High-speed antiferromagnetic domain walls driven by coherent spin waves

TL;DR

This work demonstrates that ultrafast circularly polarized light can generate coherent in-plane spin waves in an easy-plane antiferromagnet to drive domain-wall motion. In Sr2Cu3O4Cl2, antiphase Néel walls are created and their spatiotemporal dynamics are mapped with time-resolved SHG, revealing wall velocities up to about 50 km/s driven by coherent magnons near 2.8 GHz. The motion is bidirectional, governed by the pump helicity and the wall winding (w = -1/2), and reproduced by a one-dimensional φ(x,t) model with a trapping potential that returns the wall to equilibrium. This mechanism applies to other easy-plane AFMs and offers a pathway to ultrafast coherent AFM spintronics without Joule heating.

Abstract

The ability to rapidly manipulate domain walls (DWs) in magnetic materials is key to developing novel high-speed spintronic memory and computing devices. Antiferromagnetic (AFM) materials present a particularly promising platform due to their robustness against stray fields and their potential for exceptional DW velocities. Among various proposed driving mechanisms, coherent spin waves could potentially propel AFM DWs to the magnon group velocity while minimizing dissipation from Joule heating. However, experimental realization has remained elusive due to the dual challenges of generating coherent AFM spin waves near isolated mobile AFM DWs and simultaneously measuring high-speed DW dynamics. Here we experimentally realize an approach where ultrafast laser pulses generate coherent spin waves that drive AFM DWs and develop a technique to directly map the spatiotemporal DW dynamics. Using the room-temperature AFM insulator Sr$_2$Cu$_3$O$_4$Cl$_2$, we observe AFM DW motion with record-high velocities up to ~50 km/s. Remarkably, the direction of DW propagation is controllable through both the pump laser helicity and the sign of the DW winding number. This bidirectional control can be theoretically explained, and numerically reproduced, by the DW dynamics induced by coherent spin waves of the in-plane magnon mode - a phenomenon unique to magnets with an easy-plane anisotropy. Our work uncovers a novel DW propulsion mechanism that is generalizable to a wide range of AFM materials, unlocking new opportunities for ultrafast coherent AFM spintronics.

Figures (12)

• Figure 1: Creation and positioning of wide antiphase Néel DWs.a, Crystal and magnetic structure of the Cu3O4 plane of Sr2Cu3O4Cl2 below TN,I. b, Creation of an antiphase DW at a 90 DW. The bottom row shows SHG images of the process at different $H$ and horizontal laser positions ($x_\text{laser}$), while the top row shows a schematic of the domain configuration. The resulting antiphase DW configuration is depicted in the bottom right. The imaging laser is horizontally polarized (along $x$ axis). c, SHG images of an antiphase DW at different $x_\text{laser}$. d, SHG image of antiphase DW with vertically polarized laser. Rotational anisotropy polar plots of the SHG intensity (for P-polarized input and output electric field polarization measured as a function of the scattering plane angle $\varphi$) are shown for selected locations. $\varphi=0$ corresponds to the $x$ direction. The solid lines are fits to a coherent superposition of crystallographic electric quadrupole (EQ) and AFM-induced magnetic dipole (MD) SHG processes, as described in ref. seylerDirectVisualization2022. The EQ and MD processes shown in the figure represent the Pin-light-induced nonlinear polarization projected along Pout. Filled and unfilled lobes indicate opposite phases. The DW SHG pattern is fit to a three-domain averaged expression, as described in Supplementary Section 1. e, Line cut of the SHG image intensity perpendicular to an antiphase DW. The solid line shows a fit to the SHG intensity of a DW profile (see Methods) with an extracted DW width $\pi\mathit{\lambda}=8.98\pm 0.22\um$
• Figure 2: Observation of helicity-dependent light-driven DW motion.a, Schematic of pump-probe SHG imaging experiment. The pump beam is focused obliquely on the sample at ∼ 10 angle of incidence, while the SHG imaging probe is near-normal incidence. b, Schematic of the DW position at different times before and after each pulse of pump excitation train. c, SHG images of an antiphase DW at selected pump-probe time delays for linear, left circular, and right circular pump polarizations. The dashed oval indicates the pump excitation spot, which is offset from the DW center. The DW configuration is displayed on the right. d, Extracted dynamics of the DW shape for left and right circular pump. The solid curves are fits to a Voigt profile. The data at different delays are vertically offset for clarity. e, Pump photon polarization dependence of the maximum DW position (at $x=0$) for different time delays. The horizontal axis runs from $\theta=\ang{-90}$ to $\theta=\ang{90}$, where $\theta$ is the linear polarization angle before entering a quarter-wave plate with fixed fast axis at 0. The solid curves are fits proportional to $\sin(2\theta)$. f, SHG images at $t=200\ps$ for different pump photon helicities and DW winding numbers $w$. The dashed lines indicate the initial DW position at $t < 0$. The additional intensity outside the DW location arises due to scattered SHG from the pump beam as well as a contribution from spin wave precession for $t>0$ (see main text). The vertical scale of each image is 20.
• Figure 3: Mechanism of light-driven DWs.a, Schematic of Néel-type DW at different times ($t_0$, $t_1$, $t_2$) after clockwise rotations of $\mathbf{n}$ for negative (left panel) and positive (right panel) winding number. b, Schematic of coherent in-plane magnon induced by a circularly polarized laser pulse. $\mathbf{m}_1$ and $\mathbf{m}_2$ indicate the sublattice moments and $m_z$ is the out-of-plane moment induced by laser. The Néel vector $\mathbf{n} = (\mathbf{m}_2 – \mathbf{m}_1)/2$ initially rotates in the clockwise direction and its full trajectory is a back-and-forth oscillation in the $xy$ plane (inset). c, SHG transients for linear pump excitation with the probe beam linearly polarized in the $\hat{x}$ (circular markers) or $\hat{y}$ (square markers) direction, giving SHG proportional to $n_y^2$ or $n_x^2$ respectively. The black curves are guides to the eye. d, Same as c but with circularly polarized pump. The black curves are fitted to the square of a damped sine wave: $I_\text{SHG}=I_0(\sin{(2\pi f t+\phi_0)}e^{-t/\tau})^2$, where $I_0$ is the intensity, $f$ is the frequency of oscillation, $\phi_0$ is a phase offset, and $\tau$ is the decay time constant. e, Extracted change in Néel vector angle, $\Delta\phi(t)$, using the case of circular pump with $\hat{x}$ probe in d. Since the SHG cannot distinguish between positive and negative $\Delta\phi$, we have chosen the signs of the data to give agreement with $f\approx 2.8\GHz$ found in d. The black curve is a fit to a damped sine wave.
• Figure 4: Quantitative extraction of DW dynamics and comparison to simulation.a, b, Position (a) and velocity (b) of the DW center (at the location of maximum displacement) over time for different pump polarizations. Error bars represent one standard error for the fitted DW positions and the corresponding propagated error for calculated velocities. c, d, Simulated time dependence of the DW position (c) and velocity (d) for positive and negative initial angular velocities of the Néel vector.
• Figure S1: SHG rotational anisotropy polar plot (with P-polarized input and output electric field polarization). The solid line is a fit to a three-domain averaged intensity, where each domain has SHG arising from a coherent superposition of electric quadrupole and magnetic dipole SHG processes, which are illustrated on the right. Filled and white lobes indicate opposite phase. The RA patterns on the right show the single domain data and fits.