Researchers have uncovered a deeper connection between light and magnetism than Michael Faraday demonstrated in 1845. New calculations show that light's magnetic component plays a significant role in the Faraday effect, challenging long-held assumptions. This discovery could lead to advances in spin-based technologies.
In 1845, physicist Michael Faraday conducted an experiment that provided the first direct evidence linking electromagnetism and light. He shone light through a piece of glass laced with boracic acid and lead oxide, immersed in a magnetic field. The light emerged with its polarization reorientated, demonstrating what is now known as the Faraday effect.
For 180 years, scientists have understood this effect as resulting from the interaction between the magnetic field, electric charges in the material, and the electric component of light—an electromagnetic wave. The magnetic component of light was assumed to play no effective role. However, Amir Capua and Benjamin Assouline at the Hebrew University of Jerusalem in Israel have shown this is not always the case.
“There is a second part of light that we now understand interacts with materials,” says Capua. Previously, researchers overlooked this due to the relative weakness of magnetic forces in materials like Faraday’s glass compared to electric forces, and because the quantum spins in magnetized materials are typically out of sync with light's magnetic component.
Capua and Assouline realized that when light's magnetic component is circularly polarized—swirly or corkscrew-like—it interacts intensely with the material's magnetic spins. They noted that light's magnetic component naturally consists of several such corkscrew waves.
Their calculations indicate that repeating Faraday’s experiment with Terbium Gallium Garnet (TGG) instead of glass would show the magnetic interaction accounting for 17 percent of the Faraday effect with visible light, and up to 70 percent with infrared light.
Igor Rozhansky at the University of Manchester, UK, describes the calculations as convincing and suggests they could enable new ways to manipulate spins in materials. Capua envisions applications in spin-based sensors and hard drives. The findings were published in Scientific Reports (DOI: 10.1038/s41598-025-24492-9).