Experiments with Tunnel Junctions Using Ferromagnetic Metals

Results on tunnel current flow in junctions using ferromagnetic metals as electrodes or as added impurities at the barrier electrode interface are reported in this paper. Oxides of the ferromagnetic metal and aluminum oxide have been used to form the insulating barrier in combination with a second electrode made from aluminum, silver, or iron. Complete data are reported for various types of junctions using iron and iron oxide and covering a zero-bias resistance range at 4.2°K from ${10}^{-1\mathrm{}}$ Ω to above ${10}^8$ Ω. Preliminary results on junctions using cobalt and nickel are also reported. The most general feature of interest is a very strong non-Ohmic behavior at low bias voltage and a very rapid temperature dependence of tunnel conductance, particularly in the helium temperature range. Data obtained on Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al junctions doped with iron impurities at the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al interface show a strong zero-bias anomaly corresponding to a resistance maximum. This behavior appears to be generally explained by an anomalous scattering mechanism near zero bias and has been compared to the behavior of identical undoped Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al junctions. The experiments on junctions using iron oxide barriers also show strong zero-bias anomalies, but in addition show a more complex temperature dependence and an unusual breakdown effect at high voltages. These latter features are possibly associated with special properties of magnetic iron oxide ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$. The data on junctions with iron oxide barriers have also been compared to the behavior predicted by standard tunnel theory, and areas of qualitative agreement can be found if a relatively low barrier height is assumed. The appropriate barrier height has been estimated as 0.025 eV from Fowler-Nordheim plots of the data. The preliminary data on junctions made with $\mathrm{Co}{\mathrm{O}}_x$ and $\mathrm{Ni}{\mathrm{O}}_x$ barriers also show some similar effects, but they are generally of smaller magnitude. The experimental data will be presented in detail along with a summary of the relevant theories.