Observation of Thickness-Dependent Exchange Interaction in EuO Ultrathin Films

The electronic structure of single-crystalline EuO (100) ultrathin films, together with their exchange splitting energy ( ΔE EX ) and ferromagnetic phase transition temperature ( T C ), was investigated by temperature- and thickness-dependent angle-resolved photoemission spectroscopy. Both ΔE EX and T C decreased monotonically with decreasing film thickness. The band shift showed an opposite thickness dependence at the Γ and X points, reflecting the balance of the hybridization between the Eu 4 f and O 2 p states (super-exchange interaction) and between the Eu 4 f , O 2 p , and Eu 5 d states (indirect-exchange interaction). The observed transition from an indirect energy gap in the bulk to a direct gap in the ultrathin films of the ferromagnetic semiconductor EuO could be potential in future spintronic devices.