Temperature and bias-voltage dependent transport in magnetic tunnel junctions with low energy Ar-ion irradiated barriers

Abstract

Magnetic tunnel junctions $({\mathrm{Mn}}_{83}{\mathrm{Ir}}_{17}{/\mathrm{C}\mathrm{o}}_{70}{\mathrm{Fe}}_{30}{/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}{/\mathrm{N}\mathrm{i}}_{80}{\mathrm{Fe}}_{20})$ are investigated, whose barriers are irradiated by a low energy ${\mathrm{Ar}}^{+}$ ion beam immediately after plasma oxidation of the aluminum film. The tunneling magnetoresistance prior to irradiation is up to 71% at 10 K. The ion irradiation increases the area resistance product up to a factor of 40 for ion energies up to 150 V. Further, the tunneling magnetoresistance and the dielectric stability is strongly reduced with increasing ion energies. From the analysis of the temperature and the voltage dependence of the tunneling magnetoresistance we conclude that this is due to an irradiation induced local change of the coordination of the Al and O atoms in the barrier. This leads to a thicker barrier and an increase of the precursor density for the dielectric breakdown. Further, an increase of hopping conductance through localized states is discussed. At energies larger than 150 V the resistance breaks down rapidly and the tunneling magnetoresistance vanishes completely. This results from the enhanced intermixing and sputtering of the barrier and electrode material. The results are also supported by investigations of the magnetic and the noise properties of the junctions and the Cu-${\mathrm{K}}_{\ensuremath{\alpha}}$-reflectivity of ${\mathrm{AlO}}_{x}$ multilayers.