Magnons and fundamental magnetic interactions in a ferromagnetic monolayer: The case of the Ni monolayer

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.