Experimental observation of the optical spin transfer torque

TL;DR

This paper reports the first experimental observation of optical spin transfer torque in a ferromagnetic semiconductor, demonstrating ultrafast magnetization control via circularly polarized laser pulses, bridging optical and spintronic research fields.

Contribution

It provides the first experimental evidence of optical spin transfer torque, connecting optical excitation with spin transfer phenomena in magnetic materials.

Findings

Observation of coherent spin precession induced by circularly polarized light.

Quantitative microscopic interpretation of optical spin transfer torque.

Demonstration of ultrafast magnetization dynamics at short time scales.

Abstract

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.