# Investigation on the Effect of Femtosecond Laser Induced Spin Transfer Torque of GdFeCo Alloy
## 3 Results and Discussions
The results show that within a certain range of the laser power density, the higher the incident laser power density, the lower the injection current power density required for switching, and there may be an optimal injection current density window, making it possible for switching to occur and be able to achieve stability at the fastest speed; this is of great significance to the reading and writing of high speed all-optical magnetic recording devices in actual application. The simulated results are of the same order of magnitude as [PERSON] et al. had estimated photoelectric current density in GdFeCo alloy, which further indicates that the light induced spin current in GdFeCo alloy may indeed exist in the process of photoinduced magnetic transformation. To further understand the physical mechanism, it is necessary to study the magnetization dynamics of the two sublattices under the influence of spin transfer moment effect after analyzing the change of the normalized magnetization intensity with time during the photomagnetization process of Gd\({}_{25}\)Fe\({}_{75}\) alloy. In the following calculation, the power density of the incident laser is 20 mJ/cm\({}^{2}\) and the injection current density is 0\(\sim\)10\({}^{7}\) A/cm\({}^{2}\). When the injection current density does not reach the threshold value, as shown in Figure 5a,b, the magnetization of the two sublattices does not switch. When the injection current density exceeds the threshold value, as shown in Figure 5c,d, the higher the current density, and the faster the magnetization switching of Fe sublattice will occur. Fe and Gd sublattices will all transfer to a paramagnetic state due to the temperature rise up, but as a result of the atomic magnetic moment of Fe being smaller, the Fe sublattice will precede Gd melts to a paramagnetic state. The magnetization reversal of Gd sublattice will lag behind that of Fe sublattice, and the lag time is related to the magnitude of the injection current density. Calculation results are consistent with the work of [PERSON] et al. [5]. The experimental results of [PERSON] et al. are compared with our simulation results in Figure 6.  
Figure 7 shows the time-resolved variation process of the normalized spin transfer torque in the case of reverse inversion of sublattice magnetization (10\({}^{6}\)-10\({}^{7}\) A/cm\({}^{2}\)). Where \(\tau_{x}\), \(\tau_{y}\) and \(\tau_{z}\) are respectively the components of the normalized spin transfer torque in three directions in the Cartesian coordinate system; \(T_{e}\) is the normalized electronic temperature; \(J_{e}\) is the normalized current density. It can be found that the time for the current to reach the peak lags behind the time for the electron temperature to reach the peak, and when the current density gradually decays to 0, the spin transfer torque effect also stops. In addition, the component of spin transfer torque in three directions has no obvious relationship with the injection current. This is because the torque is jointly affected by the current and the spin magnetic moments of the two sublattices. The constant precession of the spin moment will lead to constant change in the direction of the spin transfer torque, and then the spin transfer torque will react on the atomic spin. The magnitude of the current is the critical factor affecting the strength of this interaction. Figure 5: Time-resolved magnetization dynamics process of two sublattices at different current densities. For the situation **(a,b)**, the magnetization of Fe and Gd is not reversed. The magnetization of two sublattices switching at the same time almost in **(c)**. The switching time of Gd magnetization lags about several picoseconds behind Fe in **(d)**. **(a)** The injection current density is 0 A/cm\({}^{2}\); **(b)** the injection current density is \(10^{5}\) A/cm\({}^{2}\); **(c)** the injection current density is \(10^{6}\) A/cm\({}^{2}\); **(d)** the injection current density is \(10^{7}\) A/cm\({}^{2}\).