Magnetic Memory Effects in Coherent Magnetization Dynamics of GaMnAs: From Non-equilibrium to Thermal Regime

III-Mn-V heterostructures exhibit rich spin memory effects and their magnetic properties show strong responses to external stimuli (light, electrical gate and current) via carrier density tuning. Here we report on the coherent magnetization dynamics due to laser excitation of transient carriers in GaMnAs, distinctly depending on the initial magnetic state the system is prepared in. We identify two distinct temporal regimes that reveal a complex scenario of spin reorientation, marked by the transition from a highly non-equilibrium, non-thermal, transient carrier-mediated regime ($<$ 300 femtosecond), to a thermal, lattice-heating regime (on the picosecond time scale). The ultrafast, sub-picosecond response can be used as footing for ultrafast optical detection of magnetic memory states. This observed femtosecond cooperative magnetic phenomenon may represent an as-yet-undiscovered universal principle in all carrier-mediated ferromagnetic materials, offering potential perspectives for terahertz (10$^{12}$ Hz) speed ``spintronic'' devices and functional systems.