This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In some materials, physical properties don't emerge from their individual particles, but from the collective behavior of their quantum spins. Now, researchers led by Pengcheng Dai at Rice University in the U.S. have discovered that in one compound containing the rare-earth element ytterbium, these spins can behave much like the molecules in a liquid crystal: favoring a certain direction without lining up to create magnetism on larger scales.
Their research has been published in Physical Review X. Spin is a quantum property of particles including electrons, essentially turning them into tiny bar magnets pointing in specific directions. When multiple spins interact, the patterns they form can strongly influence how a material conducts electricity and heat.
In ordinary magnets, spins line up to create an overall magnetization—but this order breaks down above a certain temperature, leaving the spins pointing in random directions. Previous studies suggested that some magnetic compounds may host an in-between "spin nematic" state. Much like the rod-shaped molecules in a liquid crystal display, the spins share a preferred direction, but without forming magnetic order.
Dai's team considered whether this state could exist in YbMnBi₂, a compound made from ytterbium, manganese and bismuth. In a magnetic field, currents of electricity and heat flowing through the material are pushed sideways far more strongly than expected, even at temperatures where its magnetic order has vanished. So far, this behavior has lacked a convincing explanation.
To investigate, the researchers fired beams of neutrons at crystals of YbMnBi₂ and at a closely related compound where ytterbium is replaced by calcium. Since neutrons carry spins of their own, the team could control their orientation, then measure how they scattered off the spins inside each crystal. This allowed them to compare how the spins fluctuated along different directions while varying the temperature and the strength of an applied magnetic field.
As they cooled the ytterbium compound from 450 K (about 177°C), the team found that at around 400 K (260°F), its manganese spins began fluctuating more strongly in some directions than others—even though the material was still too warm for magnetic order to set in at around 290 K (62°F). The calcium compound showed no sign of this effect, providing strong evidence for a fluctuating spin nematic state. Based on their result, Dai's team now proposes that the state arises because the heavy ytterbium atoms tie the motion of electrons closely to their spins.
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