Low-resistance magnetic tunnel junctions by in situ natural oxidation

Abstract

Spin-dependent tunneling has received much attention in recent years. In this study, low-resistance exchange-biased magnetic tunnel junctions (down to 2 $\mu {\mathrm{m}}^2)$ were fabricated in a self-aligned way. The insulating barrier was achieved by in situ natural oxidation of very thin Al layers (<1.3 nm). The resistance and magnetoresistance of tunnel junctions is studied as a function of the aluminum thickness for different oxidation times, leading to resistance-area products from 0.1 to 5 ${\mathrm{k\Omega \mu m}}^2,$ and tunnel magnetoresistance (TMR) ratios up to 20%. The decrease in TMR for the lowest resistance barriers in the data set is explained by the nonuniformity of the initial Al layer, which has been characterized using different methods. In this case, the optimal oxidation state cannot be reached. The barrier oxidation state can be improved by thermal treatment, by which an increase in TMR is observed for temperatures up to 275 °C.