STM-induced reversible switching of local conductivity in thinAl2O3films

The local electron transport properties of thin aluminum oxide layers used for magnetic tunnel junctions were studied in situ by scanning tunneling microscopy (STM) and spectroscopy under ultrahigh-vacuum conditions. The STM images of the oxide films reveal a granular structure, down to atomic resolution. A reversible switching of the conductive properties of grains, attributed to a charge redistribution, is observed during scanning. We demonstrate the possibility of intentionally switching a grain to the low-resistance state by exposing it to a high current density. We conjecture that the observed switching behavior may be considered as the precursor of an electric breakdown in tunnel junctions.