The plasma treatment with O2/Ar mixture was applied to reduce the surface roughness of the L10-FePt electrode in perpendicular magnetic tunnel junctions. The surface roughness (Rrms) of the L10-FePt layer was decreased from 1.2 nm at the as-deposited state to 0.52 nm after the plasma treatment with the ratio of O2 to Ar equal to 0.5. The low surface roughness accompanying the formation of an oxide layer led to magnetic decoupling between free and reference layers after the plasma treatment on the surface of FePt layer. The existence of the oxide layer was confirmed by using transmission electron microscopy and x-ray photoelectron spectroscopy. The interfacial Pt-oxide layer plays a significant role in the magnetic decoupling.