Scientists have recently discovered the interaction between electromagnetic waves and their own magnetic components as they pass through materials, updating a 180-year-old hypothesis that only considered the interaction between light and its electric field.
This phenomenon, called the Faraday Effect (FE), was first described by Michael Faraday in 1845 and provides some of the earliest evidence of interactions between magnetic and light waves.
It describes how a light beam passing through a transparent material is affected when the material is subjected to a magnetic field. Specifically, this changes the polarization direction of the beam.
To simplify viewing angles, light can be unpolarized or polarized. When light is unpolarized, its electromagnetic oscillations occur in all directions (perpendicular to its plane of propagation).
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However, when light is polarized, these oscillations line up in one direction—imagine taking a frilly, furry sweater out of the closet and smoothing its fibers.
It has long been thought that the impact of the Faraday effect on light polarization is simply a matter of the interaction of the electrical component of the electromagnetic ripples with the material’s magnetism and additional magnetic fields.
Last year, a research group at the Hebrew University of Jerusalem experimentally demonstrated the subtle but noticeable effect of the magnetic plane opposite to FE, whereby the polarization of light creates a magnetic moment in the material.
In their new study, the researchers combined their experimental results with complex calculations based on the Landau-Lifshitz-Gilbert equation, which describes the dynamics of magnetism in solid materials, to determine whether this subtle interaction might also play a role in the Faraday effect itself.
They used a physical model of terbium-gallium garnet, a magnetizable crystal commonly used in fiber optics and telecommunications technology, as the basis for their calculations.
Calculations show that light’s magnetic field contributes about 17% of the FE in visible wavelengths and 70% in infrared wavelengths—far from being as insignificant as previously assumed.
It turns out that the finite element is directly affected by the oscillating magnetic field of light and not just the electric field, as was thought.
“Light not only illuminates matter, it also has a magnetic effect on it. Static magnetic fields ‘twist’ the light, which in turn reveals the material’s magnetic properties,” explains physicist Amir Capua.
“We found that the magnetic part of the light has a first-order effect that is surprisingly active in the process.”
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So this study found another way in which light’s magnetic field interacts with matter—not by interacting with the charge of electrons, but by interacting with another fundamental aspect of electrons, their spin, since every electron in every piece of matter has a charge and a spin.
Capua describes ScienceAlert’s breakthrough:
“At the heart of this effect are fundamental principles that we have identified. Broadly speaking, you can think of the spin of an electron as a tiny charge spinning around its axis, almost like a miniature top. In order to interact with the ‘spinning electron’ and change the direction of its spin axis, the magnetic field it interacts with also needs to ‘spin’, i.e. it needs to be circularly polarized.”
Capua added that this “creates a very balanced picture: the electric field exerts a linear force on the charge, while the ‘rotating’ circularly polarized magnetic field exerts a torque on the electron spin.”
Discovering this overlooked interaction in established finite elements could provide scientists with a way to more precisely control light and matter, potentially enabling advances in sensing, storage and computing, such as quantum computer innovations through more precise control of spin-based qubits.
Additionally, the field of spintronics uses electron spin rather than electric charge to store and manipulate information.
“This discovery shows that you can control magnetic information directly with light,” said electrical engineer Benjamin Assouline.
In the end, this work is tantalizing because it reminds us of one of the cornerstones of science, which is that researchers may discover other unknown properties of light or other electromagnetic phenomena at any time, even within well-established models.
This research was published in scientific report.
