Micromagnetic Studies of Laser-induced Magnetization Dynamics in FePt-C Films.

Laser-induced magnetization switching has been studied extensively for its applications in heat-assisted magnetic recording (HAMR) and all-optical switching (AOS), since Beaurepaire etal. found that magnetizations can respond to the femtosecond time scale laser pulse [1]. With the help of heat and assistance of external field or helicity-dependent magneto-optical effect, magnetizations switch in an ultrafast regime of picoseconds [2]. Recently, micromagnetic simulations based on LLB equation have been used to study the magnetization dynamics and switching in FePt films [3], [4]. In this work, the hybrid Monte Carlo (HMC) micromagnetics developed by the authors [5], [6] will be utilized to analyze the laser-induced magnetization switching. In this work, we built a model of FePt-C granular film with Voronoi polycrystalline structure. Some magnetic and structural parameters are derived by experimental measurements [7]. The total simulated area is 32nm $\times 32$ nm $\times 8$ nm, divided into a regular mesh of 2.5nm $\times 2.5$ nm $\times 2$ nm micromagnetic cells and the number of crystalline grains is 48. At $\mathrm {T}=0\mathrm {K}$, in crystalline grains, the saturation $M_{s}(0)$ is 1300 emu/cc, the exchange constant $A_{1}^{\ast }$ is $6.8 \times 10 ^{-7}$ erg/cm and the anisotropy energy $K(0)$ is $4.5 \times 10 ^{7}$ erg/cm 3; at disorder grain boundary, $M'_{s}(0)$ is $0.1 M_{s}(0)$, $A_{2}^{\ast }$ is $0.2 A_{1}^{\ast }$ and $K'(0)$ is 0.1 K(0) [7]. In simulation of laser-induced magnetization dynamics, the temperature profile of the laser shooting process is vitally important. In LLB micromagnetics, two-temperature model (2TM) is commonly used to get the profile of the electron temperature, which is then introduced in LLB equation via the spin coupling parameter $\lambda $. But some parameters in 2TM model are quite arbitrary. In HMC micromagnetics, we use the time-resolved magneto-optical kerr effect (TR-MOKE) to get the spin temperature profile directly. The time resolved kerr rotation, which is proportional to magnetization, is measured with an external magnetic field 2 T applied in the direction of initial magnetization to ensure that all magnetizations are saturated in one direction. The measurement and simulation results are shown in Fig. 1 and Fig. 2. The measured kerr rotation signal is shown in Fig.1(a). In Fig. 1(a), in the process of $\sim 290$ fs laser shooting, the kerr rotation decreases rapidly and then recovers slowly, which results from the rapid demagnetization and slower recovery of magnetization. Additionally, with higher laser power, the kerr rotation decreases to a critical minimum point where the magnitude of magnetization gets closely to zero, so that we can get the scaling relationship between the kerr rotation signal and the magnetization. We assume a spin temperature profile in Fig.1(b), and the HMC micromagnetics is performed with $M_{s}(T)$ determined by the mean field Brillouin function based on the temperature profile $T(t)$ in Fig.1(b) [8]. The simulated averaged M(t) curves fit well with the measurements in Fig.1(a), which in turn convince the correct choice of the spin temperature profile in Fig.1(b). So that HMC micromagnetics can be further utilized to study the laser-induced magnetization dynamics with various external magnetic fields or circularly polarized light.