ns × nm imaging of current-induced domain wall dynamics at low current density

There have been numerous experimental and theoretical studies on the motion of magnetic domain walls (DW) under applied currents, and many attempts have been made to visualize the dynamics. DW motion at high current density (flow regime), dominated by current-induced spin torques, has been studied using various imaging techniques based on stop-motion method. However, far less focus has been given to low current density (pinned and creep regime), where the effects of DW pinning become pronounced, and as a result cannot be effectively captured unless using time-resolved imaging techniques.

In this study, we investigate current-induced DW dynamics in room-temperature ferromagnetic (Fe_0.63Ni_0.3Pd_0.07)₃P by utilizing time-resolved Lorentz TEM imaging based on ultrafast electron microscopy, at spatiotemporal resolution better than 40 nm and 2 ns. Under the injection of pulsed current with a duration of 100 ns, transient DW displacement is observed, which can be interpreted as the interplay between spin-transfer torque and pinning. DW displacement elevates drastically from a few nanometers to tens of nanometers once a specific threshold current density is exceeded, suggesting the transition from pinned to creep regime. By examining the transient process of DW motion, we identified non-trivial behaviors appearing at current density slightly above the threshold. In this presentation, we will discuss the mechanism behind these nonlinear dynamics.