Mechanism of Magnetic Orientation Reversal Induced by Ultrafast Laser

Although a phenomenological description, the inverse Faraday effect (IFE) is generally adopted for all optical magnetic switching (AOS) study, and has been successfully incorporated into the dynamics of AOS. The origin of IFE has been investigated theoretically by Pitaevskii[1], and predictions made by the study have been proved by experiments in 1966[2]. However, the effective field of IFE is deduced within the nanosecond time scale, which means the approximations valid in nanosecond scale are not quite suitable for femtosecond scenario. Further, the ~10-times mismatch between the duration of IFE in magnetic materials and the width of the incident laser pulse remains an open question for magnetic materials which exhibit AOS[3]. We investigate the process of AOS with quantum mechanical descriptions. The interactions between the incident femtosecond laser and magnetic materials are attributed to Rashba effect and stimulated Raman scattering (SRC): the incident femtosecond laser breaks the space inversion symmetry and splits the energy band of the electrons of magnetic material into two due to Rashba effect, which in turn maintains a quasi-stationary state for SRC (The three level system is depicted in Fig. 1). The fine energy structure is spin dependent, so each sub-band is exclusive for spin up or spin down. The adiabatic transition of the states of electrons among the fine energy bands and the excited state gives rise of the spin direction switching, which switches the orientation of the magnetization of the material. The process discussed above can be described by Rabi model[4]. Simulation carried on this model shows that by tuning the center frequency of the incident femtosecond laser regarding the energy gap of the ground state and the excited state, the spin flip could be realized, results are shown in Figure 2. The further investigation will be focused on the details of decay rate for the two sub-bands.