THz and sub-THz antiferromagnetic magnons via magnetoacoustic resonances excited by picosecond strain pulses in NiO

Antiferromagnets can host ultrafast spin dynamics in the terahertz (THz) frequency range, but an energy-efficient generation of THz magnons necessary for high-speed information processing devices is challenging. Fortunately, some antiferromagnetic compounds possess substantial magnetoelastic coupling between spins and strains, which opens the way for their excitation by mechanical stimuli. Here, we report a theoretical study of the spin dynamics excited in single-crystalline NiO by picosecond acoustic pulses, which can be created by optomechanical transducers driven by femtosecond laser pulses. To describe the interrelated spin and strain dynamics in this antiferromagnet distinguished by a strong magnetoelastic coupling, we carry out micromagnetoelastic simulations based on the numerical solution of the Landau-Lifshitz-Gilbert equation for sublattice magnetizations and the elastodynamic equation for mechanical displacements. The simulations show that the propagating "bipolar" pulse of the longitudinal strain generates correlated clockwise and counterclockwise precessions of the sublattice magnetizations, which have a complex spatial distribution in the region behind the pulse front. The spatiotemporal analysis of the simulation data reveals that the spin dynamics excited by the pulses with durations $\tau$ smaller than about 7 ps comprises a monochromatic spin wave with the frequency $\nu \approx 450$ GHz. Moreover, a second monochromatic spin wave having the frequency $\nu \approx 2$ THz emerges at $\tau \le 3$ ps. By comparing the calculated dispersion of spin and elastic waves in NiO we demonstrate that the revealed monochromatic magnons originate from two magnetoacoustic resonances existing in this antiferromagnetic compound. Importantly, the acoustic pulses with durations $\tau \le 3$ ps appear to be capable of creating antiferromagnetic magnons with THz and sub-THz frequencies in the absence of external magnetic fields. Our findings shed light on the magnetoacoustic phenomena in antiferromagnets and indicate that the single-crystalline NiO is a promising material for the development of ultrafast magnonic devices with a low power consumption.