Kagome magnets are frustrating materials. Their properties pivot on the nature of their crystal lattice wherein intersecting webs of corner-sharing triangles allow electrons to traverse the structure and lead to intriguing quantum states known as frustrated, correlated, and topological. An international team has now demonstrated that the kagome ferromagnet Fe3Sn2 exhibits an electronic state that couples unusually strongly to an applied magnetic field. This can be rotated in any direction giving rise to a "giant" magnetization-driven electronic energy shift. That energy shift sheds a little light on the presence of spin-orbit coupling and topological spin textures in the kagome lattice. This spin-orbit activity was previously unknown.

"We found out two things," explains Boston College's Ziqiang Wang. "The first one is that the electronic state of Fe3Sn2 is nematic, a state that spontaneously breaks the rotation symmetry. The electrons behave as a liquid crystal inside this magnet, presumably due to the strong electron-electron interaction," he adds. "The second is that you can manipulate and make big changes to the electron energy structure through tuning the magnetic structure by applying a magnetic field." [Jin, Y-X et al., Nature (2018) 562(7725), 91 DOI: 10.1038/s41586-018-0502-7

The team includes other researchers from Boston College, Princeton University, the Chinese Academy of Sciences, Renmin University, and Peking University. They used theoretical studies as well as scanning tunneling microscopy (STM) and vector-magnetic-field tools to observe this spin-orbit coupling and reveal the exotic characteristics of this material and to explain it theoretically.

"What our colleagues found is that by changing the direction of the magnetic field, they saw changes in the electronic states that are anomalously large," explains Wang. "The shifts of the bands - there are band gaps, forbidden regions in quantum mechanics where electrons cannot reside - those regions can be tuned enormously by the applied magnetic field."

The "band shift" sees the band gap expanding and contracting depending on the direction of the applied magnetic field. This effect was 150 times stronger in the kagome ferromagnet than in conventional materials. By probing the interference patterns of the electron's quantum mechanical wave functions the team was also able to reveal consistent spontaneous nematicity. This, they explain, is an indication of an important electron correlation that causes the rotation symmetry-breaking of the electronic state in the material. Such spin-driven giant electronic responses suggest that there exists an underlying correlated magnetic topological phase. Moreover, because the properties of the kagome magnet can be tuned this reveals a strong interplay between an externally applied magnetic field and nematicity and could open up a new approach to controlling spin-orbit properties and so facilitate the exploration of emergent phenomena in topological and quantum materials. Inevitably, implications for magnetic memory devices and sensing technologies come to the fore.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. You can see more of his macro and other photography via his website.