Figure 5: Schematic illustration of DW motion and energy landscape.

(a) A magnetic DW in a nanowire and its collective coordinates, the position q and the tilting angle φ. (b,c), Energy landscape for the DW motion in the presence (b) and absence (c) of d.c. current. Here, the periodic potential along q direction is not the exact energy barrier but an illustrative one to represent typical energy barrier. In (b), the d.c. current tilts the potential in the φ direction. Once the DW is excited above the intrinsic barrier (indicated by the red solid arrow), it evolves dynamically along the red dotted curve and eventually overcomes the extrinsic barrier, which is lowered by the tilting. Note that the threshold is determined by the intrinsic barrier when the intrinsic barrier is lower than the extrinsic one. Thermal excitation overcoming the extrinsic pinning potential along q axis (blue arrow) is blocked due to high-energy barrier. In (c), there is no tilting in the energy landscape. When the intrinsic barrier is lower than the extrinsic one, the DW may easily overcome the intrinsic barrier (red arrow), but this does not lead to the depinning from the extrinsic barrier since it does not change q. The DW depinning from the extrinsic pinning occurs only when it overcomes the full extrinsic barrier (blue arrow).