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Modeling Spin-Orbit Torques
in Advanced Magnetoresistive Devices

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8.2 In-Plane Switching

The simplest case of SOT magnetization reversal is in-plane switching. In-plane switching was first experimentally demonstrated by Liu et al. [40], and requires only the typical CIP NM/FM bilayer SOTs to switch the magnetization, as magnetic shape anisotropy can be utilized to align the easy axis along the direction of the spin polarization generated by the SHE and RE. Figure 8.1a shows the structure of a typical in-plane SOT-MRAM device. In this device, the typical MTJ is placed on top of a rectangular NM wire with a strong SOC, with the FL interfacing the NM, thus forming a bilayer. The write current flows through the NM layer, generating SOTs on the FL. The magnetization of the FL is kept in the idle state along the \({\mathbin {\textpm }} y\) direction by an IMA realized through an elliptical FL with a high eccentricity.

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(a) Device
   

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(b) Mesh
Figure 8.1: Schematic of an in-plane SOT-MRAM device with IMA (a), and the corresponding mesh used for the simulations (b). Red arrows indicate the magnetization direction of the FL and RL. The dashed lines indicate the path of the read and write currents.

Without a current flowing through the MTJ, there are no STTs, and the TB, RL can be excluded from the simulations to reduce the computational load. The effect of the RL stray field on the FL is assumed to be negligible, and since it is not necessary for the switching, it is also excluded from the simulations. Figure 8.1b shows the mesh used for the FEM simulations. The NM layer measures \(170\,\si {nm}\) along both the length and width, with a thickness of \(4\,\si {nm}\). The FL is \(1\,\si {nm}\) nm thick and has an elliptical cross-section with a major axis of \(120\,\si {nm}\) and a minor axis of \(40\,\si {nm}\). The mesh is more refined in the FM layer to improve the accuracy of the magnetization dynamics, while a coarser mesh is used in the NM layer to further reduce the computational cost. The resulting mesh consists of approximately \(28000\) elements. The parameters for Pt and CoFeB are used for the NM and FM layers, respectively.

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Figure 8.2: The \(y\)-component of the volume averaged magnetization and the applied electrical current density as a function of time. The red curve shows the switching from \(+y\) to \(-y\) with a current along \(+x\) (a), while the blue curve shows the switching from \(-y\) to \(+y\) with a current along \(-x\) (b), representing the switching of the MTJ from an antiparallel to a parallel state and vice versa.

A bias voltage of \(0.11\,\si {V}\) is applied across the SOT generating Pt layer, resulting in a current density of \(j_\mathrm {SOT} \approx 4.5 \times 10^{8} \, \mathrm {A/cm^2}\) along the \(+x\) direction. Figure 8.2 shows the magnetization switching of the FL driven by SOTs using a \(0.4\) ns current pulse. The current pulse is applied along the \(+x\) direction, generating SOTs that switch the magnetization from the initial state along \(+y\) to the final state along \(-y\). Since the spin polarization generated by the SHE and RE is collinear with the initial magnetization direction, one would expect a long incubation time before switching starts, similar to STT switching. However, as there is no TB along the write path, higher current densities can be used, reducing the incubation time. Furthermore, the magnetization on the edges of the FL is slightly tilted away from the \(y\)-axis due to the demagnetizing field, allowing the SOTs to initiate switching of the magnetization immediately. Switching in the other direction is achieved by reversing the current direction; the process is identical but mirrored.

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Figure 8.3: The spatial distribution of the \(y\)-component of the magnetization in the center of the FL at different time steps during the switching process. Each snapshot is taken at 25 ps intervals from the moment the pulse is applied. The yellow arrows indicate the direction of the current pulse.

Figure 8.3 shows the spatial distribution of magnetization at various time steps during switching for both directions. The process is mirror-symmetric, characterized by Néel domain walls nucleating at the edges along the minor axis of the FL and forming a central domain that expands outward along the long axis until switching is complete. Nucleation occurs at the edges due to the aforementioned magnetization tilt induced by the demagnetizing field. Along the FL center, the magnetization initially experiences no torque because it is collinear with the current-induced spin polarization, whereas regions farther from the central axis experience increasing torque as the magnetization tilts away from the y-axis. This torque gradient triggers domain-wall nucleation at the edges, followed by their propagation toward the center and along the long axis until the reversal is complete.