Direct imaging of current-driven domain walls in ferromagnetic nanostripes

To better understand the response of domain walls to current-induced spin transfer torques, we have directly imaged the internal magnetic structure of domain walls in current-carrying ferromagnetic nanostripes. Domain wall images were acquired both while a constant current was flowing through the wire, and after applying current pulses. Domain walls ranging from vortex walls in wide (1 mu m) wires to transverse walls in narrow (100 nm) wires were quantitatively analyzed using scanning electron microscopy with polarization analysis. The domain wall motion is characterized by strong interactions with random pinning sites along the wire. The walls either jump with the electron flow between pinning sites, or the pinned walls are distorted by the current. The domain wall propagation is also associated with transverse motion of the vortex core.