Giant resistance switching in metal-insulator-manganite junctions: Evidence for Mott transition

Heteroepitaxial ${\mathrm{CeO}}_2\left(80\mathrm{nm}\right)∕{\mathrm{L}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3\left(400\mathrm{nm}\right)$ film structures have been pulsed laser deposited on ${\mathrm{LaAlO}}_3\left(001\right)$ single crystals to fabricate two terminal resistance switching devices. $\mathrm{Ag}∕{\mathrm{CeO}}_2∕{\mathrm{L}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ junctions exhibit reproducible switching between a high resistance state (HRS) with insulating properties and a semiconducting or metallic low resistance state (LRS) with resistance ratios up to ${10}^5$. Reversible electrical switching is a polar effect achievable both in continuous sweeping mode and in the pulse regime. Successive temperature crossover of electronic transport from the thermal activation of the deep levels $(E_a=320\mathrm{meV})$ at high temperatures to thermal activation of the shallow levels $(E_a=40\mathrm{meV})$ and finally at low temperatures to the regime of temperature independent resistance, usually associated with quantum tunneling, has been found for the insulating HRS. The temperature dependence of the LRS reveals a para-to-ferromagnetic phase transition in the ${\mathrm{L}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ (LCMO) electrode at $T_c=260\mathrm{K}$ and an anomaly at lower temperatures $\sim 200\mathrm{K}$ corresponding to the Curie temperature of the ${\mathrm{Mn}}^{4+}$ depleted part of the LCMO film. Current-voltage characteristics in the LRS are highly nonlinear, and show negative differential conductivity (NDC). We suggest that the reversible resistance switching ocurrs due to the electric field induced nucleation of filament-type conducting valence-shifted ${\mathrm{CeO}}_x$ domains inside the insulating ${\mathrm{CeO}}_2$ matrix. The abrupt insulator-to-metal transition is the result of localization of $4f$ electronic states in ${\mathrm{Ce}}^{3+}$ ions and the subsequent appearance of hole conductivity in the oxygen $p$-bands. NDC at low temperatures is relied upon the interband scattering of ${\mathrm{CeO}}_x$ carriers from a low energy, high mobility valley into a high energy valley with low mobility.