  Memristors are receiving keen interest because of their potential varied
applications and promising large-scale information storage capabilities.
Tantalum oxide is a memristive material that has shown promise for
high-performance nonvolatile computer memory. The microphysics has been elusive
because of the small scale and subtle physical changes that accompany
conductance switching. In this study, we probed the atomic composition, local
chemistry and electronic structure of functioning tantalum oxide memristors
through spatially mapped O K-edge x-ray absorption. We developed a
time-multiplexed spectromicroscopy technique to enhance the weak and possibly
localized oxide modifications with spatial and spectral resolutions of <30 nm
and 70 meV, respectively. During the initial stages of conductance switching of
a micrometer sized crosspoint device, the spectral changes were uniform within
the spatial resolution of our technique. When the device was further driven
with millions of high voltage-pulse cycles, we observed lateral motion and
separation of ~100 nm-scale agglomerates of both oxygen interstitials and
vacancies. We also demonstrate a unique capability of this technique by
identifying the relaxation behavior in the material during electrical stimuli
by identifying electric field driven changes with varying pulse widths. In
addition, we show that changes to the material can be localized to a spatial
region by modifying its topography or uniformity, as against spatially uniform
changes observed here during memristive switching. The goal of this report is
to introduce the capability of time-multiplexed x-ray spectromicroscopy in
studying weak-signal transitions in inhomogeneous media through the example of
the operation and temporal evolution of a memristor.
