Toronto Physicists Detect Hidden 'Octupolar' Magnetism by Shining Rotating Light on Vibrating Atoms
The eight-pole magnetic order is invisible to standard probes, but it leaves a signature in how a crystal's atoms vibrate, which could lead to new kinds of computer memory.
Physicists at the University of Toronto have found a way to see a kind of magnetism that standard instruments cannot detect, by using rotating light to read the vibrations of a crystal's atoms.
Most magnets are dipolar. They have a north and a south pole, like a bar magnet or a fridge magnet. Some materials instead order themselves into octupolar magnetism, a higher-order pattern with eight poles. Because the magnetic fields from those patterns cancel at a distance, octupolar order does not respond to the tools that reveal ordinary magnetism, and it has been hard to confirm in real materials.
The Toronto team, led by physics professor Arun Paramekanti, found that octupolar order leaves a distinct fingerprint in the vibrations of the crystal lattice, called phonons. In an octupolar material, those vibrations pick up a handedness the researchers call pseudo-chiral. Rotating, or circularly polarized, light can pick up the difference between right-handed and left-handed motion, so the order shows up in an optical measurement.
The study was published in Physical Review Letters. Rory Sutcliffe, a Ph.D. candidate, is the lead author, and Swati Chaudhary of the University of Tokyo and Ph.D. candidate Kathleen Hart are co-authors.
"Our research opens up the possibility for using higher-order magnets in several applications including controllable read-write memory elements found in everyday computers," Paramekanti said.
The reason higher-order magnets interest engineers is that they pack information in a more compact way. Dipolar bits spill magnetic fields into their neighbors, which limits how closely they can be packed and creates cross-talk. A bit stored in an octupolar pattern would produce almost no stray field, so in principle many more could be placed side by side without disturbing each other. The same property makes them hard to read and write, which is why a clean, optical way to detect the order matters.
The researchers also point to quantum technologies as a possible use, since controlling hidden magnetic order is a path toward new kinds of devices.
The next test is whether the vibrational signature shows up in candidate crystals. If it does, it would give physicists a practical way to map an entire class of magnetic order that has until now been mostly inferred indirectly.
The approach also fits a broader shift in condensed-matter physics, where researchers increasingly treat lattice vibrations as more than background noise. Phonons that carry handedness have become a tool for probing magnetism, superconductivity and other ordered states, because the atoms feel the electrons' arrangement and respond to it. Reading that response with light is quick and does not require touching the sample, which makes it attractive for screening many materials. The paper is available in Physical Review Letters under DOI 10.1103/n11w-csdh, and an earlier version was posted to the arXiv preprint server.
