# Ultrafast Modulation of Magnetization Dynamics in Ferromagnetic (Ga, Mn)As Thin Films
## 1 Introduction  
Investigation of multiple functionalities of ferromagnetic semiconductors in the picosecond range lays the ground for future opportunities for high-speed spintronic devices. Time-resolved magneto-optical Kerr effect (MOKE) spectroscopy has been widely used for studying ultrafast photon-induced collective spin dynamics in ferromagnetic semiconductors. Based on this technique, Mn-based III-V ferromagnetic semiconductors have been extensively investigated from the point of view of both fundamental physics and spintronic applications [1; 2; 3; 4; 5; 6]. Ultrafast magneto-optical investigation of collective spin excitations in ferromagnetic (Ga,Mn)As thin films has demonstrated the possible multi-functionality of ferromagnetic semiconductors when incorporated in spintronic devices [4; 7; 8; 9]. Transient modulation involving either thermal or non-thermal mechanisms investigated by laser excitation has been shown to contribute to ultrafast coherent precession of magnetization [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Modulation of magnetic anisotropy stimulated by transient increases of either local temperature or photocarrier density in (Ga,Mn)As has demonstrated multiple possibilities of modulating the precession of magnetization by linearly polarized pump pulses.  
Recently, several types of ultrafast mechanisms for manipulating coherent spin excitations have been discovered in (Ga,Mn)As, such as optical spin transfer torque (OSTT), in which the angular momentum of photo-generated carriers is transferred to the collective magnetization of the system [10]; and optical spin-orbit torque (OSOT), in which torque is exerted on the local magnetic moments of Mn by non-equilibrium spin polarization of photo-excited holes through spin-orbit coupling [19]. Photoionization-like excitation of Mn\({}^{2+}\) has been recently proposed to account for the photoinduced modulation of perpendicular effective field [23]. Intrinsically, one or more of such mechanisms could simultaneously trigger a collective precession of magnetization [4; 7; 14; 21], and they would compete with each other as to how they contribute to the precession process. On the other hand, they may also compensate each other in their contributions to data storage in the magnetic recording. Previous studies reported that hole cooling after pulse excitation is one of the limiting factors for ultrafast control of carrier-mediated magnetization precession, while thermal dissipation after laser heating is considered as the barrier for the high speed of data storage [11; 12; 13; 14; 24; 25; 26; 27]. In this sense, the interplay between such thermal and non-thermal modulations upon laser pulse excitation is one of key issues in the context of ultrafast manipulation of magnetization.  
In this paper, we report a study by means of time-resolved magneto-optical spectroscopy of photo-induced collective spin dynamics triggered by both thermal and non-thermal mechanisms. Using different intensities of linearly polarized optical excitation, we could relate the influence of both laser heating and photo-carrier generation on the precession of magnetization precession, and show how this is connected to modulations of the in-plane and out-of-plane anisotropy fields that result from such excitation. Specifically, for moderately-doped (Ga,Mn)As films with in-plane compressive strain, it is shown that the variation of precession frequency and precession amplitude depend on the superposition of both thermal and non-thermal effects through anisotropy fields of this material. Theoretical calculations show that the out-of-plane anisotropy field via non-thermal mediation mainly contributes to nonlinear variation of magnetization precession, thus pointing to the important role of ultrafast optical spin manipulation in ferromagnetic materials via non-thermal paths.