Fyzikální ústav Akademie věd ČR

A direct transfer of angular momentum from a circularly polarized light to spins allows to excite a magnet from its equilibrium state at sub-picosecond time scales. The discovery, allowing to manipulate spins in a magnet by short laser pulses, was reported by scientists from the joint Laboratory of Opto-Spintronics at the Faculty of Mathematics and Physics, Charles University and the Institute of Physics, Academy of Sciences of the Czech Republic. The work was published on April 1st 2012 in the journal Nature Physics (DOI: 10.1038/ NPHYS2279).

Angular momentum transfer from a spin-polarized electrical current to magnetization in a ferromagnet is the so called spin-transfer-torque phenomenon. It allows, for example, to write information in the latest generation of magnetic random access memories whose development is expected to result in the construction of instant on-and-off computers. The timescales for exciting magnetization by the current induced spin-transfer-torque are nanoseconds. Physicists from the joint Laboratory of Opto-Spintronics observed an optical variant of the phenomenon, the so called optical spin-transfer-torque, in which magnetization is excited in a magnetic semiconductor by polarized photo-carriers at timescales which are several orders of magnitude shorter. The material for the experiment was a GaAs semiconductor doped with manganese, which was prepared by atomic layer-by-layer growth in the Institute of Physics of the Academy of Sciences. Femtosecond pump-and-probe technique for optically exciting and detecting magnetization dynamics was employed by scientists at the Faculty of Mathematics and Physics of the Charles University.

„Our work combines the photo-effect, a phenomenon which is at the very heart of semiconductor opto-electronics, with the spin-transfer-torque which is the key phenomenon for spintronics and magnetic memories. We have, therefore, built a new bridge between these two modern fields of research in microelectronics“, say Petr Němec and Tomáš Jungwirth from the join Laboratory of Opto-Spintronics.

Figure shows the physical principle of the optical spin-transfer-torque and the experimental observation of the optical excitation of the ferromagnetic semiconductor by short, circularly-polarized laser pulses allowed by the new phenomenon.

For detailed information contact Tomáš Jungwirth from the Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 162 53 Praha 6.