Antiferromagnetic skyrmion as a magnonic lens

TL;DR

This work addresses spin-wave control by exploiting natural magnetic textures, showing that an antiferromagnetic skyrmion can function as a magnonic lens. The method maps spin-wave dynamics to magnon kinetics in a DM-induced pseudo-magnetic field $b=b^0+b^D$, and identifies a threshold $D_t$ above which focusing occurs. Key findings include a focal point on the skyrmion's flank, a focal length that scales with spin-wave frequency and increases with DM strength, and the ability to realize a coaxial lens set by two skyrmions. This demonstrates a new approach to shaping magnons purely through the DM interaction, enabling compact, energy-efficient magnonic devices.

Abstract

A lens, a device transforming propagation directions in an organized fashion, is one of the fundamental tools for wave manipulation. Spin wave, the collective excitation of ordered magnetizations, stands out as a promising candidate for future energy-saving information technologies. Here we propose theoretically and verify by micromagnetic simulations, that an antiferromagnetic skyrmion naturally serves as a lens for spin wave, when the Dzyaloshinskii-Moriya strength exceeds a threshold. The underlying mechanism is the spin wave deflection caused by Dzyaloshinskii-Moriya interaction, a mechanism that is ordinarily overshadowed by the magnetic topology.

Figures (6)

• Figure 1: Schematics of a skyrmion-based magnonic lens. (a) Spin wave transformation through an antiferromagnetic skyrmion. Red and blue arrows denote two sublattice magnetizations of an antiferromagnetic skyrmion, and stripes indicate the spin wave with solid/dashed lines depict the isophase lines. (b) Magnon trajectories across pseudo-magnetic field induced by the magnetic skyrmion. The background blue/red colors encode the pseudo-field of opposite polarities induced by the skyrmion. Orange/green lines represent the magnon trajectories with pseudo-charges of sign $\sigma=\pm 1$.
• Figure 2: Spatial profiles of magnetization configuration and pseudo-magnetic field within an antiferromagnetic skyrmion. (a) Magnetization distribution in an antiferromagnetic skyrmion. Red and blue arrows indicate the sublattice magnetizations, and blue shading indicates the skyrmion region. Black dashed lines indicate the radial direction $\mathbf r$, orange/green dashed lines represent the characteristic radius $R$ and width $W$. (b-d) Cross-sectional distributions of magnetization parameter $\Theta$ and the pseudo-magnetic field. Lines are for theoretical modeling in Eq. \ref{['eqn:skyrmion_profile']} and Eq. \ref{['eqn:b_fields']}, and dots are extracted from micromagnetic simulations. (e-g) Spatial distributions of the topological component $b^0$, the chiral component $b^D$ and the total pseudo-magnetic field $b$. Red/blue background color encode the positive/negative values of the pseudo-magnetic field.
• Figure 3: Deflection of magnons with positive pseudo-charge $\sigma=+1$ and the corresponding scattering of right-circular spin wave by antiferromagnetic skyrmions of polarity $\chi=+1$ (a-c) and $\chi=-1$ (d-f). In (a, d), red/blue color encodes the positive/negative values of pseudo-magnetic fields induced by skyrmion. In (b, e), red/blue color encodes the spin wave $\delta n_x$, the orange color represents the magnitude of polarized flux $j$, and red/blue circles sketch the landscape of pseudo-magnetic field in (a, d). In (c, f), red/blue lines plot the positive/negative values of the path-integrated field $B(y)$, respectively.
• Figure 4: Magnon lensing by antiferromagnetic skyrmions. (a-d) Spin wave propagation modulated by a single skyrmion. (e, f) Spin wave propagation modulated by a skyrmion pair. In all panels, the background orange/green colors encode the spin wave fluxes of left/right-circular components extracted from the micromagnetic simulations, orange/green lines represent the magnon rays with pseudo-charge $\sigma = \pm 1$, and red/blue circles sketches the landscapes of the pseudo-magnetic fields.
• Figure 5: Characteristic lengths and pseudo-magnetic field patterns as function of DM strength $D$. Orange/green lines are for the skyrmion radius $R$ and the chiral length $\xi$. In upper insets, the red/blue arrows depict two sublattice magnetizations. In lower insets, the red/blue colors encodes the positive/negative values of the pseudo field.