Edge atoms do all the work. 3D view of an STM image of (a) one monolayer high Co and (b) one monolayer high islands with a Pt core and an approximately 3 atom wide Co shell (deposition of 0.2 ML Pt at 130 K and annealing to 760 K generates the quasi hexagonal non-magnetic core; subsequent deposition of 0.2 ML Co at 220 K creates the Co rim).
