Ultrafast laser-induced coherent spin dynamics in ferromagneticGa1−xMnxAs/GaAsstructures

Abstract

Ultrafast pump-probe magneto-optical spectroscopy is used to study coherent spin dynamics in ferromagnetic semiconductor Ga 1−x Mn x As systems at excitation photon energies E ph both above and below the band gap E g of GaAs. Above E 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 ph Ͻ E g . Below the Curie temperature T C of the Ga 1−x Mn x 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 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.

The essential role of carefully optimized synthesis for elucidating intrinsic material properties of (Ga,Mn)As

“…Hand-in-hand with the optimization of the material synthesis, we have developed experimental capabilities based on the MO pump-and-probe method that allows us to simultaneously determine the magnetic anisotropy, Gilbert damping and spin stiffness constants from one consistent set of measured data. The possibility to excite and detect precession of ferromagnetic Mn moments in (Ga,Mn)As by this method has been extensively discussed in previous MO studies (see Methods and Supplementary Notes 6-9 for the detailed description of the method) [30][31][32][33][34][35][36][37][38][39][40][41] . All experiments presented below were performed at 15 K. The anisotropy constants, shown in Fig.…”

Ultrafast probes of nonequilibrium hole spin relaxation in the ferromagnetic semiconductor GaMnAs

“…5(a). Here the main feature is the oscillation of the Kerr signal caused by the oscillatory component of magnetization M Z due to the collective precession of the magnetization around its inplane equilibrium orientation at a frequency of ∼4.2 GHz (see inset), as discussed in the literature [22][23][24][25][26][27][28] . The observed precession frequency in the low field limit is well described analytically by the formula,…”

“…This includes femtosecond Mn spin canting induced by spin-orbit torques 14,15 via photoexcited non-thermal "transverse" hole spins involving the interplay between spin-orbit and magnetic exchange interaction 16,17 ; femtosecond demagnetization (i.e. decrease of Mn spin amplitude) via dynamical polarization of "longitudinal" holes spins [18][19][20] ; picosecond photoinduced ferromagnetism 21 ; and magnetization precession [22][23][24][25][26][27][28] .…”

Collective magnetization dynamics in ferromagnetic (Ga,Mn)As mediated by photoexcited carriers

“…However, in earlier studies different conclusions were reported regarding the dominant effect responsible for the transient modulation of magnetic anisotropy that triggers the observed precession of magnetization. 8,14,25 Although the previous wavelength dependent studies of the photoinduced magnetization precession suggested a difference between the precession frequencies for excitations above and below the band gap energy 1,4,26 , there is a lack of systematic spectral investigation of the competing influence of the transient increase of local lattice temperature and hole concentration on precession frequency 15 . Under the description of mean-field theory, pump intensity and ambient temperature dependent experimental studies of magnetization precession in (Ga,Mn)As show that laser heating induced by the optical pumping primarily contributes to changes of magnetic anisotropy 26,27 ,while Hashimoto et al concluded that non-thermal increases in photo-excited hole concentration is responsible for such changes.…”