Dielectric breakdown in magnetic tunnel junctions having an ultrathin barrier

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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 (R×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.