A recent study led by Dr. Amir Capua and Benjamin Assouline at the Hebrew University of Jerusalem found that the magnetic part of light plays a direct, previously overlooked role in the Faraday effect - challenging nearly 180 years of common perception in optics and magnetism.
The Faraday effect is a classic phenomenon where the polarization of light rotates as it passes through a material in the presence of a static magnetic field. Historically, this rotation was explained almost entirely by the electric field of light interacting with charges in the material, while the magnetic field of light was thought negligible at optical frequencies. This new research demonstrates that the oscillating magnetic field of light itself exerts a torque on atomic spins in the material and contributes meaningfully to the rotation observed.
To analyze this process, the researchers utilized the Landau–Lifshitz–Gilbert (LLG) equation, which is a standard tool for modeling spin dynamics. Their calculations revealed that the optical magnetic field couples to the magnetization inside the material, inducing a torque that accumulates over time much like a static magnetic field, but uniquely driven by light's own magnetic component.
To quantify this effect, the team examined Terbium Gallium Garnet (TGG), a standard crystal used in Faraday rotators and optical isolators. Using TGG’s known magnetic susceptibility and permittivity, they determined that the optical magnetic field contributes about −14 rad/(m·T) to the Verdet constant, compared to a measured value of roughly −80 rad/(m·T) at 800 nm - constituting roughly 17.5% of the total effect at visible wavelengths. In the infrared, where the conventional electric-field-driven contribution drops, their model shows that the magnetic part of light can account for as much as 70% of the observed Faraday rotation.
This discovery means that light interacts with matter not only through its electric field but also through its magnetic field in surprisingly strong, first-order ways. Recognizing and leveraging this magnetic channel of light–matter interaction could lead to new approaches in controlling spins and magnetization with light, beyond established electric-based mechanisms. This insight also bridges two historically separate frameworks: the conventional Faraday effect and ultrafast, optically induced magnetization phenomena like the inverse Faraday effect and all-optical switching.
A stronger role for the optical magnetic field opens new opportunities in photonic and spintronic device design, including direct light-based manipulation of magnetic states. In fields such as optical data storage and magneto-optic components (isolators, circulators, modulators), accounting for this contribution could improve performance at longer wavelengths and inform the engineering of materials with tailored magnetic response to light.
Looking ahead, the ability to steer spins with the magnetic part of light may be crucial for spin-based quantum technologies, where precise, ultrafast, and low-dissipation control of spin states is essential. Since Michael Faraday first discovered the rotation of light in a magnetic field in 1845, the effect has stood as a cornerstone of magneto-optics. This new research adds a missing piece by quantifying the direct impact of the optical magnetic field, reframing a classic phenomenon with modern spin-dynamics and expanding the possibilities for light-driven magnetism.