First discovered by Edwin Hall in 1881, the anomalous Hall effect describes how the charges of currents change when a perpendicular magnetic field is applied. A century-long expedition in both experiment and theory has finally led us to a comprehensive microscopic understanding of the origin of the Hall effect. Just when the Hall effect was almost fully understood, new surprises came.
In 2014, Hua Chen with Qian Niu and Allan MacDonald at UT Austin, theoretically predicted that there can be large anomalous Hall effect in certain antiferromagnets, in which the total magnetization vanishes . Given the conventional wisdom about these effects, this was initially unexpected. In this more recent paper, a team formed by Prof. Zhi-Qi Liu at Beihang University and his colleagues on the experiment side, and Hua Chen (CSU) and Allan MacDonald (UT Austin) on the theory side, have successfully demonstrated the anomalous Hall effect in Mn3Pt, one of the first anomalous Hall antiferromagnets predicted in the 2014 paper. Moreover, Mn3Pt has material properties that enable this anomalous Hall effect to be controllably switched on and off though multiple techniques. This is the first time that electric switching of the anomalous Hall effect is realized in an antiferromagnet and this discovery may lead to novel device concepts in future electronics and spintronics.
Z. Q. Liu, Hua Chen, J. M. Wang, J. H. Liu, K. Wang, Z. X. Feng, H. Yan, X. R. Wang, C. B. Jiang, J. M. D. Coey, and A. H. MacDonald, “Electrical switching of the topological anomalous Hall effect in a non-collinear antiferromagnet above room temperature”, Nature Electronics, 1, 172-177 (2018). Accompanying News & Views by Christoph Sürgers. Hua Chen, Qian Niu, and Allan H. MacDonald, “Anomalous Hall effect arising from noncollinear antiferromagnetism”, Phys. Rev. Lett. 112, 017205 (2014).
The Landau-Lifshitz-Gilbert (LLG) equation is routinely used to study magnetization dynamics. LLG reflects the fact that magnetization and the angular momentum of electrons are really the same thing. In most cases, the quantum mechanical underpinnings of this equation can be approximated away to get a “classical” equation. However, in certain cases, the quantum mechanical nature of the underlying electrons contributing to the magnetism will manifest in the magnetization dynamics. In this paper, the authors provide examples of nontrivial modifications to the classical equation due to the dynamic coupling between magnetization and the electrons. These aren’t just details, as this theory points the way to new potential device architectures in information storage, processing, and retrieval.
Bangguo Xiong, Hua Chen, Xiao Li, and Qian Niu, “Geometric Dynamics of Magnetization: Electronic Contribution”, Phys. Rev. B 98, 035123 (2018)