Illuminating quantum magnets: Light unveils magnetic domains

Scientists from Osaka Metropolitan University and the University of Tokyo have successfully used light to visualize tiny magnetic regions, known as magnetic domains, in a specialized quantum material. Moreover, they successfully manipulated these regions by the application of an electric field. Their findings offer new insights into the complex behavior of magnetic materials at the quantum level, paving the way for future technological advances.

Most of us are familiar with magnets that stick to metal surfaces. But what about those that do not? Among these are antiferromagnets, which have become a major focus of technology developers worldwide.

Antiferromagnets are magnetic materials in which magnetic forces, or spins, point in opposite directions, canceling each other out and resulting in no net magnetic field. Consequently, these materials neither have distinct north and south poles nor behave like traditional ferromagnets.

Antiferromagnets, especially those with quasi-one-dimensional quantum properties — meaning their magnetic characteristics are mainly confined to one-dimensional chains of atoms — are considered potential candidates for next-generation electronics and memory devices. However, the distinctiveness of antiferromagnetic materials does not lie only in their lack of attraction to metallic surfaces, and studying these promising yet challenging materials is not an easy task.

“Observing magnetic domains in quasi-one-dimensional quantum antiferromagnetic materials has been difficult due to their low magnetic transition temperatures and small magnetic moments,” said Kenta Kimura, an associate professor at Osaka Metropolitan University and lead author of the study.

Magnetic domains are small regions within magnetic materials where the spins of atoms align in the same direction. The boundaries between these domains are called domain walls.

Since traditional observation methods proved ineffective, the research team took a creative look at the quasi-one-dimensional quantum antiferromagnet BaCu₂Si₂O₇. They took advantage of nonreciprocal directional dichroism — a phenomenon where the light absorption of a material changes upon the reversal of the direction of light or its magnetic moments. This allowed them to visualize magnetic domains within BaCu₂Si₂O₇, revealing that opposite domains coexist within a single crystal, and that their domain walls primarily aligned along specific atomic chains, or spin chains.

“Seeing is believing and understanding starts with direct observation,” Kimura said. “I’m thrilled we could visualize the magnetic domains of these quantum antiferromagnets using a simple optical microscope.”

The team also demonstrated that these domain walls can be moved using an electric field, thanks to a phenomenon called magnetoelectric coupling, where magnetic and electric properties are interconnected. Even when moving, the domain walls maintained their original direction.

“This optical microscopy method is straightforward and fast, potentially allowing real-time visualization of moving domain walls in the future,” Kimura said.

This study marks a significant step forward in understanding and manipulating quantum materials, opening up new possibilities for technological applications and exploring new frontiers in physics that could lead to the development of future quantum devices and materials.

“Applying this observation method to various quasi-one-dimensional quantum antiferromagnets could provide new insights into how quantum fluctuations affect the formation and movement of magnetic domains, aiding in the design of next-generation electronics using antiferromagnetic materials,” Kimura said.