# Optimization of Bifurcated Switching by Enhanced Synthetic Antiferromagnetic Layer
## 2 Results and Discussion
### Mechanism Analysis  
Bearing in mind that the WER test and the actual use of the MRAM chip are both focused on the MTJ pillar level, the \(H_{\text{C}}\) of the MTJ cell was measured to evaluate the magnetic properties of the free layer at the device level. The approximate average value of \(H_{\text{C}}\) and its distribution before and after optimization are shown in Figure 4a,b. As with the results in the film level, the \(H_{\text{C}}\) of the MTJ devices hardly changes after inserting a thin Pt layer between the SAF layer and the seed layer. Meanwhile, the stray field of the reference layer to the free layer in the vertical direction does not change, because the non-magnetic film (Pt) in SAF is the only changed layer. To verify the influence of SAF enhancement on the write-error rate, the dependence of the WER on the voltage pulse was measured, and the results are shown in Figure 4c. In contrast to the previous MTJ stack with slightly bifurcated switching, the optimized MTJ stack exhibited no anomalous switching, neither back-hopping nor balloon effect. Benefiting from the improved WER, the write voltage determined by the WER at the \(10^{-6}\) level was reduced by 18.6%, which improved endurance by at least two orders of magnitude and reduced the write consumption by 33.7% (P = V\({}^{2}\)/R). It is generally believed that the stray field induced by the SAF is one of the main reasons for the ballooning of the WER [36]. The unswitched domain can be trapped around the edge of the free layer due to the nonuniformity of the stray field edge, causing the free layer to return to its original state after the switching current or voltage is turned off. However, only the out-of-plane z direction was considered. In order to further explore the internal mechanism of SAF improvement in WER optimization, a new switching model was developed, as shown in Figure 4d. The magnetic moments of SAF and RL can be decomposed into horizontal and vertical components. Affected by thermal disturbance, the magnetic moments of both the free layer and the reference layer have a certain component in the horizontal direction. Typically, the reference layer has a stronger PMA and, therefore, has a smaller horizontal component of the magnetic moment, which has less influence on the free flip. When the PMA of the reference layer weakens, the horizontal component of its magnetic moment also increases. The weakened SAF and RL exhibit larger horizontal moments and smaller vertical moments. The stray field of horizontal moments induces the inhomogeneity of free-layer moment. Figure 3: The minor loops (**a**) and the major loops (**c**) of optimized pMTJ measured by MOKE in blanket wafer level. (**b**) Comparison of \(H_{\text{C}}\)/\(H\) offset distribution before and after SAF optimization. (**d**) Comparison of \(H\)/\(H\) distribution (**e**), \(\Delta H_{0}\) (**e**), and \(H_{\text{K}}\) (**f**). On the other hand, when its horizontal component is opposite to the horizontal component of the free layer at the minimum probability, difficulties can arise in flipping the free layer, and this then results in the bifurcated-switching phenomenon. Also, further increasing the horizontal component of the reference layer leads to the back-hopping abnormality. Another possibility comes from the FL-induced stray field. The RL weakens after exposure to the stray field in the horizontal direction, causing the polarization to decrease and the voltage to increase. The increased switching voltage finally leads to ballooning of the WER. A similar phenomenon can be seen in [PERSON]'s research, where there are additional stagnation points on the surface of the sphere representing the magnetization vector that acts as a trap for the magnetization and causes a delay in switching at the perturbing fields applied along directions off the perpendicular easy axis [37]. The errors in switching are significantly increased by trapping the magnetization into those stagnation points during STT-induced dynamics.