The spin transfer torque is a phenomenon in which angular momentum of a spin polarized electrical current entering a ferromagnet is transferred to the magnetization. The effect has opened a new research field of electrically driven magnetization dynamics in magnetic nanostructures and plays an important role in the development of a new generation of memory devices and tunable oscillators. Optical excitations of magnetic systems by laser pulses have been a separate research field whose aim is to explore magnetization dynamics at short time scales and enable ultrafast spintronic devices. We report the experimental observation of the optical spin transfer torque, predicted theoretically several years ago building the bridge between these two fields of spintronics research. In a pump-and-probe optical experiment we measure coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by circularly polarized laser pulses. During the pump pulse, the spin angular momentum of photo-carriers generated by the absorbed light is transferred to the collective magnetization of the ferromagnet. We interpret the observed optical spin transfer torque and the magnetization precession it triggers on a quantitative microscopic level. Bringing the spin transfer physics into optics introduces a fundamentally distinct mechanism from the previously reported thermal and non-thermal laser excitations of magnets. Bringing optics into the field of spin transfer torques decreases by several orders of magnitude the timescales at which these phenomena are explored and utilized.Comment: 11 pages, 4 figure

We observe a singularity in the temperature derivative drho/dT of resistivity at the Curie point of high-quality (Ga,Mn)As ferromagnetic semiconductors with Tc's ranging from approximately 80 to 185 K. The character of the anomaly is sharply distinct from the critical contribution to transport in conventional dense-moment magnetic semiconductors and is reminiscent of the drho/dT singularity in transition metal ferromagnets. Within the critical region accessible in our experiments, the temperature dependence on the ferromagnetic side can be explained by dominant scattering from uncorrelated spin fluctuations. The singular behavior of drho/dT on the paramagnetic side points to the important role of short-range correlated spin fluctuations.

We report dynamics of the transient polar Kerr rotation (KR) and of the transient reflectivity induced by femtosecond laser pulses in ferromagnetic (Ga,Mn)As with no external magnetic field applied. It is shown that the measured KR signal consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in (Ga,Mn)As. The origin of the light-induced magnetization precession is discussed and the magnetization precession damping (Gilbert damping) is found to be strongly influenced by annealing of the sample.Comment: 6 pages, 4 figures. accepted in Applied Physics Letter

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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

We employ Faraday and Kerr effect spectroscopy in the infrared range to investigate the electronic structure of Ga_{1-x}Mn_{x}As near the Fermi energy. The band structure of this archetypical dilute-moment ferromagnetic semiconductor has been a matter of controversy, fueled partly by previous measurements of the unpolarized infrared absorption and their phenomenological impurity-band interpretation. Unlike the unpolarized absorption, the infrared magneto-optical effects we study are intimately related to ferromagnetism, and their interpretation is much more microscopically constrained in terms of the orbital character of the relevant band states. We show that the conventional theory of the disordered valence band with an antiferromatnetic exchange term accounts semiquantitatively for the overall characteristics of the measured infrared magneto-optical spectra.

The nonlinear magnetoresistance of the two-dimensional electron gas measured in GaAs/ Al~aaq As heterostructures is attributed to the breakdown of edge-state electrons into the partly filled Landau level at the bulk part of the sample. Based on a simplified model, the transition between edge and bulk states is described with use of the electron mean free path at the edge channels. It was found that the mean free path scales exponentially with the product of the Hall voltage and the number of edge channels.High-mobility samples of GaAs/Al Gaq As heterostructures generally exhibit the strongly asymmetric line shapes of Shubnikovde Haas (SdH) oscillations in p(B) at high magnetic fields. Published data on this effect show a reduced resistivity (or conductivity) on the low-energy (i.e., high-magnetic-field) side of the peaks.~S uch an asymmetry can be suppressed or even reversed by several factors, including an increase of the width of the conducting channel in very narrow samples, enhancement of the measuring current and/or temperature, and the application of a backgate voltage to the structure. 2 It seems to be evident from the published data that there are two distinct types of asymmetry observed. The first of them concerns the form of individual subpeaks corresponding to one particular orientation of the spin within a Landau level. In extreme conditions (very narrow samples, low current density and temperature) these spin-resolved peaks take a sawtooth form. Several alternative explanations of this asymmetry have been suggested s depending on the particular conditions of the experiment and we will not deal with this effect here.The second type of asyrrimetry concerns the line shape of the double peak connected with one Landau level that consists of two more-or-less overlapping spin-resolved subpeaks. It has been observed that the relative heights of the two subpeaks depend strongly on the measuring current density j (Ref. 2) due to their widely different current dependence. This is the effect we want to address in this paper.In Fig. 1 we present the p(B) dependence for three different values of the measuring current I. The data have been taken at T = 1.3 K on a GaAs/Al Gaq As heterostructure in the form of a common Hall bar with the overall width of' the conducting channel n = 400 pm and the distance between voltage contacts along the sample I, = 1100 pm. The basic parameters of the 1.2 0. 8-C: 4g 0. 6-0. 4-2T l t I s IJ& 1«& iliad(I 10 12 B (T) FIG. 1. Dependence of the Shubnikovde Haas oscillations at T = 1.3 K on the measuring dc current I: I=0.7 pA (full line), I= 7 pA (dashed line), and I= 70 pA (dotted line), respectively.two-dimensional electron gas (2DEG) in the samplethe carrier density and mobilityhave been found to be n, = 5.5 x 10t~c m~and p = 3.04 x 10s cm2/V s, respectively. The most striking effect is a remarkable increase with I of the peak value for the spin-up subpeak (as compared to the spin-down one) for all Landau levels where the spin splitting has been observed. It is worth noting that with...

We have found that Fermi contours of a two-dimensional electron gas at GaAs/AlxGa1−xAs interface deviate from a standard circular shape under the combined influence of an approximately triangular confining potential and the strong in-plane magnetic field. The distortion of a Fermi contour manifests itself through an increase of the electron effective cyclotron mass which has been measured by the cyclotron resonance in the far-infrared transmission spectra and by the thermal damping of Shubnikov-de Haas oscillations in tilted magnetic fields with an in-plane component up to 5 T. The observed increase of the cyclotron effective mass reaches almost 5 % of its zero field value which is in good agreement with results of a self-consistent calculation. 72.40, 72.20