All-optical four-state magnetization reversal in (Ga,Mn)As ferromagnetic semiconductors

Abstract

Using density matrix equations of motion and a tight-binding band calculation, we predict alloptical switching between four metastable magnetic states of (III,Mn)As ferromagnets. This switching is initiated non-thermally within 100fs, during nonlinear coherent photoexcitation. For a single optical pulse, magnetization reversal is completed after ∼100 ps and controlled by the coherent femtosecond photoexcitation. Our predicted switching comes from magnetic nonlinearities triggered by a femtosecond magnetization tilt that is sensitive to un-adiabatic light-induced spin interactions. The goal of THz magnetic switches underlies the entire field of spin-electronics and challenges our understanding of fundamental non-equilibrium spin processes. The reading and writing of bits rely on reversing the magnetization direction between "up" and "down". In conventional switching, the magnetization moves out of equilibrium via laser heating. A magnetic field then exerts a torque that reverses the magnetization within few ns [1,2]. This speed can be improved by using coherent spin rotation, via precession of the entire memory cell around a magnetic field pulse-precessional/ballistic switching [1][2][3][4]. This pulsed field must have duration of at least half the precession period (100's of ps), which sets a fundamental limit of the magnetization reversal time. This speed is further limited by randomness [4] or weak precession damping allowing back-switching of magnetic elements ("ringing") [2,3]. Faster switching, within 100fs, could be explored by using laser pulses to inject spin-polarized carriers [5]. This is important for meeting the demand for improved read/write speeds, bit density, and reliability of current magnetic devices.Magnetic properties of (III,Mn)V ferromagentic semiconductors exhibit sensitive response to carrier density tuning via light, electrical gates, or spin currents. This holds promise for high-speed magnetic switches that combine information processing and storage on a single chip device with low power consumption [6]. The femtosecond photoexcitation of GaMnAs revealed distinct transient magneto-optical responses: (i) ultrafast decrease of the magnetization amplitude [7] within ∼ 100fs (demagnetization) [8][9][10], (ii) enhancement of magnetic order on ps timescale [11], (iii) magnetization re-orientation within ∼100fs, followed by a distinct ps regime of coherent precession [12,13]. Such non-equilibrium magnetic effects appear to be universal [7,14,15]. The pioneering work of Bigot and collaborators [7], who observed demagnetization on a ∼100fs timescale much shorter than the spin-phonon relaxation time, thus evolved into a new field of femto-magnetism. However, the many-body theory of femto-magnetism remains controversial [5,15] and must ultimately engage the elements of transient coherence, correlation, and nonlinearity on an equal footing.Here we present such a mean field theory and propose a nonthermal mechanism for achieving ultrafast all-optical magnetization switching in ferromagnetic (Ga,Mn)A...

Manipulating femtosecond spin-orbit torques with laser pulse sequences to control magnetic memory states and ringing

“…1,3,4,17,40,41,49,50 This work contributes to the debate of how fs coherent photoexcitation could drive and control ultrafast switchings 1,12 and magnetic ringing. 46 We consider the very early non-thermal and coherent temporal regimes and focus mostly on magnetization changes during the fs laser pulse.…”

Modeling of coherent ultrafast magneto-optical experiments: Light-induced molecular mean-field model

“…45 One may also mention the theoretical studies devoted to the coherent photo-excitation of (Ga,Mn)As ferromagnetic semiconductors using linearly polarized light where the interplay between the optical nonlinearities, the spin-orbit interaction and the resulting optically induced magnetic anisotropy were treated microscopically. [47][48][49] Let us also underline that we have worked with only one mode x of the electromagnetic field without considering the spectral distribution Dx of the laser pulse. Similarly, our description of the pump-probe configuration is clearly less developed than the reference work of Ref.…”

Dilute ferromagnetic semiconductors: Physics and spintronic structures

“…A subsequent theoretical work proposed a protocol for all-optical switching between four metastable magnetic states in DFSs (Kapetanakis et al, 2011).…”