  Surface alloys are highly flexible materials for tailoring the spin-dependent
properties of surfaces. Here, we study the spin-dependent band structure of a
DyAg$_2$ surface alloy formed on an Ag(111) crystal. We find a significant
exchange spin-splitting of the localized Dy 4f states pointing to a
ferromagnetic coupling between the localized Dy moments at $40\,$K. The
magnetic coupling between these moments is mediated by an indirect, RKKY-like
exchange coupling via the spin-polarized electrons of the hole-like Dy-Ag
hybrid surface state.
