Nanometric cartography of tunnel current in metal-oxide junctions

Recently, ferromagnet/insulator/ferromagnet trilayer junctions were shown to exhibit large magnetoresistance effects. However, these effects proved to be poorly reproducible from sample to sample. To get a nanoscopic insight on the origin of these fluctuations, we have used a combined atomic force microscope/scanning tunneling microscope setup to map the tunnel current that flows through metal/oxide stacks. The current histogram, which extends over 1-2 orders of magnitude, is found to be well described by a simple model which takes into account tiny spatial fluctuations of tunnel barrier thickness (typically, 0.1 nm). Moreover, our analysis shows that the total conductance of imperfect metal/oxide junctions tends to be dominated by very few sites. This result allows us to relate the sample-to-sample conductance fluctuations to slight local variations of the tunnel barrier parameters. (C) 1998 American Institute of Physics. [S0021-8979(98)47511-X].