Universal spin wavepacket transport in van der Waals antiferromagnets

TL;DR

This paper reveals that in van der Waals antiferromagnets, spin wavepackets propagate primarily through magnetic dipole-dipole coupling, a universal mechanism dictated by Maxwell's equations, crucial for future quantum information applications.

Contribution

The study demonstrates that long-range dipole-dipole interactions, not elastic modes, govern spin wavepacket propagation in vdW antiferromagnets, providing a universal understanding of the mechanism.

Findings

Propagation driven by magnetic dipole-dipole coupling

Mechanism is a consequence of Maxwell's equations

Implications for optimizing vdW magnonic devices

Abstract

Antiferromagnets (AFMs) are promising platforms for the transmission of quantum information via magnons (the quanta of spin waves), offering advantages over ferromagnets with regard to dissipation, speed of response, and immunity to external fields. Recently, it was shown that in the insulating van der Waals (vdW) semiconductor, CrSBr, strong spin-exciton coupling enables readout of magnon density and propagation using photons of visible light. This exciting observation came with a puzzle: photogenerated magnons were observed to propagate 10$^3$ times faster than the velocity inferred from neutron scattering, leading to a conjecture that spin wavepackets are carried along by coupling to much faster elastic modes. Here we show, through a combination of theory and experiment, that the propagation mechanism is, instead, coupling within the magnetic degrees of freedom through long range dipole-dipole coupling. This mechanism is an inevitable consequence of Maxwell's equations, and as such, will dominate the propagation of spin at long wavelengths in the entire class of vdW magnets currently under intense investigation. Moreover, identifying the mechanism of spin propagation provides a set of optimization rules, as well as caveats, that are essential for any future applications of these promising systems.