# Optimization of Bifurcated Switching by Enhanced Synthetic Antiferromagnetic Layer
## 2 Results and Discussion
### MRAM Device Performance
That is,  
\[WER(I)=1-\exp\left(-\frac{\Delta\tau^{2}j}{4\left(\exp\left(2\left(t_{pw}/t_{0} \right)j\right)i-1\right)}\right) \tag{4}\]  
where \(i=I/I_{\text{C0}}\) and \(j=i-1\) is called \"overdrive\". In addition, \(t_{0}\) is the characteristic relaxation time of the magnetic moment, \(\Delta\) is the data retention, and \(t_{pw}\) is the applied voltage pulse width. The value of \(t_{0}\) is given by:  
\[t_{0}=\frac{1+\alpha^{2}}{\alpha\gamma H_{K}} \tag{5}\]  
Note that for \(\alpha<<1\), \(t_{0}\) decreases significantly with increasing \(H_{K}\) and damping, and a smaller \(t_{0}\) will be beneficial in achieving a smaller WER. Therefore, the WER can be improved by increasing \(H_{K}\), which is also verified by our other experiment. However, a change in \(H_{K}\) and other magnetic parameters of the free layer makes big changes in the device's performance, such as increases in data retention and program voltage. As seen in the \(H_{\text{C}}\) and \(H_{K}\) results above, the free layer shows good uniformity and PMA, regardless of the film level or the MTJ pillar. Therefore, it is increasingly necessary to search for an effective way to improve the WER without changing the free layer. To further optimize the WER while maintaining chip performance, the non-magnetic layer in the SAF was chosen to improve the write-error rate without obviously changing the magnetic moment. The minor hysteresis loops and the major hysteresis loops tested by MOKE, after the insertion of a thin Pt layer between the seed layer and the SAF layer, are shown in Figure 3a,c. The magnetic anisotropy of Co/Pt multilayers is strongly influenced by the crystallographic orientation. The highest perpendicular magnetic anisotropy was observed in Co/Pt multilayers with Pt (111) aligned parallel to the film plane. Therefore, the inclusion of a Pt layer can enhance the perpendicular magnetic anisotropy of synthetic antiferromagnetic structures (SAFs) [34, 35]. As can be seen, there are similar switching curves for similar MTJ stack structures. The \(H_{\text{C}}\) in the film level, which refers to the stability of the free layer, keeps invariability (~150 Oe) for the fixed free layer. Similar performance is not only related to the mean value but also related to the distribution (see Figure 3b). After the change in the SAF, the Hex exhibited little change, while the \(H\!f\) significantly improved from ~5700 Oe to ~7500 Oe, but the distribution remained unchanged. Visual comparisons before and after optimization are shown in Figure 3b,d. As discussed above, the \(H_{K}\) and damping can change the phenomenon of WER. FMR spectrum measurements on the optimized MTJ stack were employed to verify the effect of SAF tuning on \(H_{K}\) and damping. The dependence of the resonant field \(H\) on frequency \(f\) is shown in Figure 3e, and the extracted \(H_{K}\) and damping compared to the baseline SAF are shown in Figure 3f. As can be seen, there are small differences in \(H_{K}\) (5020 Oe vs. 5048 Oe) in the MTJ stacks before and after optimization. However, the fitted damping and \(\Delta\)H0 exhibit little difference. The post stack exhibits greater damping (0.0054) and a smaller \(\Delta H_{0}\) (218 Oe). A smaller \(\Delta H_{0}\) indicates better uniformity, which is beneficial for less bifurcation switching in the WER test.