Pushing boundaries: Detecting the anomalous Hall effect without magnetization in a new class of materials

An international research team led by Mayukh Kumar Ray, Mingxuan Fu, and Satoru Nakatsuji from the University of Tokyo, along with Collin Broholm from Johns Hopkins University, has discovered the anomalous Hall effect in a collinear antiferromagnet. More strikingly, the anomalous Hall effect emerges from a non-Fermi liquid state, in which electrons do not interact according to conventional models. The discovery not only challenges the textbook framework for interpreting the anomalous Hall effect but also widens the range of antiferromagnets useful for information technologies. The findings were published in the journal Nature Communications.

Spins are intrinsic properties of electrons, typically described as being either “up” or “down”. In ferromagnets, spins align in the same direction, magnetizing the material. This magnetization can lead to a voltage perpendicular to the electric current even without an external magnetic field; this is the anomalous Hall effect. In contrast, antiferromagnets feature spins that are aligned in opposite directions, effectively canceling out magnetization. Thus, it should follow that the anomalous Hall effect does not emerge in antiferromagnets. Yet it does.

“There have been previous reports on the anomalous Hall effect appearing in a certain class of collinear antiferromagnets,” says Nakatsuji, the principal investigator. “However, the observed signals were extremely weak. Identifying a truly magnetization-free anomalous Hall effect was of broad scientific and technological interest.”

This endeavor required coordination across various groups. Fu and her colleagues were responsible for the experimental setup to measure the effect. They used a family of materials called transition metal dichalcogenide (TMD) as two-dimensional (2D) building blocks. By inserting magnetic ions between the atomic layers, the researchers could control the movements and interactions of electrons. The modified structure, now in 3D, had the potential to exhibit new behaviors that could not have been possible in only 2D. At last, the researchers could make measurements of the anomalous Hall effect across a wide range of temperatures and magnetic fields. In addition, Broholm’s group’s provided microscopic evidence confirming the collinear antiferromagnetic structure of the material. The results were then combined with the theoretical analysis and calculations done by Ryotaro Arita’s group at UTokyo.

“One of the main challenges in our research project has been constructing a coherent scientific narrative from our observations,” says Fu, a co-lead of the paper. “Each step required careful interpretation, especially due to the structural disorder commonly found in transition metal dichalcogenide (TMD) systems.”

The resulting measurement is the first strong experimental evidence for the anomalous Hall effect observed in collinear antiferromagnets. As the anomalous Hall effect is commonly believed to go hand in hand with magnetization, the detection suggests that something far beyond the standard understanding is at play. Researchers suspect the phenomenon is rooted in the unique structure of the material’s electron bands, causing a large “virtual magnetic field” and boosting the anomalous Hall effect in the absence of magnetization. Nakatsuji explains the next steps.

“We are seeking experimental confirmation for this hypothesis and actively pursuing a range of follow-up studies using complementary techniques, including Raman spectroscopy, to uncover the underlying mechanisms.”

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