Antiferromagnetic magnons and spin pumping generated by elastic waves injected into NiO/Pt bilayers

Elastic waves propagating in magnetic materials with significant coupling between spins and strains are able to efficiently generate coherent magnons, which represent advantageous signal carriers for information processing devices. Such strain-driven excitation of spin waves is especially important for antiferromagnets, where magnon frequencies can reach the terahertz (THz) range, thus enabling ultrafast data processing. Here, we report the micromagnetoelastic modeling of the spin dynamics arising in single-crystalline NiO films and NiO/Pt bilayers traversed by monochromatic longitudinal and transverse elastic waves, which can be generated by an attached optomechanical transducer. The strong magnetoelastic interaction inherent in this collinear antiferromagnet is described using the magnetoelastic energy density written in terms of the N\'eel-vector direction cosines. The simulations show that a monochromatic elastic wave propagating across the (001)-oriented NiO film creates a ``forced'' antiferromagnetic spin wave with the length and velocity controlled by the driving strain wave at any frequency of the latter. In addition, a ``free'' spin wave compatible with the dispersion of antiferromagnetic magnons is generated at the NiO surface. This wave is evanescent at excitation frequencies $\ensuremath{\nu}$ below the frequency ${\ensuremath{\nu}}_{\text{AFMR}}$ of the antiferromagnetic resonance, but it becomes propagating at $\ensuremath{\nu}>{\ensuremath{\nu}}_{\text{AFMR}}$. As the upper branch of the antiferromagnetic dispersion curve crosses the dispersion of longitudinal elastic waves, magnetoelastic waves with frequencies close to the crossing-point frequency can be generated in NiO as well. Performing the modeling of the NiO/Pt bilayer, we also determine the oscillating spin current created in Pt by the correlated precession of sublattice magnetizations in NiO. It is shown that the acoustically excited NiO can efficiently generate a spin current with a THz frequency, which gives rise to an alternating charge current in the Pt film and a THz electromagnetic wave above its free surface. Our theoretical results provide useful information for the development of energy-efficient magnon generators, spin injectors, and emitters of THz electromagnetic waves based on dynamically strained antiferromagnets.