Characterization of an AlO$_{\rm x}$ Tunneling Barrier in a Magnetic Tunnel Junction by a Surface Plasmon Resonance Spectroscopy Technique

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 AlO
_x
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 AlO
_x
barrier. We checked this result by measuring the junction resistances in parallel and antiparallel states.