Significant Magnetoresistance Enhancement due to a Cotunneling Process in a Double Tunnel Junction with Single Discontinuous Ferromagnetic Layer Insertion

Abstract

We fabricate $\mathrm{C}\mathrm{o}\mathrm{F}\mathrm{e}/{\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{C}\mathrm{o}\mathrm{F}\mathrm{e}/{\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{C}\mathrm{o}\mathrm{F}\mathrm{e}$ ferromagnetic double tunnel junctions and observe spin-dependent tunneling phenomena. A middle CoFe layer becomes discontinuous by forming CoFe particles two dimensionally, of which the average diameter is evaluated to be 2.0--4.5 nm from cross-sectional transmission electron microscopy images. Below 50 K, a Coulomb gap is observed in current-voltage curves, and both magnetoresistance ratios and resistances are found to increase significantly with decreasing temperature. This indicates that a cotunneling process is dominant within the gap, which agrees very well with theoretical prediction [Phys. Rev. Lett. 80, 1758 (1998)].