Screw-pitch effect and velocity oscillation of a domain wall in a ferromagnetic nanowire driven by spin-polarized current.

We investigate the dynamics of a domain wall in a ferromagnetic nanowire with spin-transfer torque. The critical current condition is obtained analytically. Below the critical current, we get the static domain wall solution, which shows that the spin-polarized current cannot drive a domain wall moving continuously. In this case, the spin-transfer torque plays both the anti-precession and anti-damping roles, which counteracts not only the spin precession driven by the effective field but also Gilbert damping of the moment. Above the critical value, the dynamics of the domain wall exhibits the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width, respectively. Both the theoretical analysis and numerical simulation demonstrate that this novel phenomenon arises from the conjunctive action of Gilbert damping and spin-transfer torque. We also find that the roles of spin-transfer torque are completely opposite for the cases below and above the critical current.