When Youn Jue Bae and her team at Cornell University trained a laser on a flake of chromium sulfide bromide, they weren’t just illuminating a material—they were steering its very magnetism with light. In a breakthrough that redefines the role of excitons, those fleeting electron-hole pairs long used as passive probes in magnetic materials, the researchers discovered these quantum entities can actively push and pull on magnetic spins, opening a new frontier in the control of spin with light. This isn’t just observation; it’s intervention. At the heart of their discovery lies a surprising force: excitonic spin torque.
Published in Nature Materials on June 15, the study reveals that in the two-dimensional magnetic semiconductor CrSBr, light-generated excitons do more than report on magnetic order—they reshape it. “Excitons have been very useful for watching what spins are doing in magnetic materials,” said Bae, assistant professor of chemistry and chemical biology. “What we show here is that excitons can also act back on the spins. They are not just spectators; they can help drive the magnetic motion.” This shift from observer to actor could accelerate the development of spintronic and optospintronic technologies, where information is carried not by electric charge but by the quantum spin of electrons—promising faster, cooler, more efficient computing.
The team, co-led by doctoral candidate Nicholas Brennan and postdoctoral researcher Jiacheng Tang, used a pump-probe technique to track how spins evolved after a laser pulse created an exciton reservoir in CrSBr. At low light levels, the magnetic oscillations were smooth and predictable. But at higher excitation, the waveform turned sharply asymmetric—sawtooth-shaped—a telltale sign of nonlinear torque. Unlike conventional spin torques driven by electric currents in metallic layers, this one emerged from within the material itself, sustained even after the initial laser pulse faded. The excitons, though electrically neutral and once thought incapable of exerting torque, were exchanging energy with the spin system, either damping its motion or injecting energy to amplify it.
CrSBr proved an ideal testbed: air-stable, easily exfoliated into atomically thin sheets, and naturally hosting both magnetic order and robust excitons. These properties make it a powerful platform for exploring the interplay of light, charge, and spin in two-dimensional materials. The researchers observed spins flipping between canted antiferromagnetic, ferromagnetic, and switched antiferromagnetic states—demonstrating precise, light-driven control over magnetic configurations.
Beyond faster memory devices, this work hints at applications in brain-inspired computing, where nonlinear, adaptive responses are essential. By turning excitons into active control knobs, Bae’s team has illuminated not just a new physical phenomenon, but a path toward computing that mimics the complexity and efficiency of the human brain.