Scanning tunneling microscopy measurements on Co nanoclusters grown on Au(111) revealed the presence of two peaks in the electronic states, below and above the Fermy energy. Ab initio calculations show that both peaks are due to spin-polarized surface states. The crystallographic stacking of the Co nanoclusters are shown to influence strongly the energy shift of filled surface states, providing a way to increase the low bias spin current of magnetic tunnel junctions. Due to the small lateral dimensions of the nanoclusters ( 3nm), edge effects dominate the amplitude of the empty states peak.