Topological surface magnon-polariton in an insulating canted antiferromagnet

TL;DR

The paper addresses realizing and controlling topological surface magnons in insulating antiferromagnets by coupling magnons to photons on-chip in hematite. Using a semiclassical Landau-Lifshitz-Maxwell framework, it demonstrates a strong bulk magnon-photon coupling that opens a topological gap with a bulk invariant $C_+ = 1$, and predicts a nonreciprocal surface magnon-polariton confined to the hematite-vacuum interface. The surface mode resides in the bulk gap and features high group velocity and long propagation length, with tunability via external magnetic fields, suggesting practical routes for on-chip, long-range magnonic information transfer. Overall, the work provides a general principle for optomagnetic control in antiferromagnets and outlines concrete strategies for excitation and detection of topological surface states in hematite and related materials.

Abstract

Excitation and control of antiferromagnetic magnon modes lie at the heart of coherent antiferromagnetic spintronics. Here, we propose a topological surface magnon-polariton as a new approach in the prototypical magnonic material hematite. We show that in an insulating canted antiferromagnet, where strong-coupled magnon-photon modes can be achieved using electrical on-chip layouts, a surface magnon-polariton mode exists in the gap of the bulk magnon-photon bands. The emergence of surface magnon-polariton mode is further attributed to the nontrivial topology of bulk magnon-photon bands. Magnon-photon coupling enhances the Berry curvature near the anticrossing points, leading to a topological bulk Chern band associated with the surface magnon-polaritons. Our work provides a general principle for the utilization of optomagnetic properties in antiferromagnets, with an illustration of its experimental feasibility and wide generality as manifested in hematite.

Figures (7)

• Figure 1: Modelling bulk magnon-photon bands in the DMI-canted antiferromagnets. (a) The DMI canted model of hematite with the frame of reference setting, sublattices $\boldsymbol{m}_1$, $\boldsymbol{m}_2$, net magnetic moment $\boldsymbol{m}=(\boldsymbol{m}_1+\boldsymbol{m}_2 )/2$, Néel vector $\boldsymbol{n}=(\boldsymbol{m}_1-\boldsymbol{m}_2)/2$, and an oscillating magnetic field $\boldsymbol{b}$ coupling to the in-plane polarized precession. (b) Component-projected dispersion relations of magnon-polaritons. The blue (red) circles denote the photon (magnon) component at the given wavevector and frequency, where the radius of the circle is proportional to the ratio of the corresponding component. (c) Zoomed-in view of the dispersion relations in the region marked by the dashed box in (b), where the blue (red) dashed lines denote the photon (magnon) dispersion without anticrossing. We use parameters that give a coupling strength $2g = 1.9~\text{GHz}$ to qualitatively fit the previous experimental observations wang2025long.
• Figure 2: Distribution of frequency difference $(f_1 - f_2)$ and Berry curvature in reciprocal space. (a) Distribution of frequency difference in reciprocal space with a color scale. The minima occur around the anticrossing points. (b) Distribution of the Berry curvature in reciprocal space. The Berry curvature is enhanced around the minima of bulk band gaps.
• Figure 3: Reciprocal bulk magnon-polariton and nonreciprocal surface magnon-polariton. (a) The effective DOS (eDOS) of magnon-polariton with a bulk Ansatz (see Appendix \ref{['surface_A']}) which reproduce the magnon-photon band and its coupling strength in a color scale. The yellow color indicates a mode locating at the given wavevector and the frequency with a similar data process procedure in Ref. macedo2019engineering. (b) The eDOS of surface magnon-polariton with a finite evanescent wavevector $q$ and an $x$-direction propagation at the upper surface in a color scale with the same meaning of (a). The surface magnon-polariton appears within the bulk band gap and possesses nonreciprocity which can be tuned by the external magnetic field.
• Figure 4: Anisotropic coupling strength of the bulk magnon-photon bands. Dispersion relations showing the coupling strength along different wavevector directions: (a) for $\boldsymbol{k}//\hat{\boldsymbol{x}}$, (b) for $\boldsymbol{k}//\hat{\boldsymbol{y}}$ and (c) for $\boldsymbol{k}//\hat{\boldsymbol{z}}$. The minimal band gap occurs along the $k_y$ direction.
• Figure 5: Wavevector-dependent polarization of bulk and surface magnon-polaritons. The ratio of the semi-major to semi-minor axis of the precession ellipse of the net magnetic moment $\boldsymbol{m}$, plotted as a function of the wavevector $k$, quantitatively characterizing the polarization of the elliptical precession across different propagation directions.