Memristive devices, which emulate the synaptic behavior of biological systems, are at the forefront of next-generation memory and neuromorphic computing technologies. Here, we investigate bipolar resistive switching in Pt/polycrystalline ErMnO3/Ti/Au memristive devices and show how mixed orthorhombic and hexagonal ErMnO3 polymorph films can be engineered to optimize the device performance. The two crystalline phases are evidenced by a combination of correlative microscopies (scanning electron microscopy, optical microscopy and conductive atomic force microscopy) and Raman spectroscopy. The devices exhibit high ROFF/RON ratios (~105) and ultra-low RON resistances (~10 Ω). The resistive switching is the result from the formation and rupture of an oxygen-vacancy-based conductive filament, which likely occurs either in the orthorhombic phase or at the boundary between the two polymorphs. An increased fraction of orthorhombic phase strongly reduces the operating voltage (down to VSet ~ −2.07 V) and its variability. The presence of the hexagonal phase, which is much less conductive than the orthorhombic one, reduces leakage currents in the devices, that otherwise would not exhibit switching behavior.
