Ultrafast laser-induced coherent spin dynamics in ferromagnetic ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{As}/\text{GaAs}$ structures

Ultrafast pump-probe magneto-optical spectroscopy is used to study coherent spin dynamics in ferromagnetic semiconductor ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{As}$ systems at excitation photon energies $E_{\text{ph}}$ both above and below the band gap $E_{\text{g}}$ of GaAs. Above $E_{\text{g}}$, the temporal Kerr rotation signal is found to be strongly dependent on pump photon polarization. This polarization dependence, persisting to room temperature, is attributed to spins of electrons photoexcited to the conduction band, and disappears for $E_{\text{ph}}<E_{\text{g}}$. Below the Curie temperature $T_C$ of the ${\text{Ga}}_{1-\text{x}}{\text{Mn}}_{\text{x}}\text{As}$ samples, the temporal Kerr rotation acquires an additional oscillatory component with a period of the order of 100 ps, attributed to the precession of the ferromagnetically coupled Mn spins. This precession is observed for excitation both above and below $E_{\text{g}}$, regardless of the pump polarization states. The detailed characteristics of this ferromagnetic precession are discussed in terms of the Landau-Lifshitz-Gilbert model. In discussing the observed results, special attention is given to the process of the magnetization precession due to excitation of the pump, to its dependence on the pump intensity, and ambient temperature, and to the relationship between the damping of the magnetization precession and the defects characteristic of ferromagnetic GaMnAs.

Figure 1

Schematic representation of the experimental configuration for time-resolved Kerr rotation measurements.

Figure 2

Time-resolved Kerr rotation data for ferromagnetic samples A and B measured at 10 K and at photon energy 1.57 eV for different pump polarizations. Open circles are for excitation by linearly polarized laser pump pulses.

Figure 3

(a) Temporal Kerr rotation profiles (excluding the oscillatory component) at several ambient temperatures for the annealed sample A; (b) temporal Kerr rotation profiles (excluding the oscillatory component) at two ambient temperatures for the annealed sample A (open circles) and its corresponding as-grown sample (continuous curves).

Figure 4

(a) Temporal Kerr rotation profiles measured at 10 K at photon energies below band gap of GaAs for sample B; (b) temporal Kerr rotation profile at photon energy $E_{\text{ph}}=1.38\text{eV}$ from (a) in the short-time domain. The dotted line is zero time delay where the pump and probe cross-correlation profile peaks. All results were obtained using a constant pump fluence $I_0$.

Figure 5

Precession frequency $\omega$ in sample B as a function of (a) pump intensity at $T_0=10\text{K}$; and (b) ambient temperature at $I=I_0$, respectively. Photon energy dependence of the precession frequency is also shown in (a). Note the difference between the precession frequencies for excitations above and below the band-gap energy.

Figure 6

Amplitude $A$ in sample B as a function of (a) pump intensity at $T_0=10\text{K}$ and $E_{\text{ph}}=1.41\text{eV}$, (b) ambient temperature at $I=I_0$ and $E_{\text{ph}}=1.41\text{eV}$, and (c) photon energy at $T_0=10\text{K}$ and $I=I_0$, respectively.

Figure 7

Gilbert damping coefficient for sample A (annealed) as a function of (a) pump intensity $I$ at $T_0=10\text{K}$, and (b) ambient temperature $T_0$ at $I=I_0$. Here, $E_{\text{ph}}=1.41\text{eV}$.

Figure 8

Gilbert damping coefficient as a function of Mn concentration $x$ for four ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{As}$ samples prepared under the same annealing conditions at $E_{\text{ph}}=1.41\text{eV}$, $T_0=10\text{K}$, and $I=I_0$.

Figure 9

Gilbert damping coefficient for sample B (as-grown) as a function of (a) pump intensity at $T_0=10\text{K}$, and (b) ambient temperature at $I=I_0$. Here, $E_{\text{ph}}=1.41\text{eV}$.