  Ferroelectric memristors are intensively studied due to their potential
implementation in data storage and processing devices. In this work we show
that the memristive behavior of metal/ferroelectric oxide/metal devices relies
on the competition of two effects: the modulation of metal/ferroelectric
interface barriers by the switchable ferroelectric polarization and the
electromigration of oxygen vacancies, with the depolarizing field playing a
fundamental role in the latter. We simulate our experimental results with a
phenomenological model that includes both effects and we reproduce several
non-trivial features of the electrical response, including resistance
relaxations observed after external poling. Besides providing insight into the
underlying physics of these complex devices, our work suggests that it is
possible to combine non-volatile and volatile resistive changes in single
ferroelectric memristors, an issue that could be useful for the development of
neuromorphic devices.
