Researchers from Cornell University, together with collaborators from Columbia University and the University of Delaware, have demonstrated excitonic spin torque in the 2D magnetic semiconductor CrSBr. The work shows that excitons generated by light can directly drive and control magnetization dynamics, rather than only probing them, and establishes a new optical pathway to manipulate spins in magnetic semiconductors.
In the study, the team used ultrafast pump-probe measurements on the van der Waals antiferromagnet CrSBr. A short laser pulse creates a reservoir of tightly bound excitons in the material, and this exciton population exerts a spin torque on the underlying antiferromagnetic order. The torque has both damping-like and anti-damping-like components and drives the spins along a non-trivial trajectory on the magnetic energy landscape.
At low excitation densities, the researchers observed nearly harmonic magnon oscillations. At higher excitation, the dynamics became strongly anharmonic, with a sawtooth-like waveform, which the team attributed to repeated energy exchange between the exciton reservoir and the spin system. The excitonic spin torque persists for about 1 ns, well beyond the sub-picosecond duration of the pump pulse, and can repeatedly switch the system between different magnetic configurations, including canted antiferromagnetic, ferromagnetic and switched antiferromagnetic states.
CrSBr is an air-stable layered A-type antiferromagnetic semiconductor that can be exfoliated down to a few atomic layers and that hosts strongly coupled excitons whose properties are sensitive to the magnetic order. Previous work mainly used this coupling to optically “read” spin states or magnons in CrSBr; in the present work, the exciton reservoir plays an active role and reshapes the dissipation landscape that governs the magnon dynamics.
The authors note that excitonic spin torque provides a complementary control mechanism to conventional spin-transfer and spin-orbit torques in metallic heterostructures, as well as to strong-field THz and microwave driving. Because the effect is optical and can be confined to a diffraction-limited spot, it could be useful for future optospintronic devices, spintronic memory elements and quantum transduction schemes that couple spin or magnon degrees of freedom to light. The group also points to potential applications in neuromorphic computing using magnetic systems.
The experiments were carried out in part at the Cornell Nanoscale Facility and the Cornell Center for Materials Research and were supported by the U.S. Department of Energy, NASA, the U.S. Air Force Office of Scientific Research and the National Science Foundation.