Nonuniform switching of the perpendicular magnetization in a spin-torque-driven magnetic nanopillar

Time-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of $~1.3\hspace{0.5em}\mathrm{ns}$, a $100\times 300\hspace{0.5em}{\mathrm{nm}}^2$ ellipsoidal device switches in $~1\hspace{0.5em}\mathrm{ns}$ via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods.

Figure 1

Schematic of the STXM experiment performed on a [Co/Ni]-based nanopillar spin valve. The pulse sequence (current versus time) is shown on the right. Letters (a), (b), (d), and (f), respectively, represent the STXM measurements corresponding to the images shown in Fig. 2.

Figure 2

Experimental STXM images of the magnetic contrast in a $100\times 300\hspace{0.5em}{\mathrm{nm}}^2$ ellipsoidal nanopillar spin valve. Images (a)–(e) have been taken at different times during the CIMS reversal based on the setup shown in Fig. 1. Image (a) is the initial state, and (f) is the final state. The color scale corresponds to the perpendicular component of the free-layer magnetization, from parallel (P)(red) to antiparallel (AP)(blue) with respect to the reference layer.

Figure 3

A series of micromagnetic simulations showing the free-layer configuration at different times (a)–(f) during the CIMS process in a $100\times 300\hspace{0.5em}{\mathrm{nm}}^2$ PMA nanopillar. The color scale is the same as in Fig. 2 and corresponds to the perpendicular component of the free-layer magnetization, from parallel (P)(red) to AP (blue) with respect to the reference layer. Times are given with respect to domain nucleation.

Figure 4

(a) Experimental image of the magnetization along the PMA axis in the free layer at $-250\hspace{0.5em}\mathrm{ps}$ where the color scale ranges from parallel (red) to AP (blue) and the negative sign indicates that the image was taken prior to nucleation. (b) The simulated magnetization in the free layer at $-350\hspace{0.5em}\mathrm{ps}$ where the length of the yellow arrows corresponds to the amplitude of the in-plane magnetic component. (c) Simulated component of the magnetization along the PMA direction ($M_z$, left vertical axis) as well as the two in-plane components ($M_x$ and $M_y$, right vertical axis).