Tohoku researchers unlock layer-based design strategy for topological magnets

For decades, materials scientists have hunted for topological quantum materials—substances that combine unusual electronic behavior with magnetism or superconductivity—hoping they might power the next generation of quantum computers and advanced electronics. The search has been largely trial-and-error, each new candidate a gamble. Now researchers at Tohoku University have found a systematic way to design these materials by simply changing how many layers a crystal contains.

The breakthrough builds on a principle that has worked well in other fields. Chemists have long used homologous series—families of compounds built from identical structural units but stacked in different numbers—to fine-tune the properties of functional oxides, including the high-temperature superconductors that revolutionized materials science in the 1980s. No one had successfully applied this layer-based strategy to topological quantum materials until now.

Professor Hideaki Sakai and his team at Tohoku University's Institute for Materials Research focused on magnetic materials containing square-net layers, atomic arrangements that host topological Dirac electrons—exotic quantum states that behave in ways impossible for ordinary particles. Between these square nets sit spacer layers that provide magnetic properties. The insight was simple but powerful: by varying how many spacer layers exist between the square nets, the researchers could tune both the electronic and magnetic behavior simultaneously.

To test the idea, the team synthesized Ce₃Au₄Ge₂Bi₄, a compound that had never been made before. This material contains two spacer layers between its square nets, positioning it as a bridge between two known compounds: CeAuBi₂, which has a single spacer layer, and CeAu₂Ge₂, which has infinite layers. Neutron scattering experiments revealed that this intermediate compound behaves magnetically in a fundamentally different way. The single-layer material exhibits antiferromagnetic order, where magnetic moments point in opposite directions and cancel each other out. The new compound, by contrast, shows ferrimagnetic order—opposing moments that do not fully cancel, leaving behind a spontaneous magnetization that can be measured and potentially harnessed.

The electronic structure changed as well. Using theoretical calculations and high-magnetic-field measurements, the researchers discovered that the Dirac bands in Ce₃Au₄Ge₂Bi₄ form what physicists call a highly tilted type-II Dirac cone, an electronic configuration that cannot exist for relativistic particles in a vacuum. This exotic state emerges only in specially engineered materials, and its presence suggests new physics waiting to be explored.

Sakai described the significance plainly: by keeping the essential square-net structure intact while varying the number of layers, the team could tune magnetic and topological properties together. This transforms materials discovery from a search for needles in a haystack into a methodical design process. The homologous-series framework provides a roadmap. Researchers can now predict which combinations of properties might emerge at different layer numbers, then synthesize and test those specific materials rather than hoping to stumble upon them.

The implications extend beyond topological magnets alone. Combining layer-number control with elemental substitution—swapping one atom for another in the crystal structure—could open up even more possibilities for creating materials with tailored combinations of magnetic and topological properties. Such materials could prove essential for spintronics, a field that exploits electron spin to store and process information, and for quantum technologies that rely on precisely controlled quantum states.

The team's next steps are already mapped out. They plan to investigate how electrons actually move through the highly tilted Dirac bands and how those movements couple with the spontaneous magnetization. Spectroscopic measurements will probe the microscopic electronic structure in finer detail. Guided by the homologous-series framework, they will synthesize compounds with additional crystal structures and hunt for new combinations of topological and magnetic states. The study appeared in the Journal of the American Chemical Society on July 1, 2026.