Determination of chirality and density control of Neel-type skyrmions with in-plane magnetic field

Magnetic skyrmions are topologically protected nanoscale spin textures exhibiting fascinating physical behaviors. Recent observations of room temperature Neel-type skyrmions in magnetic multilayer films are an important step towards their use in ultra-low power devices. Here, we investigate the magnetization reversal in [Pt/Co/Ta](n) multilayer samples under a tilted magnetic field using in-situ Lorentz transmission electron microscopy. On decreasing the magnetic field, individual skyrmions appear to subsequently evolve into snake-like structures growing in the direction opposite to the in-plane magnetic field. We show that this unusual relation between the velocity vector and the magnetic field is dominated by the chirality of the Neel-type skyrmions. It allows one to extract the sign of the Dzyaloshinskii-Moriya constant. We also demonstrate that high concentration of skyrmions can be achieved on increasing the in-plane component of the field or increasing the disorder of the film. Our micromagnetic simulations agree with our experimental results.