A small squeeze reveals new clues about an unusual kind of magnet

Scientists at Rice University have uncovered new insights into the workings of a rare type of magnet by applying gentle pressure to a crystal of iron sulfide. Their research demonstrates that squeezing the material alters two of its unique properties simultaneously, its faint magnetic signal and the way electric currents flow through it. This discovery provides a more detailed understanding of altermagnets, a newly identified category of magnetic substances, and offers a straightforward method for manipulating their behavior. The study focused on a hexagonal variant of iron sulfide, or FeS, which exhibits characteristics that blend aspects of two well-known types of magnets. Similar to antiferromagnets, its internal magnetic fields largely cancel each other out, preventing the formation of a strong external magnetic field. However, unlike traditional antiferromagnets, FeS still influences the movement of electrons in ways that might prove beneficial for future electronic technologies. One notable feature of FeS is its production of an unusual electrical phenomenon called the anomalous Hall effect, where a small voltage develops perpendicular to the flow of current, even in the absence of an external magnetic field. Pengcheng Dai, Rice’s Sam and Helen Worden Professor of Physics and Astronomy and a co-author of the study, explained that the simultaneous observation of the material’s weak magnetic signal and the anomalous Hall effect under pressure provides crucial insight into their relationship. “When we apply pressure to the crystal in one direction, both signals decrease in unison,” he said. “That indicates the two phenomena are closely related.” To conduct the experiment, the researchers constructed a specialized device capable of applying controlled compression to the crystal. As the pressure increased, the material’s residual magnetic moment weakened alongside the anomalous Hall effect. However, the broader magnetic order within the material remained largely unaffected. To investigate further, the team employed neutron beam analysis at Oak Ridge National Laboratory, revealing that while the fundamental magnetic structure stayed intact, the pressure altered the prevalence of different magnetic orientations within the crystal. In essence, the crystal possesses multiple nearly identical configurations for its magnetic moments to align. Due to the minimal energy differences between these configurations, even slight pressure can influence which orientations dominate. This characteristic makes FeS particularly responsive to mechanical tuning. The findings also contribute to a longstanding debate regarding the cause of the anomalous Hall effect in FeS. While one prevailing theory attributes the effect to the movement of electrons through the material’s electronic structure, a concept described by physicists using the term “Berry curvature”, the Rice experiments suggest a connection between the effect and the material’s minute magnetic moment. “We aren’t dismissing the electronic explanation,” said Weiliang Yao, lead author of the study. “Our results indicate that the tiny magnetic moment and the anomalous Hall effect are strongly correlated in this material. Determining precisely why they are linked is now a key area of investigation.” The potential to manipulate these effects using minimal mechanical force could hold promise for the emerging field of spintronics, which seeks to harness the magnetic properties of electrons for data storage and processing. Such devices might offer advantages over conventional technologies by reducing unwanted magnetic interference and lowering energy consumption. Dai emphasized the importance of controllability in potential applications. “For practical uses, having the ability to fine-tune these properties is critical,” he noted. “This experiment illustrates that a relatively simple mechanical adjustment can significantly impact the material’s behavior.”