Direct observation of the coherent precession of magnetic domain walls propagating along permalloy nanowires

Spintronic devices, whose operation is based on the motion of a magnetic domain wall (DW), have been proposed recently. If a DW could be driven directly by flowing an electric current instead of a magnetic field, the performance and functions of such device would be drastically improved. Here we report real-space observation of the current-driven DW motion by using a well-defined single DW in a micro-fabricated magnetic wire with submicron width. Magnetic force microscopy (MFM) visualizes that a single DW introduced in the wire is displaced back and forth by positive and negative pulsed-current, respectively. We can control the DW position in the wire by tuning the intensity, the duration and the polarity of the pulsed-current. It is, thus, demonstrated that spintronic device operation by the current-driven DW motion is possible.

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins to a spatial and time varying magnetization \({\bf M} ({\bf r},t)\) within the time-dependent semiclassical transport theory. We show that the induced non-equilibrium conduction spin density in turn generates four spin torques acting on the magnetization--with each torque playing different roles in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques that has not been previously identified is crucial to consistently interpret experimental data on domain wall motion.