  The experimental investigations of the magnetic interactions in an atomically
thin magnetic layer are essential to understand the physics of low-dimensional
magnets. The full spectrum of collective magnetic excitations (magnons) would
provide an access to these fundamental interactions on the atomic scale. Here
in order to be able to excite the magnons by means of spin-polarized electrons
we couple a Ni monolayer to one and two atomic layers of Co and probe the full
experimental magnon dispersion relation up to the Brillouin zone boundary.
Comparing to the results of ab initio calculations we quantify the complex
pattern of the magnetic exchange interaction in the Ni monolayer. We show that
although the magnons in this system are rather stiff, the Heisenberg exchange
coupling between the Ni spins is weak. We unravel the origin of the observed
large magnon stiffness constant being a consequence of the small spin density
of the Ni atoms.
