Most magnets - like those on a fridge door or in a compass - have two poles: north and south, or positive and negative, in a familiar arrangement called a "dipole."
But researchers are increasingly uncovering more complex forms of magnetism.
Recent advances in quantum mechanics have revealed higher orders of magnetism, including the existence of an "octupolar order" in which a pattern of particles arranged in a crystalline structure within a material behaves as if it has eight magnetic poles rather than the familiar two. However, detecting and controlling these elusive magnetic states has been a significant challenge.
Now, a team of researchers led by quantum physicists at the University of Toronto has established a new method for observing quantum magnetic states using light to probe the atomic vibrations produced as electrons spin. The work is a critical first step toward harnessing multi-polar magnetism for practical technologies, including next-generation data storage and computing devices.
"We identified new signatures of a hidden type of magnetic state which cannot be detected using ordinary probes," says Arun Paramekanti , a professor in the department of physics and the Centre for Quantum Information and Quantum Control in U of T's Faculty of Arts & Science. "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 is the senior author of a study published in Physical Review Letters that describes how he and his collaborators discovered that magnetic patterns normally invisible using conventional techniques are revealed by shining light on a crystalline material and triggering tiny vibrations within its atomic structure.
The findings offer a new tool for uncovering and manipulating previously inaccessible forms of magnetism, opening new avenues for quantum technology development.
In the study, the researchers focused their attention on the packets of vibrational energy moving through a solid's crystal lattice, known as phonons. Specifically, they looked for chiral phonons - ones that don't match their own mirror image.
"Just as a left hand cannot fit cleanly over a right hand even though they are mirror images, chiral phonons have a 'handedness' and exist in distinct, non-matching forms," says lead author Rory Sutcliffe, a PhD candidate in the department of physics.
That handedness turned out to be key. The researchers found that hidden octupolar magnetic states leave a unique signature in these vibrations that were detected with light. By directing a special type of rotating light at magnetic materials, the team observed a clear optical fingerprint of the otherwise hidden magnetic order.
"We found that the onset of octupolar order can impart a distinct handedness to certain phonon modes," says Swati Chaudhary, a University of Tokyo project research associate and study co-author. "These vibrations behave differently from those found in conventional magnets, so we call them 'pseudo-chiral' phonons, and they provide a new way of identifying and studying hidden magnetic states."
Kathleen Hart, also a PhD candidate in U of T's department of physics and study co-author, explains: "Our work offers a new optical probe of hidden magnetic orders that are difficult to detect by standard techniques and lays the foundation for how such octupolar magnetism might eventually be controlled through atomic vibrations within a material."
