Xuefei Tang

Dielectric breakdown in magnetic tunnel junctions having an ultrathin barrier

Magnetic tunnel junctions have been fabricated by magnetron sputtering and patterned by deep ultraviolet photolithography. The tunnel magnetoresistance was 15%–22% and resistance times area product (R×A) 7–22 Ω μm2 for junctions having 4.75–5.5-A-thick Al layer oxidized naturally. Two types of breakdown were observed: abrupt dielectric breakdown at an effective field of 10 MV/cm determined by the thickness of the tunnel barrier, and a gradual breakdown related to defects in the tunnel barrier. After the breakdown a metallic pinhole is created, the size of which depends on the maximum current applied to the junction. The current flowing through the pinhole creates a strong circular magnetic field that curls the local magnetization in the free layer around the pinhole. The subsequent free-layer reversal is very sensitive to the pinhole location. The electric properties after breakdown can be well described by an Ohmic resistor and a tunnel magnetoresistor connected in parallel.

Two breakdown mechanisms in ultrathin alumina barrier magnetic tunnel junctions

Two breakdown mechanisms are observed in magnetic tunnel junctions having an ultrathin alumina barrier. The two breakdown mechanisms manifest themselves differently when considering large ensembles of nominally identical devices under different stress conditions. The results suggest that one type of breakdown occurs because of the intrinsic breakdown of a well-formed oxide barrier that can be described by the E model of dielectric breakdown. The other is an extrinsic breakdown related to defects in the barrier rather than the failure of the oxide integrity. The characteristic of extrinsic breakdown suggests that a pre-existing pinhole in the barriers grows in area by means of dissipative (Joule) heating and/or an electric field across the pinhole circumference.

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.