Electron　holography　was　applied　to　study　the　microstructure　evolution　of　magnetic　tunnelling　junctions　(MTJs)　CoFe/AlOx/Co　annealed　at　different　temperatures.　A　mean　inner　potential　barrier　was　observed　in　the　as-deposited　MTJ　sample,　while　it　was　changed　to　a　potential　well　after　a　200degreesC　or　a　400degreesC　annealing.　It　is　suggested　that　the　oxygen　atoms　were　redistributed　during　the　annealing,　which　left　metallic　atoms　acting　as　acceptors　to　confine　the　electrons,　leading　to　the　decrease　of　the　potential　of　the　AlOx　barrier　layer.　The　results　suggest　that　the　electron　holography　may　be　a　useful　tool　for　the　study　of　the　microstructure　of　amorphous　materials.　(C)　2003　Elsevier　B.V.　All　rights　reserved.