Ytterbium manganese dibismuthide, or YbMnBi₂, is a magnetic compound that Rice University physicist Pengcheng Dai and his team found can enter an unusual state: a magnetic liquid crystal. In a recent study published in Physical Review X, the researchers heated YbMnBi₂ until its normal magnetic order disappeared. Surprisingly, its magnetic spins did not become completely random. Even while fluctuating, they still preferred certain directions.
“In a conventional magnet, once you heat above the magnetic ordering temperature, you expect the spins to lose their directional organization,” said Dai, the Sam and Helen Worden Professor of Physics and Astronomy. “What we find in YbMnBi₂ is different. The magnetic order disappears, but the fluctuating spins still prefer certain directions. In this sense, the system behaves like a magnetic liquid crystal.”
When a magnetic field is applied to an ordinary metal, an electric current can produce a voltage across the material, known as the Hall effect. Usually, that voltage disappears when the magnetic field is removed. In some magnetic materials, however, a sideways voltage can remain even without an external magnetic field. This is called the anomalous Hall effect, and YbMnBi₂ shows an unusually large one. The researchers wanted to understand why.
One proposed explanation was that the magnetic spins in YbMnBi₂ were slightly tilted, or canted, creating an unusual electronic state known as a Weyl state. But the Rice team’s neutron measurements, carried out at high flux isotope reactor and spallation neutron source at Oak Ridge National Laboratory, showed that the spins are essentially aligned in straight lines, ruling out this proposed explanation for the bulk material.
“Several proposed explanations for the anomalous Hall effect require the magnetic spins to be canted or tilted,” said Yaofeng Xie, a Rice graduate student and co-first author of the study. “Our neutron measurements showed that the spins in YbMnBi₂ are essentially collinear, so we had to look for another explanation.”
The researchers then asked whether the same unusual spin behavior that creates the magnetic liquid crystal could also be connected to the anomalous Hall effect.
To test the role of ytterbium, the team compared YbMnBi₂ with CaMnBi₂, a very similar material in which ytterbium is replaced by nonmagnetic calcium. In CaMnBi₂, the unusual directional spin fluctuations disappeared. This showed that ytterbium is essential to the magnetic liquid crystal state.
“This comparison gave us an important clue,” said Sijie Xu, a Rice graduate student and co-first author. “When we replaced ytterbium with calcium, the unusual directional behavior disappeared. That told us that ytterbium plays an essential role.”
Further measurements showed that some of the ytterbium ions in YbMnBi₂ carry their own magnetic moments. These moments can respond to an applied magnetic field and interact with the fluctuating manganese spins.
The researchers’ theoretical calculations showed how these two ingredients can work together. Under a magnetic field, the magnetic ytterbium moments and the directional fluctuations of the manganese spins can create a special magnetic arrangement that pushes moving electrons sideways, providing a possible new explanation for the anomalous Hall effect.
“The key is that the ytterbium moments and the unusual manganese spin fluctuations work together,” Dai said. “Their interaction can change how electrons move through the material and may explain the large anomalous Hall effect.”
The work was supported by the U.S. Department of Energy’s Basic Energy Sciences program (DE-SC0012311, DE-SC0026179), the Robert A. Welch Foundation (C-1839-663), the National Research Foundation of Korea (RS-2021-NR060140, RS-2025-16065011, RS-2025-25441317), JSPS KAKENHI (24H00197, 24H02231), the RIKEN TRIP initiative, the Scientific Research Innovation CapabilitySupport Project for Young Faculty (SRICSPYF-675 ZY2025076), the National Key R&D Program of China (2025YFF0524500) and the National Natural Science Foundation of China (52401263).