Giant Response and Harmonic Generation in Néel-Torque Antiferromagnetic Resonance

Abstract

We theoretically investigate the resonant and higher order magnetic responses of a collinear antiferromagnet induced by Néel spin-orbit torques (NSOTs). By deriving the dynamical susceptibilities up to the third harmonic, we find remarkable NSOT-induced amplifications of the linear and nonlinear magnetic dynamics by orders of magnitude compared to conventional spin-orbit torques, enabling highly-efficient frequency conversion in the terahertz frequency range. From the effective dynamics, we uncover the strong coupling between the Néel vector and the driving field of NSOTs, providing a physical explanation of the gigantic responses at all orders. We then propose a multilayer antiferromagnetic nano-device leveraging the gigantic harmonic generation to achieve unprecedented frequency amplifiers and converters. Our work uncovers a previously overlooked role of the NSOTs in nonlinear dynamics.

Figures (3)

• Figure 1: Real ($\chi'$) and imaginary parts ($\chi"$) of (a) the first‐, (b) second‐, and (c) third‐order susceptibilities under the NSOT. Solid (dashed) lines denote sublattice A (B); their real and imaginary components are distinguished by colors. Insets reproduce the same susceptibilities under conventional SOT using identical line styles and colors. All harmonic responses are markedly enhanced by the NSOT compared to SOT. Parameters: $\gamma=2\pi\cdot 28.02$ GHz$\cdot$T$^{-1}$, $\alpha=0.02$, $H_0=0.2$ T, $H_J=35$ T, $H_\parallel=0.16$ T, and $f=\omega/2\pi$.
• Figure 2: Real (${\chi^{\mathrm{tot}}}'$, solid) and imaginary (${\chi^{\mathrm{tot}}}"$, dashed) parts of the total susceptibilities at their (a) first‐order, (b) second‐order, and (c) third‐order under the NSOT. Insets reproduce the same plots for conventional SOT with identical line styles and colors. All harmonic responses are markedly amplified by the NSOT compared to SOT. Parameters are the same as in Fig. \ref{['fig:chiAandB']}.
• Figure 3: Schematic of the proposed AFM frequency converter. A $\mathcal{PT}$‐symmetric AFM1 with eigenfrequency $\omega_0$ is driven by NSOTs, generating higher harmonics. Through the RKKY coupling, the $n$th harmonic of AFM1 resonantly excites a second AFM layer (AFM2) whose resonance frequency is $n\omega_0$, enabling energy transfer from AFM1 to AFM2, effectively converting a frequency source of $\omega_0$ into $n\omega_0$.