Topological Charge Analysis of Ultrafast Single Skyrmion Creation
  Magnetic skyrmions are topologically non-trivial spin textures of potential interest for future information storage applications, and for such purposes, the control and understanding of single skyrmion creation is required. A scheme is analyzed to create single N\'{e}el-type and Bloch-type skyrmions in helimagnetic thin films utilizing the dynamical excitations induced by the Oersted field and the spin transfer torque given by a vertically injected spin-polarized current. A topological charge analysis using a lattice version of the topological charge provides insight into the locally triggered transition from a trivial to a non-trivial topological spin texture of the N\'{e}el or Bloch type skyrmion. The topological protection of the magnetic skyrmion is determined by the symmetric Heisenberg exchange energy. The critical switching current density is $\sim10^{7}\thinspace\textrm{A/cm}^{2}$, which decreases with the easy-plane type uniaxial anisotropy and thermal fluctuations. In-plane spin polarization of the injected current performs better than out-of-plane polarization, and it provides ultrafast switching times (within 100 ps) and reliable switching outcomes. 
Gen Yin, Yufan Li, Lingyao Kong, Roger K. Lake, C.L. Chien and Jiadong   Zang
cond-mat.mes-hall