Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation

Abstract

We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga 0.965 Mn 0.035 As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~T C /2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We provide a physical interpretation of the observed light-induced collective Mn-spin relaxation and precession.The magnetic semiconductor GaMnAs has received considerable attention in recent years, largely because of its potential role in the development of spin-based devices 1,2 . In this itinerant ferromagnet, the collective magnetic order arises from the interaction between mobile valence band holes and localized Mn spins. Therefore, the magnetic properties are sensitive to external excitations that change the carrier density and distribution. Ultrafast pump-probe magneto-optical spectroscopy is an ideal technique for controlling and characterizing the magnetization dynamics in the magnetic materials, and has been applied to the GaMnAs system by several groups 3,4 .Although optically induced precessional motion of magnetization has been studied in 2 other magnetic systems 5 , magnetization precession in ferromagnetic GaMnAs has been observed only recently 4 and has yet to be adequately understood.In this paper, we report comprehensive temperature and photoexcitation intens ity dependent measurements of photoinduced magnetization precession in Ga 1-x Mn x As (x = 0.035) with no externally imposed magnetic field. By comparing and contrasting the temperature and intensity dependence of the precession frequency, damping, and amplitude, we identify the importance of light-induced nonlinear effects and obtain new information on the relevant physical mechanisms. Our measurements of the photoinduced magnetization show coherent oscillations, arising from the precession of collective Mn spins. Amplitude of the magnetization precession saturates above certain pump intensity is a strong indication that direction of the magnetic easy axis remains unchanged at temperatures above about half the Curie temperature (T C ). The precession is explained by invoking an ultrafast change in the orientation of the in-plane easy axis, due to an impulsive change in the magnetic anisotropy induced by the laser pulse. We also find that the Gilbert damping coefficient, which characterizes the Mn-spin relaxation, depends only weakly on the ambient temperature but changes dramatically with pump intensity. Our results suggest a general model for photoinduced precessional motion and relaxation of magnetization in the GaMnAs system under compressive strain.Time-resolved magneto-optical Kerr effect (MOKE) measurements were performed on a 300 nm thick f...

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] .…”

Coherent control of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As

“…The most intensively studied example of DMSs is (Ga,Mn)As where the ferromagnetic coupling between Mn local-moments is meadiated by spin-polarized valence band holes [5]. In the last few years, not only the ultrafast demagnetization [6] but also the ultrafast enhancement of ferromagnetism [7], the complete reversal of magnetic hysteresis loop [8] and the laser-induced precession of magnetization [9][10][11] were demonstrated in (Ga,Mn)As.Ultrashort laser pulses can be also used for coherent control of the spin precession. In this experiment the sample is excited by pairs of pump pulses.…”