Two little known methods to extract the height of tunnel barriers have been applied to magnetic tunnel junctions with barriers of aluminum oxide and tantalum oxide. These are based on the logarithmic derivative of $I-V$ curves and on their temperature dependence, which display cusps at about 1.2 times the barrier height. The benefit is a determination of the barrier height which is completely independent of the barrier thickness. The values thus found are compared to those from the conventional Brinkman fits. It is shown that the specific cusps appear if and only if the junction displays a magnetoresistance effect. If they do not, the investigation of the current-voltage and current-temperature characteristics indicates the Poole-Frenkel effect as dominant transport mechanism.

• Received 11 September 2001

DOI:https://doi.org/10.1103/PhysRevB.65.054422