Whilst materials are conventionally categorised as either electrical conductors or insulators, there are many materials that have properties that don't neatly fit into either category. For example, in topological insulators, charges can only be transported on the surface, but they have extreme mobility. Following a string of theoretical predictions and experimental discoveries of such materials, at the current frontier of this field are real materials in which topological surface states couple to the magnetic order.
The control of these phenomena could enable topological spintronic technologies, including ultra energy-efficient computation and storage, as well as new approaches to quantum computing. In this study, published in Physical Review B, an international research team focused on Eu5In2Sb6, a magnetic candidate for complex topological states that showed particularly unusual behaviour at low temperature. By using neutrons and muons at ISIS, they were able to clarify the magnetic structures in this material and demonstrate that they are indeed coupling to charge transport.
Neutron measurements using the WISH instrument at ISIS were used to understand the magnetism of Eu5In2Sb6. The team found that the material has two magnetic phase transitions where the magnetism changes. First, below -259°C the material forms a complicated pattern of magnetic order with a net magnetisation that can be thought of as a complicated ferromagnet. When the magnet gets even colder, below -266°C, the magnetic order changes to one that has no net magnetisation, more like a complicated antiferromagnet. This lower temperature magnetism has two potential explanations, both of which are interesting in their own right. However, one possibility is that it is a state known as an axion insulator, an elusive state of matter that has very different properties on the surface of the sample when compared to deeper within the material.
The team also used muon experiments at ISIS to understand Eu5In2Sb6. Using the EMU instrument, muons were implanted inside the material, where they act as miniature compasses, revealing what is happening to the magnetism inside the sample. This technique is known as muon spin relaxation.
They found that there were two different signals, one from within the plane of magnetic moments, and another from between the planes. With access to these two different signals, the researchers were able to understand what changes the magnetic structure underwent. Perhaps most intriguingly, the muon spin relaxation measurements indicated that the magnetism first appears at a higher temperature, around -238°C. Below this temperature, there are strong, short range magnetic correlations. These might be so-called magnetic polarons, bubbles of magnetic order that behave like a particle, changing optical, thermal, and electrical properties of the material.
Lead author, Marein Rahn from Technical University of Dresden, had the following to say when asked about the work: “Once again this project has taught me that with neutrons, never say never! Even though europium is an overwhelming neutron absorber, and even though we only had a small amount of Eu5In2Sb6 powder, we were able to extract all the crucial information from our data, which then guided our complementary measurements using synchrotron light. The success of the study really comes down to the design of the WISH diffractometer, which makes no compromise in signal-to-noise."
There is currently a handful of candidate families of materials in which qualitatively new electronic behaviour could be realised. The first era of research into topological matter was driven by theoretical predictions. But since magnetic correlations are hardly predictable from first principles, the work on complex magnetic phases like those in Eu5In2Sb6 is putting experimentalists in the driver's seat. The progress of the next decades will necessarily hinge on insights gained at neutron, muon and synchrotron light sources.
To
learn more, the full article can be found at https://doi.org/10.1103/PhysRevB.109.174404
