We present a study of the bias dependence of resistance and magnetoresistance ͑MR͒ in magnetic tunnel junctions both experimentally and theoretically. A modified Brinkman model has been proposed by incorporating the voltage-dependent density of states of the ferromagnetic electrodes to explain the bias dependence of magnetoresistance and resistance in a large bias range. It has been found that a reasonable variation of the effective density of states is necessary to fit the rapid decrease of resistance and magnetoresistance under large voltage bias, indicating that the most significant contribution of the bias dependence of tunneling MR comes from the electronic structure of ferromagnetic electrodes. The calculated polarizations based on these extrapolated densities of states are in good agreement with reported values by various experiments.

We have fabricated magnetic tunneling junctions by oxidizing a wedge-shaped aluminum layer to produce junctions with ideal oxidized as well as under and over oxidized junctions on a single wafer. By investigating the capacitance spectra, we are able to study the effects due to interface charge accumulation. The electron–electron interaction among accumulated interface charges leads to a voltage drop inside the magnetic electrodes, resulting in the measured capacitance differing from the geometric capacitance. We have extracted an interfacial capacitance of 16 μF/cm2 per interface and a screening length of 0.55 Å for FeNi electrodes.

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Microstructures of the NiFe/AlOx/NiFe magnetic tunnel junctions and the barrier shape profile have been studied with atomic resolution using high-resolution electron microscopy and electron holography. A clear relationship between the growth morphologies of the electrodes and the quality of the barrier has been obtained. Although the bottom interface between electrode and barrier is very sensitive to the oxidation condition, a sharp interface can be achieved in optimumally oxidized junctions. The top interface, on the other hand, is always slightly oxidized due to the three-dimensional growth of top electrode above the barrier, independent of the oxidation condition of the barrier. Furthermore, charge accumulation seems to exist at the sharp interfaces. It is also interesting, yet surprising, that both interfaces are actually sharp in underoxidized junctions. Furthermore, charge accumulation seems to exist at the sharp interfaces.

By oxidizing wedge-shaped Al films we were able to simultaneously fabricate Ni80Fe20/AlOx/Ni80Fe20 tunnel junctions with over, completely, and underoxidized barriers. We have systematically investigated TMR ratio, resistance, barrier height and thickness, and bias dependence along the wedge. The bias dependence of MR ratio is asymmetric in overoxidized regions and symmetric in underoxidized regions. The interesting bias dependences of resistance at parallel and antiparallel configurations will also be presented.

We present a study on the positions of conductance minima for the parallel and antiparallel magnetization configurations in magnetic tunnel junctions, both experimentally and theoretically. It is found that the conductance minima can have as much as a 100-mV shift from zero bias, and the shifts are spin dependent. These behaviors have not been observed in tunnel junctions based on nonmagnetic electrodes. By considering the voltage dependent density of states of ferromagnetic electrodes in the Brinkman model, the difference can be well explained. The results demonstrate that the density of states of the ferromagnetic electrodes play an important role in defining the bias dependence behaviors in magnetic tunnel junctions.

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We have investigated the bias dependence of the tunneling magnetoresistance (TMR) in Py/CoFe(t)/Al2O3/Py magnetic tunneling junctions with a wedge-shaped CoFe layer. The value of TMR increases slowly with CoFe thickness and saturates at t⩾2.3 nm, implying a significant bulk-like contribution in spin-dependent tunneling. The bias dependence of TMR also shows a similar trend as a function of CoFe thickness, and becomes independent of CoFe thickness at t⩾2.3 nm. The extracted tunneling characteristic length decreases slightly with increasing bias voltage due to the hot electron excitations.