  We report in this study the current-induced-torque excitation of acoustic and
optical modes in Ta/NiFe/Ru/NiFe/Ta synthetic antiferromagnet stacks grown on
SiO2/Si substrates. The two Ta layers serve as spin torque sources with the
opposite polarisations both in spin currents and Oersted fields acting on their
adjacent NiFe layers. This can create the odd symmetry of spatial spin torque
distribution across the growth direction, allowing us to observe different
spin-wave excitation efficiency from synthetic antiferromagnets excited by
homogeneous torques. We analyse the torque symmetry by in-plane angular
dependence of symmetric and anti-symmetric lineshape amplitudes for their
resonance and confirm that the parallel (perpendicular) pumping nature for the
acoustic (optical) modes in our devices, which is in stark difference from the
modes excited by spatially homogeneous torques. We also present our macrospin
model for this particular spin-torque excitation geometry, which excellently
supports our experimental observation. Our results offer capability of
controlling spin-wave excitations by local spin-torque sources and we can
explore further spin-wave control schemes based on this concept.
