Reliability of normal-state current–voltage characteristics as an indicator of tunnel-junction barrier quality

We demonstrate that one of the most commonly used criteria to ascertain that tunneling is the dominant conduction mechanism in magnetic tunnel junctions—fits of current–voltage (I–V) data—is far from reliable. Using a superconducting electrode and measuring the differential conductance below Tc, we divide samples into junctions with an integral barrier and junctions having metallic shorts through the barrier. Despite the clear difference in barrier quality, equally reasonable fits to the I–V data are obtained above Tc. Our results further suggest that the temperature dependence of the zero-bias resistance is a more solid criterion, which could therefore be used to rule out possible pinholes in the barrier.