Diamond uncovers mythical magnetic monopole in rust

A team of researchers has discovered magnetic monopoles – isolated magnetic charges – in a material closely related to rust, a result that could be used to power greener and faster computing technologies.

Led by researchers at the University of Cambridge in the UK, the team used a technique known as diamond quantum sensing to observe swirling textures and faint magnetic signals on the surface of hematite, a type of iron oxide.

The researchers observed that magnetic monopoles in hematite emerge through the collective behaviour of many spins (the angular momentum of a particle). These monopoles glide across the swirling textures on the surface of the hematite, like tiny hockey pucks of magnetic charge. This is the first time that naturally occurring emergent monopoles have been observed experimentally.

The researchers also uncovered the direct connection between the previously hidden swirling textures and the magnetic charges of materials like hematite, as if there is a secret code linking them together. They report their findings, which could find use in next-generation logic and memory applications, in a paper in Nature Materials.

According to the equations of James Clerk Maxwell, a giant of Cambridge physics, magnetic objects, whether a fridge magnet or the Earth itself, must always comprise a pair of magnetic poles that cannot be isolated.

“The magnets we use every day have two poles: north and south,” said Mete Atatüre, who led the research. “In the 19th century, it was hypothesized that monopoles could exist. But in one of his foundational equations for the study of electromagnetism, James Clerk Maxwell disagreed.”

Atatüre is head of Cambridge’s Cavendish Laboratory, a position once held by Maxwell himself. “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,” he said.

About 15 years ago, scientists suggested how monopoles could exist in a magnetic material. This theoretical finding relied on the extreme separation of north and south poles, so that locally each pole appeared isolated in an exotic material called spin ice.

However, there is an alternative strategy for finding monopoles, which involves the concept of emergence. This is the idea that the combination of many physical entities can give rise to properties that are either more than or different to the sum of their parts.

Working with colleagues from the University of Oxford in the UK and the National University of Singapore, the Cambridge researchers used emergence to uncover monopoles spread over two-dimensional space, gliding across the swirling textures on the surface of a magnetic material.

These swirling topological textures are found in two main types of materials: ferromagnets and antiferromagnets. Of the two, antiferromagnets are more stable than ferromagnets, but they are more difficult to study, as they don’t have a strong magnetic signature.

To study the behavior of antiferromagnets, Atatüre and his colleagues used an imaging technique known as diamond quantum magnetometry. This technique uses a single spin – the inherent angular momentum of an electron – in a diamond needle to precisely measure the magnetic field at the surface of a material, without affecting its behavior.

For the current study, the researchers used this technique to look at hematite, an antiferromagnetic iron oxide material. To their surprise, they found hidden patterns of magnetic charges within hematite, including monopoles, dipoles and quadrupoles.

“Monopoles had been predicted theoretically, but this is the first time we’ve actually seen a two-dimensional monopole in a naturally occurring magnet,” said co-author Paolo Radaelli from the University of Oxford.

“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,” said co-first author Hariom Jani from the University of Oxford. “The result is a tiny, localized stable particle with a diverging magnetic field coming out of it.”

“We’ve shown how diamond quantum magnetometry could be used to unravel the mysterious behavior of magnetism in two-dimensional quantum materials, which could open up new fields of study in this area,” said co-first author Anthony Tan from the Cavendish Laboratory. “The challenge has always been direct imaging of these textures in antiferromagnets due to their weaker magnetic pull, but now we’re able to do so, with a nice combination of diamonds and rust.”

This study not only highlights the potential of diamond quantum magnetometry but also underscores its capacity to uncover and investigate hidden magnetic phenomena in quantum materials. If controlled, these swirling textures dressed in magnetic charges could power super-fast and energy-efficient computer memory logic.

This story is adapted from material from the University of Cambridge, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.