Topological quantum materials combine unusual electronic states with properties such as magnetism or superconductivity, offering possibilities for future electronics and quantum technologies. Researchers at Tohoku University have now shown that changing the number of layers in a crystal can provide a systematic way to design topological magnets.

The strategy builds on homologous series, families of compounds made from the same structural units but with different numbers of layers. Such series have long been used to tune functional oxides, including high-temperature superconductors. Until now, a comparable layer-based design approach had not been established for topological quantum materials.

A team led by Professor Hideaki Sakai of Tohoku University's Institute for Materials Research focused on magnetic materials containing square-net layers. These layers host topological Dirac electrons, while spacer layers provide magnetic functionality. Varying the number of spacer layers can therefore change both properties.

The team synthesized Ce₃Au₄Ge₂Bi₄, a previously unknown compound containing two spacer layers between its square nets. The material connects the known single-layer compound CeAuBi₂ with the infinite-layer compound CeAu₂Ge₂, forming a new homologous series based on the same structural building blocks.

Neutron scattering experiments showed that layer number changes the magnetic order. The single-layer material has antiferromagnetic order, whereas Ce₃Au₄Ge₂Bi₄ exhibits ferrimagnetic order. In a ferrimagnet, opposing magnetic moments do not completely cancel, resulting in spontaneous magnetization.

The researchers also examined how the electronic structure changes across the series. Theoretical calculations and high-magnetic-field measurements revealed that the Dirac bands in Ce₃Au₄Ge₂Bi₄ form a highly tilted type-II Dirac cone, an electronic configuration that cannot occur for relativistic particles in vacuum.

"By changing the number of layers while keeping the essential square-net structure, we can tune magnetic and topological properties together," says Hideaki Sakai. "This gives us a framework for designing topological magnets rather than searching for each new material independently."

The findings show that homologous-series design can be applied to topological quantum materials. Combining layer-number control with elemental substitution could provide additional ways to create materials with different combinations of magnetic and topological properties, with potential relevance to spintronics and quantum technologies.

The researchers will next investigate transport phenomena associated with the highly tilted type-II Dirac bands and explore spintronic functions arising from their coupling with spontaneous magnetization. Spectroscopic measurements will also probe the microscopic electronic structure.

Guided by the homologous-series framework, the team plans to synthesize compounds with additional crystal structures and search for new combinations of topological electronic and magnetic states.

The study was published in the Journal of the American Chemical Society on July 1, 2026.