Layer Number Tunes Topological Magnet Properties
Researchers at Tohoku University have demonstrated that varying the number of layers in a crystal provides a systematic method for designing topological
A research team has developed a layer-based design strategy for creating topological magnets. The work, led by Professor Hideaki Sakai of Tohoku University's Institute for Materials Research, was published in the Journal of the American Chemical Society on September 9, 2026.
Topological quantum materials host unusual electronic states alongside properties like magnetism. These materials are promising for future electronics and quantum technologies. The new approach applies the concept of homologous series-families of compounds built from the same structural units but with different layer counts-to this class of materials. This method has been used for functional oxides but was previously unexplored for topological quantum materials.
Building a New Homologous Series
The team focused on magnetic materials containing square-net layers. These layers are known to host topological Dirac electrons. Spacer layers, positioned between the square nets, provide magnetic functionality. By varying the number of these spacer layers, researchers can potentially alter both the magnetic and topological electronic properties.
They synthesized a previously unknown compound, Ce₃Au₄Ge₂Bi₄. This material contains two spacer layers between its square nets. This new compound bridges two known structures: the single-layer compound CeAuBi₂ and the infinite-layer compound CeAu₂Ge₂. This connection establishes a new homologous series based on identical structural building blocks.
Magnetic and Electronic Changes
Experiments revealed that the number of layers directly influences magnetic order. Neutron scattering showed that the single-layer material CeAuBi₂ has an antiferromagnetic order. In contrast, the newly created two-layer material Ce₃Au₄Ge₂Bi₄ exhibits ferrimagnetic order. A ferrimagnet has opposing magnetic moments that do not fully cancel, resulting in a spontaneous magnetization.
The electronic structure also transforms across the series. Theoretical calculations and high-magnetic-field measurements indicated that the Dirac bands in Ce₃Au₄Ge₂Bi₄ form what is known as a highly tilted type-II Dirac cone. This specific electronic configuration cannot occur for relativistic particles in a vacuum and is a distinct feature of the material's topology.
A Systematic Design Framework
Professor Hideaki Sakai explained the significance of the finding. "By changing the number of layers while keeping the essential square-net structure, we can tune magnetic and topological properties together," he said. The researchers argue this provides a framework for designing topological magnets, moving beyond a trial-and-error search for each new material.
The findings demonstrate that homologous-series design can be extended to topological quantum materials. The team suggests that combining layer-number control with elemental substitution could yield further ways to create materials with different combinations of magnetic and topological properties. These materials could have potential applications in spintronics and quantum technologies.
Future Research Directions
The researchers plan to investigate transport phenomena linked to the highly tilted type-II Dirac bands in their new material. They will also explore spintronic functions that arise from the coupling of these bands with the material's spontaneous magnetization. Spectroscopic measurements will probe the microscopic electronic structure in greater detail.
Guided by the new homologous-series framework, the team intends to synthesize compounds with additional crystal structures. Their goal is to search for novel combinations of topological electronic and magnetic states, systematically expanding the library of functional quantum materials.
