Femtosecond Coherent Control of Spins in (Ga,Mn)As Ferromagnetic Semiconductors Using Light

Abstract

Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near-ultraviolet (∼3eV), coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the {111} equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. We predict nonthermal magnetization control by tuning the laser frequency and polarization direction. Our mechanism explains recent ultrafast pump-probe experiments.PACS numbers: 78.47. 78.20.Ls, 78.47.Fg, 42.50.Md Long range magnetic order arises from the interactions between itinerant and localized spins in a wide variety of systems, such as EuO, EuS, chrome spinels, pyrochlore, manganese oxides, or (III,Mn)V ferromagnetic semiconductors [1,2]. With ferromagnetic semiconductors one can envision multifunctional devices combining information processing and storage on a single chip with low power consumption. Fast spin manipulation is of great importance for such spin-electronic, spin-photonic, magnetic storage, and quantum computation applications.One of the challenges facing magnetic devices concerns their speed. The magnetic properties of carrier-induced ferromagnets respond strongly to carrier density tuning via light, electrical gates, or current [3]. While magnetic field pulses and spin currents can be used to manipulate spin on the many-picosecond time scale, femtosecond spin manipulation requires the use of laser pulses [4,5]. In ultrafast pump-probe magneto-optical spectroscopy, the pump optical pulse excites e-h coherences and corresponding carrier populations, whose subsequent interactions trigger a magnetization dynamics monitored as function of time via the Faraday or Kerr rotation [6].The physical processes leading to femtosecond magnetization dynamics (femto-magnetism) are under debate. Open questions include the possibility of direct photonspin coupling, the distinction of coherent and incoherent effects, and the exact role of the spin-orbit interaction. Following the pioneering work of Ref. [7], many ultrafast spectroscopy experiments were interpreted in terms of a decrease in the magnetization amplitude due to transient thermal effects [7,8]. Observations of lightinduced changes in the magnetization orientation were also mostly attributed to the temperature elevation, which leads to transient changes in the magnetic easy axes [9,10,11]. Most desirable is nonthermal magnetization control within the femtosecond coherent [12] temporal regime, which promises more flexibility limited only by the optical pulse duration. Experiments in ferrimagnetic garnets were interpreted in terms of an interplay between the inverse Faraday effect [13] and long-lived changes in the magneto-crystalline anisotropy [4]. In (Ga,Mn)As, Ref.[14] reported magnetization precession triggered by changes of magnetic anisotropy on a ∼100ps time scale due to carrier relaxation, while Ref.[15] demonstrated coherent ...

Manipulation of ferromagnets via the spin-selective optical Stark effect

“…[23][24][25] Photoinjected carriers induced by linearly polarized light with frequency slightly above the or L band edges have been shown to induce magnetization dynamics in GaMnAs. 23,26 In contrast, we focus here on excitation with frequencies below the fundamental band gap, which is dissipationless, since no free carriers are excited. Our approach is quite general and can be applied to arbitrary electronic structures and computed from first principles.…”

Influence of nonmagnetic impurity scattering on spin dynamics in diluted magnetic semiconductors

“…For this purpose, spintronic devices based on semiconductors are preferable to metallic structures since the dephasing time in a metal is about three orders of magnitude shorter than in a semiconductor 4 . In the context of semiconductor spintronics [5][6][7] , a particularly interesting class of materials for future applications are diluted magnetic semiconductors (DMS) [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] , which are obtained when semiconductors are doped with transition metal elements, such as Mn, which act as localized magnetic moments. While some types of DMS, such as Ga 1−x Mn x As, exhibit a ferromagnetic phase 8,23 , other types of DMS, like the usually paramagnetic CdMnTe, are especially valued for the enhancement of the effective carrier g-factor by the giant Zeeman effect that can be used, e.g., to facilitate an injection of a spin-polarized current into a light-emitting diode 24 .…”

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

“…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] .…”