Hybrid characteristic of multi-shot circularly polarized laser pulses for magnetization switching process in L10-FePt nanoparticles.

All-optical magnetic switching (AOS) observed on some materials provides a potential recording option due to its ultra-short recording-time [1]. In most studies, it is attributed to inverse Faraday effect (IFE) that circularly polarized laser will induce an opto-magnetic field [2, 3] or magnetic circular dichroism (MCD) that one magnetization orientation will absorb more energy than another [4, 5]. L10-FePt is a promising candidate for high density magnetic recording due to its high perpendicular anisotropy [6] whose magnetization is fixed out of plane either up or down. However, the mechanism about AOS on FePt is still under debate. In our research, we show that it benefits from both IFE and MCD. Taking both the thermal effect and the induced opto-magnetic field into account, we calculate the switching probability of L10-FePt nanoparticles for a single laser pulse with different opto-magnetic fields by atomistic level simulation [7], as shown in Fig. 1(a-d). We consider the effect of multi-shot pulses by an accumulative model, assuming that the switching probability after each shot is identical. Fig. 1(f) shows the normalized net magnetisation variation with different MCD ratios when the opto-magnetic field induced by left circularly laser pulse (δ−) is −0.1 Tesla. Obviously, when the MCD ratio is 2%, the final magnetisation is in agreement with the experiment result of Lambert et al. that the magnetization induced by circularly polarized laser is ~10 to 20% of saturation magnetization [8]. This MCD ratio is reasonable for FePt [5, 9]. Fig. 2(a-c) show the final magnetisation over multiple linearly (L), left (σ−) and right (σ+) circularly laser pulses respectively with different initial magnetization states, which verify that AOS is helicity-dependent but independent of the initial state. Fig. 2(d) shows the magnetisation variation over laser fluence with different opto-magnetic fields. Applying an external magnetic field 0.03 Tesla when the opto-magnetic field is −0.1 Tesla, the corresponding effective magnetic field is −0.07 Tesla. In this case, the net magnetisation roughly equals to 0 at a wide laser fluence range, which is qualitatively consistent with the results of Lambert et al. that a 700 Oe field could eliminate the all-optical switching. In addition, when the opto-magnetic fields are ±0.4 Tesla, there will be a deterministic switching that the final magnetization is greater 90% of the saturation magnetization. Taking the opto-magnetic field −0.4T as example, the net magnetisation after multiple laser pulses is shown in Fig. 2(e), and the fastest deterministic switching occurs at 34 mJ/cm2 after 34 left circularly laser pulses. Although our simulation is simplified, these results demonstrate the possibility of reaching deterministic all-optical magnetic recording by optimizing the parameters presented above.