Researchers find unexpected excitations in a layered material

Researchers from the US Department of Energy’s Ames National Laboratory have discovered an unexpected chiral excitation in the kagome layered TbMn topological magnet₆Mr₆. This chiral excitation can be seen as a localized magnetic spin or vortex. The team also confirmed the existence of localized flat band magnons, a new excitation associated with the frustrated geometry of the kagome lattice.

Over the past few years, a team of Ames Lab scientists, led by Rob McQueeney, has been investigating TbMn₆Mr₆ to learn more about the material, its properties and how it behaves at different temperatures and magnetic fields. According to McQueeney, the material has been known for a long time, but it appeared on researchers’ radars about five years ago. “This material has been a goldmine of interesting phenomena,” said McQueeney.

In their investigation of this material so far, the team has mapped its magnetic excitations at low energies and studied how these excitations evolve through a temperature-dependent magnetic spin reorientation transition. Their latest findings come from studying high-energy excitations of the material.

The study is titled “Flatband and Chiral Magnetic Quasiparticles in Ferromagnetic and Metallic Kagome Layers,” and is published in the journal Nature Communications.

McQueeney explained that based on theoretical models, they expected to find localized flat-band magnons due to the special geometry of the kagome lattice. Magnons are quasi-particles, or elementary excitations, that control the magnetization of a material.

“Flatband excitations are localized, so the magnetic excitation doesn’t propagate as a wave, it just sits localized in a single hexagon. And an excitation in another hexagon in the lattice doesn’t talk to the other, they just behave . sit there and they swing in these hexagons,” McQueeney said. “In a kagome layer, those flat bands are guaranteed by the mesh geometry.”

In addition to observing these localized, flat-band magnons, the team was surprised to discover another type of localized excitation.

“We saw another harassment that shouldn’t have been there,” McQueeney said. “When we looked at the momentum distribution, this had the character of what they call a chiral excitation. Chirality means it has one-handedness, so there are left and right-handed versions of the excitation. And we found that it was also localized in a hexagon , just like the flat belt.”

He further explained that TbMn₆Mr₆ there are ferromagnetic kagome layers, but the chiral excitations suggest that a chiral counterferromagnetic instability is nearby. The magnetic state of a material determines its functional properties. For example, ferromagnets strongly attract other magnetic materials, while antiferromagnets do not have a magnetic field outside the material (so they do not attract other magnets).

In this material, the ferromagnetic state is more stable than the chiral antiferromagnetism the team discovered. However, McQueeney said that since the chiral excitations are there, perhaps changing the chemical composition of the material can stabilize the chiral order.

“People are always looking for chiral ground states,” McQueeney said. “The reason we use the concept of quasiparticle here is because it’s a way to transmit energy or information, like an electron is a quasiparticle, and we can send it from point A to point B, carrying some information.

“A chiral quasiparticle would have other attributes to it. For example, it would have a reach, and so you could think of new ways of, say, transmitting information from point A to point B, which doesn’t involve moving a charge but moving a chiral signal.”

The discovery of this new chiral excitation was particularly exciting for McQueeney, “You don’t expect it to be there,” he said. “And we still don’t understand why it’s there. In fact, we’re designing other experiments to look for it in other materials.”