Ultrafast Enhancement of Ferromagnetism via Photoexcited Holes in GaMnAs

We report on the observation of ultrafast photoenhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100 ps time scale, after an initial subpicosecond demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature $T_c$ and dominates the demagnetization component when approaching $T_c$. We attribute the observed ultrafast collective ordering to the $p\mathrm{\text{−}}d$ exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20 K and a transient increase in $T_c$.

Figure 1

(a) Illustration of hole-density-tuning effects via external stimuli in III–V FMSs seen in the static experiments [9, 10]. FM: ferromagnetism. $△p$ is hole density change. (b) Schematic diagram of the spin-dependent density of states in GaMnAs. Femtosecond pump pulses create a transient population of holes in the valence band. (c) Time-resolved MOKE dynamics at 70 K and under 1.0 T field. Transient enhancement of magnetization, with $\sim 100\mathrm{ps}$ rise time, is clearly seen after initial fast demagnetization. Thick line is the fit described later in the text.

Figure 2

(a),(b) Temporal traces of photoinduced magnetization changes at different temperatures. All traces are intentionally offset for clarity. Inset: the first 2 ps demagnetization dynamics at 15 K. (c) Temperature dependence of magnetization changes at different time delays—600 fs (squares), 3 ps (diamonds) and 240 ps (circles), respectively.

Figure 3

Decomposed demagnetization $-△{\theta }_k^d$ (a) and enhanced magnetization $△{\theta }_k^m$ (b) components are plotted as a function of temperature. The static magnetization curve under 1.0 T field is shown as the inset in (a). The simulation $△M/M_0$ (thick line, $△T_c=1.1\mathrm{K}$ and the hole-Mn ratio of 0.06) and experimental values (circles, normalized by static ${\theta }_k$ at 5 K) of the photoenhancement peak $\sim 20\mathrm{K}$ are shown as the inset of (b).

Figure 4

Magnetic field dependence of photoinduced magnetization enhancement $△{\theta }_k^m$ and demagnetization $△{\theta }_k^d$ at 78 K.