Scientists can learn a lot about a quantum material by watching how it responds to light. In magnetic semiconductors, one especially useful messenger is the exciton: a pairing of a negatively charged electron and the positively charged “hole” it leaves behind.
Until now, excitons in magnetic materials have mostly been used as reporters. They could reveal how spins were arranged or how magnetic waves moved through a material. But Cornell researchers have shown that excitons can do more than observe magnetism. They can actively steer it.
In “Excitonic Spin Torque in a Magnetic Semiconductor,” published June 15 in Nature Materials, Youn Jue (Eunice) Bae, assistant professor of chemistry and chemical biology in the College of Arts and Sciences, and colleagues report that excitons created by light can exert a spin torque in the two-dimensional magnetic semiconductor chromium sulfide bromide, or CrSBr. The finding establishes excitons as a new way to control magnetic motion with light.
“Excitons have been very useful for watching what spins are doing in magnetic materials,” Bae said. “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.”
The work pushes forward a central goal in spintronics, a field that seeks to use electron spin, rather than electronic charge, to store and process information. Because spin-based devices could operate with less wasted heat than conventional electronics, researchers are searching for new ways to control spins quickly, precisely and efficiently.
Traditional approaches to spin torque often rely on metallic heterostructures, interfaces and electrical currents. Bae’s group found a different route: using light to create an exciton reservoir inside the magnetic semiconductor itself.
The result was surprising because excitons are not obvious candidates for pushing spins around. An exciton is electrically neutral, and optically bright excitons are often thought of as having little or no net angular momentum.
“So it was conventionally thought that it shouldn’t exert torque,” Bae said.
But in CrSBr, the exciton does not simply act like a mechanical shove. Instead, it changes how the spin system gains and loses energy as it moves. Depending on the orientation of the spins, the exciton can either damp the motion or feed energy back into it.
“Exciton-generated torque is unlike conventional torques,” said Nicholas Brennan, a doctoral candidate and co-first author of the study with postdoctoral researcher Jiacheng Tang. “It’s not a simple linear torque like turning a lever. It’s nonlinear; the torque changes depending on the direction of the spin.”
That nonlinear behavior showed up clearly in the team’s measurements. At low excitation, the magnetic motion behaved like a regular oscillation. At higher excitation, the motion became sharply asymmetric, forming a sawtooth-like waveform. The team traced this unusual behavior to excitonic spin torque: the light-created exciton reservoir was repeatedly exchanging energy with the spin system.
The timing is part of what makes the effect powerful. The laser pulse that creates the excitons is extremely short, but the exciton reservoir can continue influencing the spins after the initial pulse is gone.
The researchers observed that this sustained interaction could drive the magnetic system far from equilibrium. At high excitation, the spins crossed repeatedly between different magnetic configurations, including canted antiferromagnetic, ferromagnetic and switched antiferromagnetic states.
The experiments used a pump-probe technique. A first pulse of light created excitons and launched magnetic motion in CrSBr. A second pulse arrived at controlled time delays to read out how the spins evolved.
“We do this in a stroboscopic fashion,” Brennan said. “Doing this repeatedly, we string together the time dynamics and watch how the material changes as a function of our excitation.”
CrSBr is a promising platform for this kind of work because it is air-stable, can be exfoliated down to atomically thin sheets, and hosts both magnetic order and strongly coupled excitons. Those properties make it a useful material for exploring how light, charge and spin interact in two-dimensional systems.
Looking ahead, Bae said excitonic spin torque could open new directions in optospintronic devices (which use both light and spin to carry and process information), spintronic memory (which uses spin for data storage) and quantum transduction (converting quantum information from one form, like light, into another, like electrical signals).
Brennan is also applying the concept toward neuromorphic computing, a form of information processing inspired by the brain, with support from a NASA Space Technology Graduate Research Opportunity fellowship.
“There is a nice analog between neurons in the brain and spins in a lattice,” Brennan said. “That’s why magnets are a compelling platform for realizing machine learning algorithms in particular.”
The experiment was performed in part at the Cornell Nanoscale Facility and made use of the Cornell Center for Materials Research.
This study was a collaborative effort with researchers at Columbia University and the University of Delaware, with support from the U.S. Department of Energy, NASA, the U.S. Air Force Office of Scientific Research and the National Science Foundation.
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