Local Polarization-dependent Electron Transport Through Ferroelectric and Multiferroic Surfaces

Electron tunneling underlies the operation of numerous devices relevant to information technology and has been proposed in futuristic applications for energy harvesting and quantum computing. Replacing a conventional insulator in the tunnel junction with electronically correlated materials can yield new types of electronic functionality. In one such concept, dubbed ferroelectric tunneling, the tunneling barrier height is controlled by the polarization of a ferroelectric oxide, enabling non-volatile conduction states that can be switched with electric field. Ferroelectric tunneling is a prototypical example and, in many ways, a stepping stone to a broad range of transport phenomena controlled by soft-phonon order parameters in complex oxide materials. Despite its conceptual simplicity, polarization-controlled transport has remained a largely theoretical concept for almost thirty years, because of the deficiency of conventional approaches to transport measurement and inherently large resistivity of many ferroic oxides.

We have recently discovered highly reproducible polarization control of local electron transport through thin Pb(Zr_0.2Ti_0.8)O₃ film. Despite being 30 nm thick, conductive atomic force microscopy revealed that the oxide film possessed spatially and temporally reproducible local conductivity. The coupling between ferroelectric dynamics and electron transport at the tip-surface junction is manifested as a giant, up to 500-fold enhancement of local conductance upon polarization reversal. The physical mechanism of the observed effect was traced to the polarization-dependence of the height and width of the metal-ferroelectric Schottky barrier, based on variable-temperature measurements of local electron transport and spatially-resolved correlation between conductivity and ferroelectricity, which also revealed the role of defects. Apart from archetypal PZT, we have revealed polarization-controlled transport in thin films of model multiferroic BiFeO₃, where coupled G-type antiferromagnetic, ferroelastic and ferroelectric order parameters coexist at room temperature. We will discuss how polarization switching and polarization-controlled transport phenomena scale with the thickness of the BiFeO₃ films down to a few unit-cells, and local conductivity of ferroelastic domain walls.