Tunneling criteria and breakdown for low resistive magnetic tunnel junctions

The tunneling criteria are evaluated using magnetic tunnel junctions having ultrathin alumina barrier with and without pinholes. It is shown that the tunneling criteria formulated by Rowell [J. Appl. Phys. 41, 1915 (1970)] clearly do not rule out the presence of pinholes in an ultrathin insulating barrier. In particular, the third criterion, a downward temperature dependence of resistance, cannot be used to decisively rule out the presence of pinholes. Examination of the breakdown mechanism will reveal the true nature of the barrier quality, and thus should be applied alongside the tunneling criteria to identify tunneling and the presence of pinholes.