Friday,
October 12, 2007 - 4:00 P.M., NSH 184
Magnetic materials displaying carrier-mediated spin-spin exchange interaction are ideal for non-thermal, potentially fast spin manipulation and detection. Prominent examples of such materials
are Mn doped III-V semiconductors, where the strong interaction of holes and Mn ions results in
high ferromagnetic transition temperature. The steady-state magneto-optical/transport
measurements reveal enhancement of ferromagnetism via external stimuli and rich magnetic
memory effects. However, no time-resolved experiments in (III,Mn)V systems have shown these
collective magnetic phenomena, and the time scales for these are completely unknown. In this
talk, I will present our recent observations in GaMnAs: (1) ultrafast enhancement of
ferromagnetism via photoexcited transient holes on a 100 ps time scale and (2) femtosecond
detection of magnetic memory states. Our measurements reveal new fundamental collective
magnetic processes at ultrafast time scales, and identify the critical roles of the Mn-hole
correlation in these photo-induced cooperative behaviors. These constitute the first evidence for
ultrafast, non-thermal manipulation of the ferromagnetic order in (III,Mn)Vs, which may represent as-yet-undiscovered universal features in all carrier-mediated ferromagnetic materials.