Physicists at Rice University have discovered that the magnetic compound ytterbium manganese dibismuthide, or YbMnBi₂, enters an unusual state resembling a magnetic liquid crystal when heated above its normal magnetic ordering temperature.
Physical Review X Study Reveals Directional Fluctuations
Published in Physical Review X, the study details how the research team heated YbMnBi₂ to dissolve its conventional magnetic order. Surprisingly, the magnetic spins did not randomize completely. Instead, they maintained preferences for specific directions while fluctuating.
“In a conventional magnet, once you heat above the magnetic ordering temperature, you expect the spins to lose their directional organization,” said Pengcheng Dai, the Sam and Helen Worden Professor of Physics and Astronomy at Rice. “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.”
Investigating the Anomalous Hall Effect in YbMnBi₂
When an external magnetic field is applied to an ordinary metal, electric currents generate a voltage known as the Hall effect, which typically vanishes when the field is removed. Certain magnetic materials retain a sideways voltage without an external field, a phenomenon called the anomalous Hall effect, which YbMnBi₂ displays to an unusually large degree.
Initial hypotheses suggested that tilted or canted magnetic spins created a Weyl state responsible for the voltage. However, neutron measurements conducted at the high flux isotope reactor and spallation neutron source at Oak Ridge National Laboratory revealed that the spins are collinear.
“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.”
Neutron Measurements Rule Out Spin Canting
To pinpoint the mechanism behind the anomalous Hall effect, the research team examined whether the directional spin fluctuations driving the magnetic liquid crystal state were also responsible for the electronic properties.
Researchers compared YbMnBi₂ with CaMnBi₂, a very similar material where nonmagnetic calcium replaces ytterbium. In CaMnBi₂, the unusual directional spin fluctuations vanished entirely, confirming that ytterbium is vital for the magnetic liquid crystal phase.
“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.”
Ytterbium Ions and Manganese Spins Interact Under Magnetic Fields
Further analysis indicated that specific ytterbium ions within YbMnBi₂ possess their own magnetic moments. These moments respond to external magnetic fields and interact directly with the fluctuating manganese spins.
Theoretical calculations demonstrated that the combination of ytterbium magnetic moments and directional manganese spin fluctuations generates a unique magnetic arrangement. This arrangement deflects moving electrons sideways, offering a novel explanation for the material’s pronounced 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.”
Frequently Asked Questions About YbMnBi₂ Research
What instruments were used to measure the spin structure of YbMnBi₂?
The team carried out neutron measurements at the high flux isotope reactor and spallation neutron source located at Oak Ridge National Laboratory.
How does CaMnBi₂ differ from YbMnBi₂ in experimental tests?
CaMnBi₂ replaces the element ytterbium with nonmagnetic calcium, which caused the unusual directional spin fluctuations to disappear during comparative testing.
Which funding agencies supported the Physical Review X study?
The research received support from the U.S. Department of Energy’s Basic Energy Sciences program under grants DE-SC0012311 and DE-SC0026179, alongside contributions from the Robert A. Welch Foundation, the National Research Foundation of Korea, JSPS KAKENHI, the RIKEN TRIP initiative, the Scientific Research Innovation CapabilitySupport Project for Young Faculty, the National Key R&D Program of China, and the National Natural Science Foundation of China.