Electron　holography　(EH)　has　been　used　successfully　for　studying　the　barrier　shape　profiles　and　barrier/electrodes　interfaces　in　a　magnetic　tunnel　junction　(MTJ)　that　was　fabricated　with　a　wedge-shaped　Al　oxide　barrier,　and　an　asymmetrical　curve　of　the　tunnelling　magnetoresistance　(TMR)　versus　nominal　Al　thickness　has　been　found.　EH　results　reveal　several　useful　features　that　are　unlikely　to　be　found　using　other　techniques.　The　asymmetrical　TMR　curve　and　the　TMR　value　unachievable　with　Julliere　model　are　mainly　due　to　the　different　barrier/electrode　interfaces　along　the　wedge.　Furthermore,　a　model　of　barrier　oxidation　has　been　built　to　explain　the　TMR　behaviour　in　MTJs.