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

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)].

Figure 1

Schematic illustrations of (a) the symmetric DTJ and (b) the hybrid junction. The arrows indicate the magnetization of the electrodes. (c) Cross-sectional TEM image of the ${\mathrm{C}\mathrm{o}}_{90}{\mathrm{F}\mathrm{e}}_{10}/{\mathrm{A}\mathrm{l}\mathrm{O}}_x/{\mathrm{C}\mathrm{o}}_{90}{\mathrm{F}\mathrm{e}}_{10}/{\mathrm{A}\mathrm{l}\mathrm{O}}_x/{\mathrm{C}\mathrm{o}}_{90}\mathrm{F}{\mathrm{e}}_{10}$ DTJ region.

Figure 2

$I\mathrm{\text{−}}V$ characteristics of the hybrid junction measured at 7, 50, 150, and 300 K under the magnetic field application of 5 kOe. The inset shows the corresponding curves around $V=0$ at 7 and 300 K.

Figure 3

(a) $\log I\mathrm{\text{−}}\log V$ plots at 12, 30, 50, and 100 K under the magnetic field application of 5 kOe. Solid lines show the fitting with Eq. (3). (b) Constant ratio $(K_3/K_1)$ as a function of temperature. Experimental plots (open circles with a solid line) are obtained by fitting in (a), and the theoretical calculation (dashed line) represents $(e/2\pi k_{B}T{)}^2$.

Figure 4

(a) $\log \mathrm{R}\mathrm{A}$ and (b) $R_{\mathrm{A}}/R_{\mathrm{F}}$ (TMR) plots as a function of $\log T$. Both ${\mathrm{R}\mathrm{A}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ and ${\mathrm{R}\mathrm{A}}_{\mathrm{m}\mathrm{i}\mathrm{n}}$ indicate the maximum and minimum RA in the TMR curves at each temperature, respectively. Insets in (a) show the bias voltage dependence of the $dV/dI$ at 7 and 150 K under a 5 kOe applied field. Insets in (b) show the bias voltage dependence of the $R_{\mathrm{A}}/R_{\mathrm{F}}$ at 7 and 150 K.

Figure 5

Bias voltage dependence of the $R_{\mathrm{A}}/R_{\mathrm{F}}$ (TMR) in the bias range of $\lesssim 80\mathrm{m}\mathrm{V}$ at 7 ($k_{B}T/E_c=0.03$), 50 (0.22), 150 (0.65) and 300 K (1.3).