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We measured tunneling magnetoresistance (TMR) in a magnetic tunnel junction as a function of temperature. We used surface plasmon resonance spectroscopy (SPRS)-one of the most useful tools in the analysis of dielectric function in a thin tunneling barrier-to study the correlation of AlOx barrier quality with the temperature dependent behavior of the TMR ratio. The experimental SPRS analysis suggests that the decrease of the TMR ratio starting at 150 K is caused by residual Al metallic defects distributed within the insulating AlOx barrier. We checked this result by measuring the junction resistances in parallel and antiparallel states.