Dielectric breakdown in magnetic tunnel junctions having an ultrathin barrier

Abstract

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 $\mathrm{(R\times A)}$ 7–22 Ω μm² for junctions having 4.75–5.5-Å-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.