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

Abstract

We present a study of photo-excited magnetization dynamics in ferromagnetic (Ga,Mn)As films observed by time-resolved magneto-optical measurements. The magnetization precession triggered by linearly polarized optical pulses in the absence of an external field shows a strong dependence on photon frequency when the photo-excitation energy approaches the band-edge of (Ga,Mn)As. This can be understood in terms of magnetic anisotropy modulation by both laser heating of the sample and by hole-induced non-thermal paths. Our findings provide a means for identifying the transition of laser-triggered magnetization dynamics from thermal to non-thermal mechanisms, a result that is of importance for ultrafast optical spin manipulation in ferromagnetic materials via non-thermal paths.

Ultrafast Modulation of Magnetization Dynamics in Ferromagnetic (Ga, Mn)As Thin Films

“…Figure 1 shows a typical magnetization precession response to such a pulse at 10 K for the annealed (Ga,Mn)As sample excited by a linearly-polarized pump beam at 1.58 eV. As shown in Figure 1a, the temporal evolution of the photoexcited magnetization observed by time-resolved magneto-optical measurement in ferromagnetic (Ga,Mn)As can be fitted by an exponentially damped sine function superimposed on a pulse-like function [7,21],…”

“…Earlier studies reported that the pulse-like signal which persists above the Curie temperature is related to the recovery process of nonequilibrium electron-hole pairs in the GaAs substrate [10,28], and only the oscillatory part is directly related to the ferromagnetic order of (Ga,Mn)As [12]. The numerical fitting result by the Landau-Lifshitz-Gilbert (LLG) equation shown in Figure 1b is focused on the precession part of the observed Kerr rotation, which reflects the uniform precession of magnetization in the annealed (Ga,Mn)As film [21]. The collective spin excitation and modulation of the dynamics of precession discussed below will mainly focus on the characteristics of the oscillatory signal modulated by laser excitation.…”

“…The collective spin excitation and modulation of the dynamics of precession discussed below will mainly focus on the characteristics of the oscillatory signal modulated by laser excitation. The fitting is focused on the oscillatory part of the Kerr rotation, which represents a uniform precession of magnetization in the (Ga,Mn)As film [21].…”

“…Ultrafast magneto-optical investigation of collective spin excitations in ferromagnetic (Ga,Mn)As thin films has demonstrated the possible multi-functionality of ferromagnetic semiconductors when incorporated in spintronic devices [4,[7][8][9]. Transient modulation involving either thermal or non-thermal mechanisms investigated by laser excitation has been shown to contribute to ultrafast coherent precession of magnetization [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Modulation of magnetic anisotropy stimulated by transient increases of either local temperature or photocarrier density in (Ga,Mn)As has demonstrated multiple possibilities of modulating the precession of magnetization by linearly polarized pump pulses.…”

“…Photoionization-like excitation of Mn 2+ has been recently proposed to account for the photoinduced modulation of perpendicular effective field [23]. Intrinsically, one or more of such mechanisms could simultaneously trigger a collective precession of magnetization [4,7,14,21], and they would compete with each other as to how they contribute to the precession process. On the other hand, they may also compensate each other in their contributions to data storage in the magnetic recording.…”