Higher-Order Spin Magnetic Multipole Moments Relevant to Antiferromagnetic Spintronics

Antiferromagnets, due to their robustness against stray fields and ultrafast dynamics, are getting increasing attention from the condensed matter community as promising candidates for next generation spintronics. While conventional spintronics focuses on the spin degree of freedom only, harnessing higher order moments of spin is expected to reveal interesting phenomena that are both conceptually important and useful for applications. For describing the transport of higher order moments of spin, it is necessary to define them for individual charge carriers. We generalize the gauge invariant formula for spin magnetic multipole moments [1] to the case of degenerate bands, which enables us to examine the quadrupole as the leading order moment, with the dipole contribution vanishing. We find that PT symmetry in antiferromagnets is usually sufficient for the quadrupole to be the leading moment. We propose concrete material platforms for creating and manipulating quadrupole moments based on van der Waals magnets.

References:

[1] M.Tahir, H.Chen, Phys. Rev. Lett. 131, 106701 (2023)