# Investigation on the Effect of Femtosecond Laser Induced Spin Transfer Torque of GdFeCo Alloy
## 1 Introduction  
The exploding amount of data is devouring the capacity of storage devices and demanding ever faster read/write speed of these devices. For the time being, most of the data generated is stored on hard disk drives. However, the recording density of these drives is approaching the limit of the magnetic material due to the superparamagnetic effect [1]. Thus, turning to materials with higher coercivity is necessary for greater recording density. On the other hand, the magnetic field for writing is limited by the size of write head, which is in turn limited by the expected recording density. The writing speed of hard disk drives is also limited by the speed of magnetization switching. Therefore, it has become a significant challenge to develop new approaches suitable for scenarios featured with high coercivity media and ultrafast write speed. All-optical magnetic switching (AOS), which utilizes ultrafast femtosecond laser pulses to excite magnetic recording materials rather than traditional magnetic field, has drawn much attention from researchers [2, 3, 4, 5, 6, 7]. For one thing, data is written by one or a series of laser pulses, so it is possible to exploit some unique traits of the optical field in space; hopefully this will overcome the coercivity of the recording materials. Another reason is that theoretically the size of laser spots can be shrunk by means of surface plasmon polaritons (SPPs) [8, 9], so that the recording spot size beyond diffraction limit can be achieved, thus greatly improving the storage density of disks [10, 11]. Experiments also show that magnetization can be reversed by a singlefemtosecond laser pulse [2] repeatedly, which means the write/overwrite convenience of traditional magnetic recording is retained by AOS. According to the helicity of the excited laser pulse, AOS can be divided into two categories: helicity dependently AOS (HD-AOS) and helicity independently AOS (HI-AOS). Laser intruded magnetization switching has been observed in many rare earth-transition metals (RE-TM), including Tb/Co multilayers, TbCo alloy, TbFe alloy and GdFeCo alloy [12]. Among them all, GdFeCo is the one explored most both in HD-AOS and HI-AOS. There is no lack of studies on the internal mechanism of GdFeCo material AOS to explain its spin switching dynamic process [13; 14; 15; 16; 17], but most of these studies concluded that the magnetization switching was related to the helicity of femtosecond laser pulse because an axial magnetic field can be generated by intense circularly polarized light through what is known as the inverse Faraday effect [18; 19; 20]. [PERSON] et al. found that a single 40 fs circularly polarized laser pulse can cause GdFeCo alloy film magnetization reversal for the first time, the different phase of spin precession would be excited by circularly polarized lasers in different directions of helicity [2]. They deemed that AOS is not completed by absorbing the angular momentum of photons and there is another ultrafast non-thermal coupling between spin and photon angular momentum in GdFeCo alloy. On the other hand, some researchers discovered that AOS can be stimulated by linearly polarized single laser pulse in metallic ferrimagnet GdFeCo. The process of HI-AOS can be divided into two consecutive steps: sublattices demagnetization and sublattices spin reversal. Although there is a plethora of research devoted to the demagnetization, and many breakthroughs have been made, the physical mechanism and explanation of spin dynamics in spin reversal are still ambiguous. The thermal induced magnetization switching (TIMS) theory proposed by [PERSON] et al. can explain why the spins of different sublattices demagnetize successively, but are not able to provide a reasonable explanation for the stable switching of Fe spins. In TIMS theory, the thermal effective field is stochastic, and the thermal fluctuations on each atomic site can be described by a Gaussian white noise term.