Synthetic antiferromagnets (SAFs) recently attracted a lot of attention in the magnonics community, as they can provide additional degrees of freedom for spin wave dispersion engineering compared to conventional ferromagnetic systems [1],[2]. One of the most prominent features of dispersion relation in SAFs is non-reciprocity caused by the dipolar field interaction between the layers [3]. Spin wave non-reciprocity and symmetry breaking in material systems are of fundamental interest and can also be exploited in practical magnonic applications, such as diodes or circulators.
We report on experimental observation of unidirectional propagation of zero-momentum magnons in synthetic antiferromagnet consisting of strained CoFeB/Ru/CoFeB trilayer [4]. Inherent nonreciprocity of spin waves in synthetic antiferromagnets with uniaxial anisotropy results in smooth and monotonous dispersion relation around Gamma point (see Figure 1b), where, upon the crossing k=0, the direction of the phase velocity is reversed, while the group velocity direction is conserved. The experimental observation of this phenomenon by intensity-, phase- (see Figure 1c,d), and time-resolved Brillouin light scattering microscopy is corroborated by analytical models and micromagnetic simulations.
The unusual dispersion relation offers many possibilities for further investigations of e.g., propagation of wavepackets with bidirectional phase velocities, or spin wave accumulation on domain boundaries. In practical applications such unidirectional system can pave the way towards designing new non-reciprocal elements for signal processing and unconventional spin-wave computing.

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