Spintronics is a new dynamic branch of electronics that uses magnetic moment – spin – of charge carriers for data storage and processing. Spintronic concepts have been studied extensively since early eighties and the first components already found their place in commercial applications (read heads of hard disk drives and magnetic random access memories). Recently, utilization of light in spintronics has established its new “optospintronic” branch. Using the interaction of light with spin polarization provides a direct non-invasive method that can be used to modify and to study magnetic order in materials on ultrashort time scales (down to femtoseconds), that is by several orders of magnitude faster than other existing experimental techniques. Our research in the Laboratory of OptoSpintronic (LOS) is focused on the investigation of mechanisms that can be used for a re-orientation of magnetic moments in ferromagnetic and antiferromagnetic materials on ultra-short time scales using laser pulses.



Experimental technique

An impact of the femtosecond laser pump pulse on magnetically-ordered material induces changes of the magnetic ordering, which are detected by measuring a rotation of the polarization plane of reflected (or transmitted) linearly polarized probe pulses as a function of the time delay ∆t between pump and probe pulses. The orientation of magnetic moments in the sample is described by the polar angle j and azimuthal angle q. The external magnetic field Hext is applied in the sample plane at an angle jH.




Investigated materials

Time-resolved laser spectroscopy was used to study various magnetically-ordered (both ferromagnetically and antiferromagnetically) and non-magnetic (paramagnetic) materials.


1) Antiferromagnetic metal CuMnAs

Control and detection of spin order in ferromagnets is the main principle allowing storing and reading of magnetic information in nowadays technology. The large class of antiferromagnets, on the other hand, is less utilized, despite its very appealing features for spintronics applications. For instance, the absence of net magnetization and stray fields eliminates crosstalk between neighbouring devices and the absence of a primary macroscopic magnetization makes spin manipulation in antiferromagnets inherently faster than in ferromagnets. However, control of spins in antiferromagnets requires exceedingly high magnetic fields, and antiferromagnetic order cannot be detected with conventional magnetometry. Nevertheless, it is possible to use light for probing and modification of the magnetic order in antiferromagnets [see arXiv: 1705.10600].

We developed an optical technique which enables the experimental determination of the Néel vector orientation in thin antiferromagnetic metal films directly from the measured data without fitting to a theoretical model [see Nature Photonics 11, 91-97 (2017)]. This technique, which is based on a femtosecond pump-probe experiment using magneto-optical Voigt effect, was demonstrated in a thin film of antiferromagnetic metal CuMnAs which is the prominent material used in the first realization of antiferromagnetic memory chips.

2) Paramagnetic semiconductor GaAs

The long-range and high-speed transport of spin-polarized carriers is one of the key prerequisites of efficient spintronic logic devices. To achieve this, we used an optical excitation of single undoped GaAs/AlGaAs interface in order to create a long-lived conductive layer where the mobility is not reduced by the scattering on dopants. To demonstrate that such a structure can perform superior spin transport characteristics, we employed time and spatially-resolved pump-and-probe experiment. These experiments showed that in this structure the electronic spins can be transported over distances longer than 10 um in times as short as nanoseconds [see Scientific Reports 6, 22901 (2016)].



3) Ferromagnetic semiconductor GaMnAs

(Ga,Mn)As is very interesting material for observing optical spin-torque phenomena due to the direct-gap GaAs host and the strong exchange interaction between the carrier spins and the ferromagnetic Mn ions. However, a reproducible control of the (Ga,Mn)As growth and an accurate determination of its micromagnetic parameters (such as Gilbert damping and spin stiffness) is a rather challenging task that we accomplished only recently [see Nature Commun. 4, 1422 (2013)].




Investigation of high-quality (Ga,Mn)As epilayers using a high-precision magneto-optical experiments enabled us to identify two new phenomena – Optical Spin Transfer and Optical Spin-Orbit Torques – that open access to the re-orientation of magnetization on sub-picosecond time scales.


Optical Spin Transfer Torque

[Nature Physics 8, 411 (2012)]

The Optical Spin Transfer Torque (OSTT) represents a non-relativistic phenomenon where the angular momentum (spin) of the photo-injected electrons is transferred to the magnetization in a ferromagnet. The coupled precession dynamics of the magnetization and the photocarrier (electron) spin results in a torque on the magnetization, which is perpendicular both to the magnetization and to the sample plane, with a direction given by the light helicity.

We track the magnetization dynamics by time-resolved magneto-optical (MO) measurements that are based on the sensitivity of the reflected light polarization to the magnetization direction. The magnetization precession triggered by the OSST displays as an oscillatory MO signal where the phase of oscillations is given by the helicity of the circularly polarized pump pulses. The recorded MO signals can be modeled by a Landau-Lifshitz-Gilbert equation (LLG) that enables to reconstruct a three dimensional magnetization trajectory in time, which confirms the sub-picosecond duration of the laser-induced magnetization tilt.



OSTT can be also used to efficiently manipulate magnetic domains and domain walls (DWs), as we have shown in Nature Communications 8, 15226 (2017). If the magnetization in the ferromagnet is oriented perpendicularly to its surface, the torque induced by the OSTT is zero for perpendicular incidence of the circularly polarized light. However, at the interface between two domains with the opposite magnetization orientation (i.e., within the DW), magnetization rotates continuously from one perpendicular orientation to the opposite, and therefore has a non-zero component parallel to the surface of the sample. Here the OSTT acts with a nonzero torque that leads to the movement of the whole DW, whose direction is controlled by the helicity of the circularly polarized light. The measurement also showed that DW can be understood as a quasi-particle that moves with inertia even after the femtosecond laser pulse has faded away.


Optical Spin-Orbit Torque

[Nat. Photonics, DOI: 10.1038/NPHOTON.2013.76]

The Optical Spin-Orbit Torque (OSOT) is a relativistic phenomenon which occurs in an absence of any external spin polarizer. The OSOT effect originates solely from the spin-orbit coupling of non-equilibrium photocarriers (holes) excited by laser pulses – of any polarization – in the ferromagnet. The coupled precession dynamics leads again to a torque on the magnetization and, consequently, to a magnetization precession. However, the main difficulty in this experiment lies in a separation of the OSOT-related magnetization dynamics from that due to a thermal mechanism (i.e., due to a laser-induced heating of the material).



In order to achieve this goal, we developed a new experimental technique that enables a quantitative reconstruction of the three-dimensional magnetization trajectory in GaMnAs from the measured magneto-optical (MO) data without any numerical modeling [see Appl. Phys. Lett. 100, 102403 (2012)]. Using this technique, we identified experimental conditions when the OSOT-related signal dominates in the measured MO data. For example, for low excitation intensities a slow onset of the thermally-induced precession dominates while for higher intensities an abrupt tilt of the magnetization due to OSOT also occurs. Moreover, the initial OSOT-induced tilt of magnetization can yield precession angles that are inaccessible by the thermally-induced magnetization dynamics.



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