We demonstrate the parametric amplification of an acoustic magnon mode induced by an optical magnon mode in synthetic antiferromagnets, which was achieved by using the all-optical pump-probe time-resolved magneto-optical Kerr effect. The acoustic and optical modes with low and high frequencies, respectively, are clearly observed under different field directions and pump-laser powers. For a relatively high laser power, the acoustic mode shows a temporal increase in amplitude in the time domain; this is observed when the acoustic mode frequency is approximately half the frequency of the optical mode. Correspondingly, we also observe a large enhancement in the spectral density of the acoustic mode in the frequency domain. These data are supported by a numerical simulation using a macrospin model; furthermore, the optical mode amplitude threshold for achieving the acoustic mode amplification is also discussed. The parametric effect in synthetic antiferromagnets demonstrated here can be applied to nanoscale parametric amplifiers and oscillators of magnons, which are the building blocks for spintronic and magnonic computing beyond von Neumann architectures.
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