by Lorentz transmission electron microscopy in real space and by small-angle neutron scattering in momentum space. However, the dynamics of the skyrmion lattice have not been studied in detail because of the difficulty of inelastic neutron scattering measurements that can measure the dynamic structure factor at the microelectronvolts energy scale at small wavevectors. Here we examine the low-energy excitations of the skyrmion state in MnSi by using the neutron spin-echo technique under small-angle neutron scattering conditions. We observe an asymmetric dispersion of the phason excitations of the lattice because of the string-like structure of the skyrmion cores.