  Transition metal oxide memristors, or resistive random-access memory (RRAM)
switches, are under intense development for storage-class memory because of
their favorable operating power, endurance, speed, and density. Their
commercial deployment critically depends on predictive compact models based on
understanding nanoscale physico-chemical forces, which remains elusive and
controversial owing to the difficulties in directly observing atomic motions
during resistive switching, Here, using scanning transmission synchrotron x-ray
spectromicroscopy to study in-situ switching of hafnium oxide memristors, we
directly observed the formation of a localized oxygen-deficiency-derived
conductive channel surrounded by a low-conductivity ring of excess oxygen.
Subsequent thermal annealing homogenized the segregated oxygen, resetting the
cells towards their as-grown resistance state. We show that the formation and
dissolution of the conduction channel are successfully modeled by radial
thermophoresis and Fick diffusion of oxygen atoms driven by Joule heating. This
confirmation and quantification of two opposing nanoscale radial forces that
affect bipolar memristor switching are important components for any future
physics-based compact model for the electronic switching of these devices.
