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

Spin polarized carriers electrically injected into a magnet from an external polarizer can exert a spin transfer torque (STT) 1 on the magnetization. The phenomenon belongs to the area of spintronics research focusing on manipulating magnetic moments by electric fields and is the basis of the emerging technologies for scalable magnetoresistive random access memories.2 In our previous work we have reported experimental observation 3 of the optical counterpart of STT 4,5 in which a circularly polarized pump laser pulse acts as the external polarizer, allowing to study and utilize the phenomenon on several orders of magnitude shorter timescales than in the electric current induced STT. Recently it has been theoretically proposed [6][7][8] and experimentally demonstrated 9-11 that in the absence of an external polarizer, carriers in a magnet under applied electric field can develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) acting on the magnetization. In this paper we report the observation of the optical counterpart of SOT. At picosecond time-scales, we detect excitations of magnetization of a ferromagnetic semiconductor (Ga,Mn)As which are independent of the polarization of the pump laser pulses and are induced by non-equilibrium spin-orbit coupled photo-holes.In current induced STT, spin-polarized carriers are electrically injected into a magnetic object, such as thin ferromagnetic layer or domain wall, from another part of a non-uniform magnetic structure. The physical origin of STT is in the angular momentum transfer from the injected carrier spins to the magnetic moments. The current induced SOT, on the other hand, is observed in uniform magnets with no external source of spin polarized carriers. The non-equilibrium spin polarization of carriers producing SOT results from current induced redistribution of carrier states in the band structure of the magnet. The physical origin of SOT is the spin-orbit coupling in the carrier bands. While the seminal works on current induced STT are more than 15 years old 12,13 and the effect already plays a key role in commercially developed spintronic technologies, the research of the relativistic SOT is still at its infancy. Yet, the remarkable property of this inverse magneto-transport effect, allowing a single piece of magnet to excite itself under applied electric field, has already found practical utility. For example, when combined with the self-detection of the magnetization variations by anisotropic magnetoresistance, which is a direct magneto-transport effect based also on 2 spin-orbit coupling, an all-electric ferromagnetic resonance measurement of micromagnetic parameters can be performed on a single ferromagnetic nanostructure. 11The aim of our works reported in Ref. In the optical spin transfer torque (OSTT), observed in our previous experiments inRef. 3, the external source for injecting spin polarized carriers is provided by circularly polarized light at normal incide...

(Ga,Mn)As is at the forefront of spintronics research exploring the synergy of ferromagnetism with the physics and the technology of semiconductors. However, the electronic structure of this model spintronics material has been debated and the systematic and reproducible control of the basic micromagnetic parameters and semiconducting doping trends has not been established. Here we show that seemingly small departures from the individually optimized synthesis protocols yield non-systematic doping trends, extrinsic charge and moment compensation, and inhomogeneities that conceal intrinsic properties of (Ga,Mn)As. On the other hand, we demonstrate reproducible, well controlled and microscopically understood semiconducting doping trends and micromagnetic parameters in our series of carefully optimized epilayers. Hand-in-hand with the optimization of the material synthesis, we have developed experimental capabilities based on the magneto-optical pumpand-probe method that allowed us to simultaneously determine the magnetic anisotropy, Gilbert damping and spin stiffness constants from one consistent set of measured data.

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

Recent breakthroughs in electrical detection and manipulation of antiferromagnets have opened a new avenue in the research of non-volatile spintronic devices. 1-10 Antiparallel spin sublattices in antiferromagnets, producing zero dipolar fields, lead to the insensitivity to magnetic field perturbations, multi-level stability, ultrafast spin dynamics and other favorable characteristics which may find utility in fields ranging from magnetic memories to optical signal processing. However, the absence of a net magnetic moment and the ultra-short magnetization dynamics timescales make antiferromagnets notoriously difficult to study by common magnetometers or magnetic resonance techniques. In this paper we demonstrate the experimental determination of the Néel vector in a thin film of antiferromagnetic CuMnAs 9,10 which is the prominent material used in the first realization of antiferromagnetic memory chips. 10 We employ a femtosecond pump-probe magneto-optical experiment based on magnetic linear dichroism. This table-top optical method is considerably more accessible than the traditionally employed large scale facility techniques like neutron diffraction 11 and Xray magnetic dichroism measurements. [12][13][14] This optical technique allows an unambiguous direct determination of the Néel vector orientation in thin antiferromagnetic films utilized in devices directly from measured data without fitting to a theoretical model.Well-established optical methods 15 enable to study magnetic materials with a high spatial-resolution 16 on short time-scales. 17 In particular, Kerr and Faraday magneto-optical (MO) effects, which are linear in magnetization, are frequently used for the characterization of ferromagnets (FMs). [15][16][17] For antiferromagnets (AFs), the utilization of MO techniques is much more challenging. Several time-resolved studies have been performed on canted antiferromagnets [18][19][20] where the Dzyaloshinskii-Moriya interaction induces a canting of the two AF spin-sublattices with an angle of about 1° which leads to a small net magnetization. These canted AFs are, therefore, much easier for the experimental study because despite their antiferromagnetic ordering it is still possible to influence the spins with relatively weak magnetic fields and, moreover, Kerr and Faraday effects can be used for the characterization of their magnetic ordering. For fully compensated AFs the signals from oppositely oriented magnetic sublattices cancel for MO effects linear (i.e. odd) in magnetization which leaves * Electronic mail: nemec@karlov.mff.cuni.cz 2 only MO effects quadratic (even) in magnetization as suitable probes for these materials. 14 Quadratic MO effects have been reported for many magnetic materials. [15][16][17]21 However, practical utilization of quadratic MO effects for the characterization of magnetic materials is much less common than utilization of linear MO effects. 14,22,23 One reason for this is that quadratic MO effects are typically much weaker than linear MO effects. 14 Moreover, the experim...