New phenomena of Hall effects in noncollinear spin systems

It has now been well established that noncollinear magnetic order enables unconventional mechanisms contributing to different variants of Hall effects, such as the topological Hall effect, the anomalous Hall effect, and the spin Hall effect, etc. In this talk I will mention two new phenomena related to the broadly defined Hall effects in noncollinear spin systems. The first is the different lengths of anomalous Hall vector for distinguishing discrete degeneracies of frustrated spin systems, represented by the recently discovered metallic kagome spin ice compound HoAgGe [1]. We find that energetically degenerate ice-rule states on magnetically induced plateaus of HoAgGe, which are related to each other by transformations involving lattice distortion reversal, can have very different transport properties [2]. The second is a new quantity that we coin as the "Hall mass" that governs the transverse response of spin currents in noncollinear antiferromagnets to longitudinal spatial gradients of the order parameter [3]. We find that the Hall mass generally exists in all noncollinear antiferromagnets, especially in their polycrystals. The Hall mass can lead to transversely flowing spin currents when spins are pumped from a ferromagnet into a noncollinear antiferromagnet, and can also be detected by spectroscopic experiments.

[1] K. Zhao, H. Deng, H. Chen, K. A. Ross, V. Petříček, G. Günther, M. Russina, V. Hutanu, and P. Gegenwart, Science 367, 1218 (2020).

[2] K. Zhao, Y. Tokiwa, H. Chen, and P Gegenwart, Nat. Phys. 20, 442 (2024).

[3] L. Wernert, B. Pradenas, O. Tchernyshyov, and H. Chen, arXiv:2404.12898.