Ferroelectric Field Effect in Ultrathin Epitaxial Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ Films

We report the study of ferroelectric field effect modulation of the metal-insulator transition in ultrathin Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ (SNNO) films. We have fabricated high quality epitaxial SNNO thin films and Pb(Zr,Ti)O$_{3}$ (PZT)/SNNO heterostructures on (001) LaAlO$_{3}$ substrates using off-axis RF magnetron sputtering. X-ray diffraction and atomic force microscopy studies reveal (001) oriented films with highly crystallinity and surface roughness of 3-4 {\AA}. Thin SNNO films (4-6 nm) typically have the transition temperature $T_{MI}$ around 230 K, showing thermally activated transport below $T_{MI}$ followed by 3D variable range hoping at low temperature. Hall effect measurements reveal p-type conduction with $\sim$ 4 holes/uc in the metallic phase. Working with films one to two unit cells thicker than the electrical dead layer thickness ($\sim$ 4 nm), we have demonstrated nonvolatile, reversible ferroelectric field effect modulation of $T_{MI}$ in SNNO by up to 10 K. The maximum resistance ratio $R_{high}$/$R_{low}$ is 1.7 at 140 K, which is in the thermally activated regime. In the metallic phase, the carrier density has been modulated by 1x10$^{15}$ cm$^{-2}$, corresponding to the polarization field of PZT of 80 $\mu $C/cm$^{2}$.