Anisotropic propagation and topological transition of dipolar magnons in biaxial antiferromagnets

Coherent magnon transport is an efficient strategy to propagate spin information in spintronics and has been demonstrated to couple with superconducting qubits as a promising candidate of quantum transducer in hybrid quantum system. Among various magnets, antiferromagnets (AFMs) stand at the frontier of magnon transport, since AFMs can host magnons in terahertz frequency and is robust against external field. A recent measurement on 2D A-type AFM CrSBr shows a coherent magnon propagation beyond seven micrometers and nanosecond scale coherence time, but the magnon group velocity is anomalously large comparing to the prediction of linear spin wave theory [1], so that the mechanism of the magnon propagation remains an open question. Here, we demonstrate the magnetic dipole-dipole interaction plays an important role in the AFM magnon dispersion in the long-wavelength limit, known as the dipolar magnon. The dipolar magnon dispersion in a biaxial antiferromagnet matches perfectly with the anisotropic magnon propagation in CrSBr and quantitatively reproduces the group velocity. Furthermore, a topological transition of magnon bands is found in biaxial antiferromagnets by applying a magnetic field perpendicular to the easy axis. A symmetry-protected surface state is shown to propagate without back-scattering along the sample boundary, which opens the new possibility of dissipationless magnon transport.

[1] Y. J. Bae et al, Nature 609, 282-286 (2022).