We study spin-orbit torque (SOT) switching in W/CoFeB/MgO structures with various dot sizes (120-3500 nm) using pulsed current of various widths τ (800 ps-100 ms) to examine the time and spatial evolution of magnetization switching. We show that the switching behavior and the resultant threshold switching current density Jth strongly depend on device size and pulse width. The switching mode in a 3500nm dot device changes from probabilistic switching to reproducible partial switching as τ decreases. At τ = 800 ps, Jth becomes more than 3 times larger than that in the long-pulse regime. A decrease in dot size to 700nm does not significantly change the switching characteristics, suggesting that domain-wall propagation among the nucleated multiple domains governs switching. In contrast, devices with further reduced size (120 nm) show normal full switching with increasing probability with current and insignificant dependence of Jth on τ, indicating that nucleation governs switching.
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