  Quantum confinement leads to the emergence of several magnon modes in
ultrathin layered magnetic structures. We probe the lifetime of these quantum
confined modes in a model system composed of three atomic layers of Co grown on
different surfaces. We demonstrate that the quantum confined magnons exhibit
nonlinear decay rates, which strongly depend on the mode number, in sharp
contrast to what is assumed in the classical dynamics. Combining the
experimental results with those of linear-response density functional
calculations we provide a quantitative explanation for this nonlinear damping
effect. The results provide new insights into the decay mechanism of spin
excitations in ultrathin films and multilayers and pave the way for tuning the
dynamical properties of such structures.
