  The development of novel devices for neuromorphic computing and
non-traditional logic operations largely relies on the fabrication of well
controlled memristive systems with functionalities beyond standard bipolar
behavior and digital ON-OFF states. In the present work we demonstrate for
Ta$_2$O$_5$-based devices that it is possible to selectively
activate/deactivate two series memristive interfaces in order to obtain
clockwise or counter-clockwise multilevel squared remanent resistance loops,
just by controlling the (a)symmetry of the applied stimuli and independently of
the nature of the used metallic electrodes. Based on our thorough
characterization, analysis and modeling, we show that the physical origin of
this electrical behavior relies on controlled oxygen vacancies electromigration
between three different zones of the active Ta$_2$O$_{5-x}$ layer: a central --
bulk -- one and two quasi-symmetric interfaces with reduced TaO$_{2-h(y)}$
layers. Our devices fabrication process is rather simple as it implies the room
temperature deposition of only one CMOS compatible oxide -- Ta-oxide -- and one
metal, suggesting that it might be possible to take advantage of these
properties at low cost and with easy scability. The tunable opposite remanent
resistance loops circulations with multiple -- analogic -- intermediate stable
states allows mimicking the adaptable synaptic weight of biological systems and
presents potential for non-standard logic devices.
