Emission of fast-propagating spin waves by an antiferromagnetic domain wall driven by spin current

Antiferromagnets (AFMs) have great benefits for spintronic applications such as high frequencies (up to THz), high speeds (up to tens of km/s) of magnetic excitations, and field-free operation. Advanced devices will require high-speed propagating spin waves (SWs) as signal carriers, i.e., SWs with high $k$ vectors, the excitation of which remains challenging. We show that a domain wall (DW) in anisotropic AFM driven by the spin current can be a source of such propagating SWs with high frequencies and group velocities. In the proposed generator, the spin current, with polarization directed along the easy anisotropy axis, excites the precession of the Néel vector within the DW. The threshold current is defined by the value of the anisotropy in the hard plane, and the frequency of the DW precession is tunable by the strength of the spin current. We show that the above precession of spins inside the DW leads to robust emission of high-frequency propagating SWs into the AFM strip with very short wavelengths comparable to the exchange length, which is hard to achieve by any other method.

Figure 1

Schematic representation of the proposed ultrashort SW generator. The spin-torque source (shown by a dark blue bar with width $L$) is positioned at the device's center beneath the AFM DW. Charge current flow through a heavy metal leads to spin-current injection at the interface (due to the spin-Hall effect) with polarization $\mathbf{p}$ aligned with the easy axis. The spin-current application induces precession of the Néel vector within the DW in the hard plane with $n$-fold symmetry. This precession results in the emission of SWs, as schematically shown in the lower-left-hand corner.

Figure 2

Potential $U_{\omega }^{\pm }$ created by a precessing DW for SWs, given by Eq. (6), for a different frequency $\omega$ of a DW precession. Solid lines correspond to the positive sign of a SW frequency, while dashed lines correspond to the negative sign.

Figure 3

The results of simulations for twofold anisotropy in a hard AFM plane, $n=2$. (a) The frequency of the DW rotation (open circles) and emitted SWs (solid circles) are shown as a function of the applied current density for different values of $K_2$ anisotropy. Solid black lines are calculated analytically using Eq. (2). Angular frequency labels are employed for simplicity and correspond to the respective rotational frequencies $f=\omega /2\pi$. (b) The profiles of emitted SWs far from a DW with the extracted values of the wavelengths: $\lambda =71$ nm (red), 40 nm (green), 25 nm (blue). The applied current density and frequencies of the displayed SWs are indicated by arrows of matching colors in (a).

Figure 4

The results of simulations for fourfold anisotropy in a hard AFM plane, $n=4$. (a) The frequency of the DW rotation (open circles) and emitted SWs (solid circles) are shown as a function of the applied current density for $K_4/K=2\times {10}^{-2}$. Solid black lines are calculated analytically using Eq. (2). (b) Group velocity and (c) wavelength of the excited SWs as a function of the applied current density.