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 are difficult to measure 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.