  The propagation of magnons along a symmetry path may depend on the direction
of propagation, similar to many other quasiparticles in nature. This phenomenon
is commonly referred to as nonreciprocity. In addition to the fact that it is
of great interest to understand the fundamental physical mechanism leading to
this nonreciprocal propagation, the phenomenon of magnon nonreciprocity may be
used to design magnon-based logic devices. Recently, it has been demonstrated
that a significantly large spin-orbit coupling can lead to giant nonreciprocity
of exchange-dominated terahertz magnons, when they are excited by means of
spin-polarized electrons [Phys. Rev. Lett. 132, 126702 (2024)]. Here, by
providing experimental results on two additional systems we demonstrate the
generality of the observed phenomenon. Comparing the results of a Co/Ni bilayer
on Ir(001) to those of a Co double layer on Ir(001) and W(110) we unravel the
impact of the interfacial electronic hybridization on the observed phenomenon
and provide further insights into the microscopic mechanism leading to this
nonreciprocal magnon excitation. It was observed that the interfacial
electronic hybridization, which is governing the magnetic properties of the
interface atomic layer, is somewhat important but is not crucial for the magnon
nonreciprocity. On the other hand the choice of the incident energy of the
incoming electron beam is decisive for the observation of the effect. Our
results indicate that depending on the energy of the incident electron beam and
the scattering geometry the magnon nonreciprocity can be tuned and even be
inverted for some ranges of the magnon momentum.
