Scientists Uncover a Strong Magnetic Side to Light That Influences Atomic Spins

A new theoretical study shows that light's magnetic field — long considered negligible at optical frequencies — can exert a substantial torque on atomic spins. The finding reinterprets a classic interaction first explored by Michael Faraday in 1845 and points to fresh ways to control spin-based technologies.

Michael Faraday's 1845 experiments revealed that magnetic fields can rotate the plane of polarized light, an observation later incorporated into James Clerk Maxwell's formulation of electromagnetism. For nearly two centuries, researchers have treated the magnetic component of light as far weaker than its electric component and often ignored it when predicting light-matter interactions. The new work challenges that assumption.

What the researchers did

Physicists Benjamin Assouline and Amir Capua at the Hebrew University of Jerusalem revisited light–matter coupling using the Landau-Lifshitz-Gilbert (LLG) equation, a standard model for spin dynamics. Their calculations show that an oscillating magnetic field carried by light can produce a "magnetic torque" on atomic spins that behaves much like a static magnetic field when combined with an external field that 'twists' the light.

Applying the model to Terbium Gallium Garnet (TGG), a crystal commonly used to demonstrate magneto-optical effects, the authors estimate that the magnetic component of visible light accounts for roughly 17% of the induced atomic spin rotation. In the infrared — at wavelengths up to about 1,300 nanometers — the magnetic contribution can rise to as much as 75% of the spin response.

“The static magnetic field 'twists' the light, and the light, in turn, reveals the magnetic properties of the material,” said Amir Capua. “What we’ve found is that the magnetic part of light has a first-order effect; it’s surprisingly active in this process,” added Benjamin Assouline.

Igor Rozhansky, a physicist at the University of Manchester, commented that re-evaluating the magnetic part of light provides a new handle for manipulating atomic spins, although the magnitude of the effect will depend strongly on material properties and wavelength.

Potential implications

If validated experimentally across different materials, these magnetic-light interactions could enable more precise control of spins for sensors, quantum devices, and novel forms of spin-based data storage. The result is a reminder that even well-studied physical phenomena can reveal surprises with modern theory and careful modeling.

More work — both experimental and theoretical — will be needed to map where this effect is large enough to harness in real devices. Still, Assouline and Capua’s study opens a promising line of inquiry into the magnetic face of light and how it couples to matter.