Face centered cubic and hexagonal close packed skyrmion crystals in centro-symmetric magnets

Skyrmions are disk-like objects that typically form triangular crystals in two dimensional systems. This situation is analogous to the so-called "pancake vortices" of quasi-two dimensional superconductors. The way in which skyrmion disks or pancake skyrmions pile up in layered centro-symmetric materials is dictated by the inter-layer exchange. Unbiased Monte Carlo simulations and simple stabilization arguments reveal face centered cubic and hexagonal close packed skyrmion crystals for different choices of the inter-layer exchange, in addition to the conventional triangular crystal of skyrmion lines. Moreover, an inhomogeneous current induces sliding motion of pancake skyrmions, indicating that they behave as effective mesoscale particles.