Scientists find rare-earth compound with liquid crystal-like magnetic state

A strange magnetic state inside a compound called YbMnBi₂ could help explain one of its most unusual electrical properties.

Rice University researchers found that the material retains directional spin fluctuations even after conventional magnetic order disappears. The finding points to a previously overlooked connection between magnetic behavior and the material’s large anomalous Hall effect.

The work challenges an earlier explanation involving tilted magnetic spins. Neutron experiments instead found that the manganese spins remain essentially aligned. The team now points to interactions between manganese and ytterbium as a possible source of the unusual Hall response.

Spins keep their direction

Most magnetic materials lose their organized spin structure after heating past their magnetic ordering temperature. YbMnBi₂ does something more unusual.

Its manganese spins continue fluctuating in preferred directions after long-range magnetic order fades. That behavior resembles a liquid crystal, where molecules can move freely but still maintain directional alignment.

Rice physicist Pengcheng Dai and his team call the state a magnetic liquid crystal. Their measurements came from neutron experiments at Oak Ridge National Laboratory’s High Flux Isotope Reactor and Spallation Neutron Source.

Those measurements also addressed a long-standing question about YbMnBi₂. Some researchers had proposed that tilted spins could create a Weyl state responsible for the material’s unusual electrical response. The neutron data did not support that picture in the bulk material.

“Several proposed explanations require the magnetic spins to be canted,” said Yaofeng Xie, a Rice graduate student and co-first author. “Our measurements showed that the spins are essentially collinear.”

Ytterbium changes the picture

The researchers then tested what happens when they remove the magnetic ytterbium component. They compared YbMnBi₂ with CaMnBi₂, which replaces ytterbium with nonmagnetic calcium. The directional spin fluctuations vanished in the calcium-based material.

That comparison gave the team evidence that ytterbium plays a key role in the unusual magnetic state. Further measurements showed that some ytterbium ions carry magnetic moments of their own.

Those moments can respond to an applied magnetic field and interact with manganese spins. The team’s calculations suggest that this interaction could also affect how electrons travel through YbMnBi₂.

New route to Hall effect

The Hall effect normally produces a sideways voltage when current flows through a material under a magnetic field. Magnetic materials can generate a related response even without an external field.

YbMnBi₂ produces an especially large anomalous Hall effect. The researchers now propose that its unusual magnetic fluctuations may help explain why.

Their model links the ytterbium moments with the directionally fluctuating manganese spins. An applied magnetic field could organize those interactions in a way that deflects moving electrons.

“The key is that the ytterbium moments and manganese spin fluctuations work together,” Dai said. Their interaction could change electron motion and contribute to the large Hall response. The result gives researchers a different way to examine anomalous Hall effects. It also shows that magnetic disorder does not always mean complete randomness.

In YbMnBi₂, the spins lose long-range order but retain directional preferences. That combination creates a magnetic state that behaves more like a liquid crystal than a conventional heated magnet.

The study is published in the journal Physical Review X.