Parity-controlled spin-wave excitations in synthetic antiferromagnets

TL;DR

This study demonstrates how parity-controlled spin-torque sources can selectively excite acoustic and optical spin-wave modes in synthetic antiferromagnets, enabling advanced control of spin-wave excitations.

Contribution

It introduces a novel excitation scheme using parity-controlled spin torques in synthetic antiferromagnets, supported by a macrospin model and experimental validation.

Findings

Different excitation efficiencies for acoustic and optical modes.

Confirmation of parallel and perpendicular pumping for respective modes.

Successful macrospin modeling of the excitation geometry.

Abstract

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.