Physics of light and magnetism rewritten after almost two centuries

In 1845, physicist Michael Faraday provided the first direct evidence that electromagnetism and light are related. Now, it turns out that this connection is even stronger than Faraday imagined.

In his experiment, Faraday shone light through a piece of glass laced with boracic acid and lead oxide and immersed in a magnetic field. He discovered that this altered the light: when it emerged from the glass, its polarisation had been reorientated.

Light is an electromagnetic wave, and for the past 180 years it has been widely accepted that this “Faraday effect” demonstrates that the combined interaction of the magnetic field, the electric charges in the glass, and the electric component of light results in the light wave becoming rotated – wiggling in a different direction than before it entered the material.

Perhaps surprisingly, it has long been assumed that the magnetic component of light plays effectively no role in the Faraday effect. Amir Capua and Benjamin Assouline at the Hebrew University of Jerusalem in Israel have now shown that this is not necessarily always the case.

“There is a second part of light that we now understand interacts with materials,” says Capua.

Capua says there are two reasons why researchers did not pursue the idea that the magnetic component of light plays a part in the Faraday effect. Firstly, the magnetic forces within materials such as Faraday’s glass seem to be relatively weak compared to the electric forces. Secondly, when materials like Faraday’s glass are magnetised – which means the quantum spins of their constituent parts interact with any magnetic field like tiny magnets would – these spins are typically out of sync with the magnetic component of the light waves, which suggests the two don’t interact strongly.

But Capua and Assouline realised that when the magnetic component of light is circularly polarised – essentially swirly or corkscrew-like – it can interact with the magnetic spins in the glass a lot more intensely. They concluded that this happens even without any special effort to manipulate the light, because its magnetic component is always made up of several corkscrew waves.

The two researchers’ calculations revealed that if Faraday’s experiment is repeated with a magnetic material called Terbium Gallium Garnet (TGG) instead of glass, this magnetic interaction could actually account for 17 per cent of the resulting Faraday effect when visible light passes through the material. If infrared light is passed through the TGG material instead, the magnetic interaction would account for as much as 70 per cent of the resulting Faraday effect.

Igor Rozhansky at the University of Manchester, UK, says that the new calculations are convincing and point towards plausible experimental tests in the future. The so-far neglected magnetic component of the Faraday effect could provide a new way for researchers to manipulate spins inside materials, says Rozhansky. He adds that it is an open question whether this effect could actually be stronger than the traditional Faraday effect in some materials.

Future experiments will take the new findings from fundamental physics to applications, and Capua says that he can already envision how the discovery that the magnetic spins in some materials can interact with the light’s magnetic component could be used to manipulate them. That could eventually pave the way for new types of spin-based sensors and hard drives.

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