The potential barrier at the metal/oxide junction in a specular spin valve structure with nano-oxide layers has been mapped by off-axis electron holography in a field emission gun transmission electron microscope. A potential jump of similar to 3V across the metal/oxide junction was detected. Presence of the potential barrier confirms formation of metal/insulator/metal structure, which contributes to confinement of conductance electrons with spin polarity characteristic in the key SV structure by the specular reflection of the spin-polarized electrons at the metal/oxide junction and leads to nearly double enhancement of magnetoresistance (MR) ratio from 8% to similar to 16%.