Nonmagnetic impurities in skyrmion racetrack memory

The influence of non-magnetic defects of different sizes on the stability and anchoring of skyrmions in race track memory devices has been investigated. The energy surface of the system was built on the basis of the generalized Heisenberg model, which includes exchange, Dzyaloshinskii- Moriya interaction, anisotropy, and an external magnetic field. Minima and saddle points on the energy surface are used to estimate quantitatively the stability and pinning effects for skyrmions. The activation energies for attachment and detachment of skyrmions from defects, collapse and nucleation of skyrmions on a nonmagnetic impurity on a track of finite width are calculated. The joint effect of defects and the proximity of sample boundaries on the stability and localization of skyrmions has been studied. It is shown that skyrmion race track memory can only work if the track width is much greater than four times the skyrmion radius, and the spatial size of defects that can pin a skyrmion is small compared to its own size. Otherwise, the skyrmion will annihilate instead of moving under the action of the spin-polarized current.