Microscopic three-temperature model for all-optical switching in GdFeCo
Yao Han, Wang Sicong, Wei Chen, Cao Yaoyu, Li Xiangping     
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan University, Guangzhou, Guangdong 510632, China

Overview: All-optically manipulating the orientations of the magnetization or the spins in magnetic materials has aroused intensive research interests for the attractive applications in ultrafast data storage, spin dynamics, and magnetic holography. Among these applications, all-optical switching (AOS) has emerged as a promising alternative way to realize ultrafast perpendicular magnetic recording. Compared with magnetic switching by an external magnetic field or by a heat-assisted manner, AOS can complete the switching process within 100 ps, which has attracted extensive attention from researchers. Among the magneto-optical materials which can realize AOS, the ferrimagnetic GdFeCo has the ability to realize single-shot AOS and possesses great potential in all-optical magnetic storage. Currently, the atomic spin model and the Landau-Lifshitz-Bloch (LLB) model are the basic and frequently-used mathematical methods to describe the dynamics of GdFeCo after the laser-pulse excitation. However, these two models only use the damping parameters to phenomenologically describe the transfer process of angular momentum, and hence it is impossible to give the quantized information of angular momentum transfer during the switching process. In 2009, B. Koopmans et al. proposed a simple-form model which is called the microscopic three-temperature model (M3TM) to unify two contradictory ultrafast laser-induced demagnetization processes. This model is especially suitable for magnetic materials with the easy axis perpendicular to the surface and has been applied to calculate the ultrafast dynamics of multisublattice magnets, to demonstrate the spin-orbit enhanced demagnetization rate in Co/Pt-multilayers, and to explain the AOS in ferromagnets. In this model, the switching of electron spins is achieved by emitting or absorbing a phonon with a certain probability and hence the quantized information of angular momentum is explicitly given. In this paper, the M3TM is utilized to simulate the AOS process of GdFeCo, which is also demonstrated experimentally, under the excitation of a single laser pulse based on the heating effect. By using the M3TM, the AOS dynamics and the final magnetization states of GdFeCo induced by single laser pulses with different energy and pulse widths are calculated and analyzed concretely. Compared with the atomic spin model and the LLB model, M3TM provides a more concise time-varying expression of the magnetization of GdFeCo and explicitly addresses the dissipation of angular momentum after the laser-pulse excitation, which enables faster calculations of the heat-induced magnetization dynamics in magneto-optical materials with large areas.

Supported by National Natural Science Foundation of China (NSFC) (11604123, 61522504, 61432007), the Fundamental Research Funds for the Central Universities (21616338), and Guangdong Provincial Innovation and Entrepreneurship Project (Grant 2016ZT06D081)