# Investigation on the Effect of Femtosecond Laser Induced Spin Transfer Torque of GdFeCo Alloy
## 3 Results and Discussions  
In Figure 4, the normalized magnetization of Gd\({}_{25}\)Fe\({}_{75}\) alloy induced by different incident laser power densities and injection current densities are presented quantitatively. When the incident laser power density is higher than 20 mJ/cm\({}^{2}\), the magnetization is unstable; this may be that the laser power density is too high and the electronic temperature can't drop to below the Curie temperature in a short time. Therefore, we chose to study the case of laser power density of 10 and 20 mJ/cm\({}^{2}\) to further study the influence of spin transfer moment effect on AOS process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Symbol** & **Value** & **Unit** \\ \hline gyromagnetic ratio & \(\gamma\) & \(1.76\times 10^{11}\) & T\({}^{-1}\) S\({}^{-1}\) \\ microscopic damping parameter & \(\lambda\) & 0.01 & - \\ electron heat capacity constant & \(\gamma_{e}\) & 222 & J m\({}^{-3}\) K\({}^{-2}\) \\ electron heat capacity & C\({}_{e}\) & - & J m\({}^{-3}\) K\({}^{-1}\) \\ phonon heat capacity & C\({}_{l}\) & \(2.3\times 10^{6}\) & J m\({}^{-3}\) K\({}^{-1}\) \\ Electron-phonon coupling factor & \(G\) & \(6.6\times 10^{17}\) & W m\({}^{-3}\) K\({}^{-1}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The physical parameters in two-temperature model. The current density of 0 A/cm\({}^{2}\) means that the spin transfer torque effect brought by the current is not considered at this time, instead, the thermal effect of the laser pulse on the material is considered. The magnetic system is in a stable state in the initial condition. When the laser pulse acts (at 0 ps), the electron temperature rises rapidly, and the magnetization of Fe and Gd sublattices are close to the demagnetization state, leading to a turning angle. Because of the laser pulse effect has a shot duration, the electron temperature will drop rapidly, and the magnetization of Fe and Gd sublattices will gradually return to the initial state. This requires special attention, and the magnetization of GdFe alloy will occur a short time reversal in 0 and 1 ps intervals (Figure 4a,b) because the atomic magnetic moment of Gd is 7.63 u\({}_{\text{B}}\), far greater than 1.92 u\({}_{\text{B}}\) of Fe; thus, under the initial state, the magnetization direction of the GdFe alloy system is the same as Gd sublattice. After demagnetization, the magnetization Gd and Fe are close to zero, but a higher proportion of Fe is in the alloy system. Therefore, for a short period of time, the magnetization of the GdFe alloy system will show the same direction as Fe sublattice. Figure 4: Normalized magnetization of Gd\({}_{\text{25}}\)Fe\({}_{\text{75}}\) alloy induced by different incident laser power density and injection current density, the current density ranges from 0–10\({}^{8}\) A/cm\({}^{2}\). (**a**) when the incident laser power density is 10 mJ/cm\({}^{2}\); (**b**) when the power density of the incident laser is 20 mJ/cm\({}^{2}\). When the incident laser power is 10 mJ/cm\({}^{2}\), the injection current density of 10\({}^{6}\) A/cm\({}^{2}\) does not make the magnetization of Gd\({}_{25}\)Fe\({}_{75}\) alloy switch, but the injection current density of 10\({}^{7}\) A/cm\({}^{2}\) can make the magnetization of Gd\({}_{25}\)Fe\({}_{75}\) alloy switch, which means the threshold value of the flipped current density is between 10\({}^{6}\) and 10\({}^{7}\) A/cm\({}^{2}\). When the incident laser power is 20 mJ/cm\({}^{2}\), the injection current density of 10\({}^{6}\) A/cm\({}^{2}\) can make the magnetization of Gd\({}_{25}\)Fe\({}_{75}\) alloy reverse, and the stabilization time after the magnetization reversal becomes longer with the increase of the injection current density, and the magnetization intensity after stabilization is almost unaltered.