Study Shows Light's Magnetic Field Plays a Major Role in the Faraday Effect

Light's Magnetic Field Is More Important Than We Thought

Researchers have revised a 180-year-old assumption by showing that the magnetic component of an electromagnetic wave contributes substantially to the Faraday effect — the rotation of light's polarization as it passes through a magnetized material.

First observed by Michael Faraday in 1845, the Faraday effect has long been attributed primarily to interactions between a material and the electric field of light. Building on a 2024 experiment that revealed a magnetic influence in the inverse Faraday effect, a team at the Hebrew University of Jerusalem combined those experimental results with theoretical calculations based on the Landau–Lifshitz–Gilbert equation to test whether light's own oscillating magnetic field contributes directly to conventional Faraday rotation.

Using Terbium-Gallium-Garnet (TGG) — a magnetizable crystal commonly used in fiber optics and telecommunications — their models indicate the magnetic field of light contributes roughly 17% of the Faraday rotation at visible wavelengths and about 70% in the infrared for TGG. These contributions are far from negligible and change how we understand light–matter magnetic interactions.

How it works: Prior explanations emphasized forces on electron charge from the electric field. The new result highlights a complementary mechanism: a circularly polarized ("spinning") magnetic field exerts a torque on electron spin. As physicist Amir Capua explains, an electron's spin can be pictured like a tiny spinning top; to tilt its axis the interacting magnetic field must itself rotate.

“Light doesn't just illuminate matter; it magnetically influences it. The magnetic part of light has a first-order effect and is surprisingly active in this process,” said Amir Capua.

Implications include finer control of light–matter interactions for sensing, denser or more energy-efficient memory, direct optical manipulation of magnetic information in spintronics, and improved control over spin-based qubits for quantum computing.

The study synthesizes recent experiments and Landau–Lifshitz–Gilbert modeling applied to TGG crystals and is published in the journal Scientific Reports.