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

Abstract

We report single-color, time resolved magneto-optical measurements in ferromagnetic semiconductor (Ga,Mn)As. We demonstrate coherent optical control of the magnetization precession by applying two successive ultrashort laser pulses. The magnetic field and temperature dependent experiments reveal the collective Mnmoment nature of the oscillatory part of the time-dependent Kerr rotation, as well as contributions to the magneto-optical signal that are not connected with the magnetization dynamics.The lasting demand for increased speed of writing and retrieving magnetically stored information in computer hard drives stimulated an intense research of ultrafast magnetization dynamics. In particular, the ultrafast control of magnetization by laser pulses has gained a significant attention recently [1]. Initially, the research was focused mainly on ferromagnetic metals and half-metals where the impact of ultrashort (subpicosecond) laser pulses can lead to demagnetization [2], magnetization rotation [3] or even to modification of magnetic structure [4]. More recently, also the investigation of diluted magnetic semiconductors (DMSs) has started. 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. Each pump pulse generates a transient precession of magnetization vector and their temporal separation (i.e., the mutual phase difference between the corresponding magnetization precessions) determines if they superimpose constructively or destructively. Coherent control of magnetization was demonstrated in various types of magnetic materials -ferrimagnetic garnet [1], antiferromagnetic orthoferrites [1], half-metalic ferromagnetic CrO 2 [12], and paramagnetic (Cd,Mn)Te [13].In this paper, we report on the coherent control of magnetization precession in ferromagnetic (Ga,Mn)As. The experiments were performed on an annealed 50 nm thick ferromagnetic (Ga,Mn)As film with nominal Mn doping of 7% and the Curie temperature T C ≈ 160 K, which was grown by the low temperature molecular beam epitaxy (LT-MBE) on a GaAs(001) substrate. The sample exhibits in-plane easy axis behavior typical for stressed (Ga,Mn)As layers grown on GaAs substrates. The external magnetic field (generated by an a) Electronic mail: nemec@karlov.mff.cuni.cz 1

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] .…”

Ultrafast Coherent Control of Spin Precession Motion by Terahertz Magnetic Pulses

“…Ultrafast optical coherent control of the spin degrees of freedom is currently receiving intense attention [1][2][3][4][5][6][7][8][9]. Preceding reports on demonstrations of coherently controlling the spin systems have been conducted using indirect excitations of the spins that occur as a result of dielectric interaction with optical pulses such as electronic excitations and inverse Faraday effect [3][4][5][6][7][8][9].…”