Regular magnets have two poles, a north and a south, and their behavior is defined in classical terms by the Maxwell equations. From contemporaries of Maxwell through to modern researchers, there have been hypotheses of the existence of magnetic monopoles, fundamental particles that are just north or just south. Researchers have not found them yet, but they have measured the next best thing.
Using hematite, the rock equivalent of rust, the researchers discovered that on its surface, peculiar magnetic configurations can emerge. They measured quasiparticles – an interaction of sorts that behaves like a particle for a time – that had a single magnetic pole, two magnetic poles, and even four magnetic poles. The quasiparticles were swirling on the surface of the hematite.
“Monopoles had been predicted theoretically, but this is the first time we’ve actually seen a two-dimensional monopole in a naturally occurring magnet,” co-author Professor Paolo Radaelli, from the University of Oxford, said in a statement.
“These monopoles are a collective state of many spins that twirl around a singularity rather than a single fixed particle, so they emerge through many-body interactions. The result is a tiny, localised stable particle with diverging magnetic field coming out of it,” said co-first author Dr Hariom Jani, also from the University of Oxford.
The formation of these magnetic monopoles is connected to a property known as emergence, where the combination of many physical entities produces something new with properties that are either more or different from the sum of its parts. In this case, they ended up with something very different; the monopole and the quadrupole are from entities that have the standard magnet configuration.
“The magnets we use every day have two poles: north and south,” said Professor Mete Atatüre, who led the research. “In the 19th century, it was hypothesised that monopoles could exist. But in one of his foundational equations for the study of electromagnetism, James Clerk Maxwell disagreed.”
“If monopoles did exist, and we were able to isolate them, it would be like finding a missing puzzle piece that was assumed to be lost,” added Atatüre, who is the Head of Cambridge’s Cavendish Laboratory, a position once held by Maxwell himself.
The measurements were conducted using a diamond needle where the spin of a single electron – its intrinsic angular momentum – was used to precisely measure the magnetic field of the material. Discovering how to manipulate these peculiar quasiparticles might mean better computer technology, with fast and energy-efficient memory logic.
The study is published in Nature Materials.