The electrical conductance of magnetic tunnel junction (MTJ) devices whose ultra-thin aluminum oxide tunnel barrier was irradiated by highly charged ions (HCIs) increases linearly with the fluence of HCIs, while retaining a current-voltage relationship indicative of a tunnel junction. The slope of the MTJ conductance σc as a function of fluence varies with different tunnel barrier thicknesses d, levels of oxidation (stoichiometry) and charge state q. Since the MTJ conductance after HCI irradiation remains tunneling, the increased conductance can result from thinning the barrier, reducing the effective tunnel barrier height φ, or some of both. Measurements of the current-voltage profile provide enough degrees of freedom to substantially constrain d and φ, but the irradiation at higher charge states like Xe44+ reduces the barrier so dramatically (into the low barrier limit) that the commonly used WKB (WentzelKramersBrillouin) tunneling formalism is no longer valid. However, for the Xe32+ ions discussed here, σc is three orders of magnitude smaller, where application of WKB analysis is still reasonable and reveals a trend of decreasing d while φ changes very little.
Citation: Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions With Materials and Atoms
Pub Type: Journals
Highly charged ions, WKB, magnetic tunnel junction, potential sputtering