Surface plasmon resonance analysis of insulating AlOx thin film for magnetic tunnel junctions prepared by natural oxidation method

The AlO_x insulating layers used as barriers in magnetic tunnel junctions were carefully analyzed by utilizing a surface plasmon resonance spectroscopy (SPRS) technique in order to determine the optimum oxidation state inside the barrier. The measurement was performed in reflectivity mode with the barrier fabricated by a natural oxidation process. The SPRS technique was found to be very useful to determine optimum oxidation time and dielectric properties of the barrier. In this paper, the degree of oxidation is discussed as a function of oxidation time and dielectric properties of barrier are considered through the Kreschmann simulation. For comparison, magnetic tunneling junctions are also fabricated with the barriers obtained by the calculated SPRS results. The magneto-resistance ratio of about 17% is experimentally observed for a sample that was exposed to the atmosphere for 7 h, as expected by the SPRS results. Finally, it is found that a natural oxidation process in the atmosphere had a saturation effect at a low thickness of 8Å.