A new dimension of complexity for layered magnetic materials

When it comes to layered quantum materials, current understanding only scratches the surface; so demonstrates a new study from the Paul Scherrer Institute PSI. Using advanced X-ray spectroscopy at the Swiss Light Source SLS, researchers uncovered magnetic phenomena driven by unexpected interactions between the layers of a kagome ferromagnet made from iron and tin. This discovery challenges assumptions about layered alloys of common metals, providing a starting point for developing new magnetoelectric devices and rare-earth-free motors.

Patterns are everything. With quantum materials, it’s not just what they’re made of but how their atoms or molecules are organised that gives rise to the exotic properties that excite researchers with their promise for future technologies.

Graphene showed this to the world: arranged into single layers of a hexagonal lattice, common-or-garden carbon atoms could exhibit extraordinary electronic properties. Research over the last decade has since been dedicated to discovering whether other two-dimensional arrays of atoms, either alone or stacked into a three-dimensional material, can reveal similarly novel behaviours.

The kagome lattice, which takes its name from a type of Japanese basket woven in corner sharing triangles, is another two-dimensional pattern that has excited researchers with its ability to host exotic quantum states, ranging from superconductivity to unconventional magnetism.

Yet until now, research has focused on electronic and magnetic properties in two-dimensions of the material. The latest results in Fe₃Sn₂ - a ferromagnetic material made of iron and tin atoms arranged into the intricate kagome pattern - change that.

The holy grail of flat bands

Fe₃Sn₂ can host a variety of intriguing magnetic phenomena. At the heart of these are spin waves: collective precessions of electron spins that differ slightly in phase, adding up or cancelling out to form magnetic ripples that move through the material like waves on a pond.

Although the material has been the subject of research interest for many decades, the nature of these spin waves and their potential impact on magnetic and electronic behaviour remain experimentally unprobed. With this in mind, the team set out to investigate how the unique kagome structure of Fe₃Sn₂ shapes its magnetic properties.

In particular, the team wanted to determine whether the spin excitations in Fe₃Sn₂ formed a sought-after characteristic known as flat bands. Here, the kinetic energy of excitations becomes negligible and they become localised. The excitations then have the rare opportunity to interact strongly with each other, opening the door to a range of exotic quantum effects.

Using X-rays at the Swiss Light Source SLS, the research team probed the magnetic excitations in the material to verify whether the spin waves form flat bands, as predicted by theory.

“We were able to show experimentally for the first time that it possesses nearly flat bands,” said Yona Soh, scientist at PSI and corresponding author of the study. “But in reality, the context and origin of these bands was quite different to what anyone expected.”

‘Nearly’ gives a clue to something more

If you’re paying attention, perhaps you wonder about the word ‘nearly’. The bands showed subtle deviations from perfect flatness.

Exploring the origin of this topology, the researchers discovered that the explanation lay in the layered structure of the material. Through systematic experimental and theoretical evidence, the researchers could reveal that the flat bands were created by strong interactions not just within the layers of the kagome material, but also between adjacent layers.

“It is a very unexpected discovery to find that the two layers are strongly interacting. Typically, the main interactions would be within the layer,” said Soh.

This finding is significant: not just for the material Fe₃Sn₂, but also for other layered materials. Conventional theory largely considers such materials as two-dimensional systems, where interactions between the layers of a three-dimensional material are of minimal importance. This study shows that the reality is more nuanced.

A clean way into the magnetic structure with X-ray spectroscopy

Such surprising insights were down to the use of an unconventional experimental technique: resonant inelastic X-ray scattering (RIXS) at the ADRESS beamline of SLS. “Traditionally, inelastic neutron scattering is used to reveal the magnetic excitations in a material,” says Thorsten Schmitt, leader of the Spectroscopy of Quantum Materials group in the Center for Photon Sciences at PSI. However, inelastic neutron scattering requires grams of sample. For Fe₃Sn₂, this would mean precisely aligning hundreds of crystals.

By using RIXS, the team could study single crystals of the material. Yet the technique is sensitive not only to spin excitations, associated with the magnetic behaviour of the material, but also to electronic excitations. By using circularly polarised light, the research team were able to subtract out other types of excitations and focus exclusively on low-energy spin excitations that reveal the magnetic topology.

“This is a new development of RIXS,” says Schmitt, who is responsible for the ADRESS beamline at the SLS and led the experimental side of the study. “Our work is the first time RIXS has been used with circularly polarised light to isolate low-energy spin excitations in a ferromagnetic material with such clarity.”

New experimental possibilities, deeper fundamental insights, practical applications

The use of RIXS with circularly polarised light offers exciting opportunities for further development and applications, notably for accessing spin excitations at even lower energies. This would allow researchers to probe magnetic systems on an even finer scale, uncovering subtle magnetic phenomena that are currently out of reach. Such advancements can be looked forward to with the SLS 2.0 upgrade, which is nearing completion and includes enhancement of sensitivity at the new ADRESS 2.0 beamline.

“In order to access lower energy spin excitations, we require higher resolution,” adds Schmitt. “With the upgrade, we will be able to increase resolution by up to a factor of five, equipping us to explore magnetic phenomena in ways that have never before been possible.”

Such magnetic behaviours have very practical applications. For example, if complex magnetic and electronic phenomena can be generated in metals made from simple and abundant constituents, such as iron and tin, these could lead to sustainable alternatives to rare earth metals in magnetoelectric devices including more energy-efficient motors or new types of information transfer and data storage.

Text: Paul Scherrer Institute / Miriam Arrell.