Conduction at domain walls in insulating Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ thin films

Ferroic domain walls are intrinsically nanoscale and often present functional properties beyond those of their parent material. One of the most striking examples is the recent discovery of electrical conduction\footnote{Seidel et al., Nat. Mat. {\bf8}, 229 (2009)} at domain walls in multiferroic BiFeO$_3$. Different scenarios have been proposed to explain the observed conduction, generally relating it to the complex nature of domain walls specific to BiFeO$_3$.\footnote{Lubk et al., PRB {\bf80}, 104110 (2009); Chiu et al., Adv. Mat. {\bf23}, 1530 (2011); Farokhipoor et al., PRL {\bf107}, 127601 (2011)} Here, we report on scanning probe microscopy studies of domain-wall-specific conduction in thin films of tetragonal ferroelectric (PZT). Our measurements show nonlinear asymmetric current-voltage characteristics with strong thermal activation at $T>150$ K. Moreover, the average current signals remain stable over the duration of measurement (up to four days). In light of recent transmission electron microscopy measurements at 180$^{\circ}$ domain walls in PZT,\footnote{Jia et al., Sci. {\bf331}, 1420 (2011)} we discuss the possible conduction mechanisms, highlighting the role of electrode asymmetry and microscopic domain wall structure promoting local defect segregation.