On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.02.2021 - 15:00 German Time

Chiral magnetism: a geometric perspective

Oleg Tchernyshyov, Johns Hopkins University

Chiral ferromagnets have spatially modulated magnetic order exemplified by helices, spirals, and more complex patterns such as skyrmion crystals. The theoretical understanding of these states is based on a competition of a strong Heisenberg exchange interaction favoring uniform magnetization and a weaker Dzyaloshinskii-Moriya (DM) interaction promoting twists in magnetization. We offer a geometric approach, in which chiral forces are a manifestation of curvature in spin parallel transport [1]. The resulting theory is a gauged version of the Heisenberg model, with the DM vectors serving as background SO(3) gauge fields. This geometrization of chiral magnetism is akin to the treatment of gravity in general relativity, where gravitational interactions are reduced to a curvature of spacetime. The geometric perspective provides a simple way to define a conserved spin current in the presence of spin-orbit interaction. The gauge-dependent nature of the DM term raises questions about its recently proposed linear dependence on the (gauge-independent) spin current [2-3]. We also show that the gauged Heisenberg model in d=2 has a skyrmion-crystal ground state for a magic value of an applied magnetic field.

[1] D. Hill, V. Slastikov, and O. Tchernyshyov, arXiv:2008.08681.
[2] T. Kikuchi, T. Koretsune, R. Arita and G. Tatara, Phys. Rev. Lett. 116, 247201 (2016).
[3] F. Freimuth, S. Blügel, and Y. Mokrousov, Phys. Rev. B 96, 054403 (2017).

PDF file of the talk available here