By oxidizing wedge-shaped Al films we have fabricated magnetic tunneling junctions with over-, complete, and under-oxidized barriers. By investigating the capacitance spectra, we are able to study the effects due to interface charge accumulation. The electron-electron interaction among accumulated interface charges leads to a voltage drop inside the magnetic electrodes, resulting in the measured capacitance differing from the geometric capacitance. We have extracted an interfacial capacitance of 16 mF/cm 2 per interface and a screening length of 0.55 A for FeNi electrodes. We also observed some interesting phenomena in bias dependence of TMR along the wedge. Bias curve of MR is asymmetric in over-oxidized, and symmetric in under-oxidized regions. These behaviors have been explained using a model based on charge accumulation effect at the metal-insulator interfaces.Introduction Recently much attention has been focused on the research of magnetic tunneling junctions (MTJs) for their fertile physics and potential applications [1, 2]. The commonly accepted mechanism for tunneling magnetoresistance (TMR) is that the TMR is proportional to the product of spin polarization (P) of two ferromagnetic electrodes [3,4]. It is also well known that the surface layer affects the spin polarization dramatically. Recently, the electron-electron interaction effect due to bias induced charges built up at the metal-insulator interface has also been revealed [5][6][7][8]. The charge build-up at the interfaces leads to the well-known electron screening effect, causing electric field penetration into the metal [9, 10], i.e. a portion of the applied voltage is dropped across the metallic surface layer on the order of the screening length.This electron-electron interaction effect is manifested in the observation of the measured capacitance (C m ) deviating from the geometrical capacitance C g ¼ e i A/d, where A and d are the junction area and the thickness of insulating barrier, respectively, e i ¼ e 0 j d is the permittivity of the insulator, and j d is the dielectric constant of the dielectric layer. This phenomenon has been observed in nonmagnetic tunneling junctions [7,11,12]. In a ferromagnet, this picture becomes more complicated due to the additional exchange interaction, resulting in a spin dependent screening potential [6]. In addition to the electron-electron interaction effect, there are other fundamental problems that remain illusive, e.g. the oxidation mechanism and bias dependence. The TMR is critically dependent on the proper oxidation of the aluminum layer [13]. In order to attack this problem, we have developed a method to fabricate optimized junctions by