Unidirectionality of spin waves in synthetic antiferromagnets

We study the frequency nonreciprocity of the spin waves in symmetric Co40Fe40B20/Ru/Co40Fe40B20 synthetic antiferromagnet stacks set in the scissors state by in-plane applied fields. Using a combination of Brillouin light scattering and propagating spin-wave spectroscopy experiments, we show that the acoustical spin waves in synthetic antiferromagnets possess a unique feature if their wave vector is parallel to the applied field: the frequency nonreciprocity due to layer-to-layer dipolar interactions can be so large that the acoustical spin waves transfer energy in a unidirectional manner for a wide and bipolar interval of wave vectors. Analytical modeling and full micromagnetic calculations are conducted to account for the dispersion relations of the optical and acoustical spin waves for arbitrary field orientations. Our formalism provides a simple and direct method to understand and design devices harnessing unidirectional propagation of spin waves in synthetic antiferromagnets.