Cluster multipole theory for anomalous Hall effect in antiferromagnets

Here, the authors discover a missing link between antiferromagnetism and the Hall effect by introducing a theoretical framework based on a novel concept, cluster multipole (CMP), to characterize macroscopic magnetization of antiferromagnets. Whereas the anomalous Hall effect (AHE) is usually observed in ferromagnets and explained as an outcome of the macroscopic dipole magnetization, CMP theory reveals that a certain type of antiferromagnetic (AFM) structure induces the AHE despite no net magnetization. The new order parameters enable us to characterize the AHE in the AFM states and explain the AHE in the AFM states of Mn₃Ir and Mn₃Z (Z = Sn, Ge), for which the large AHE has recently been studied. Furthermore, the theory can deal with the AHE in antiferromagnets on an equal footing with that in simple ferromagnets. The authors compare the AHE in antiferromagnetic Mn₃Z Mn₃Z and ferromagnetic bcc Fe based on first-principles calculations and find out their similarity with respect to the CMP moments. The theory brings on a significant step forward in our current understanding of anomalous current in condensed matter, and the obtained knowledge could be crucial in the future for the design of antiferromagnetic devices, e. g. , with possible spintronics-related applications.