  We report the effect of epitaxial strain on the magnitude and retention of
the ferroelectric field effect in high quality PbZr$_{0.3}$Ti$_{0.7}$O$_3$
(PZT)/3.8-4.3 nm Sm$_{0.5}$Nd$_{0.5}$NiO$_3$ (SNNO) heterostructures grown on
(001) LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) substrates. For SNNO on LAO, which
exhibits a first-order metal-insulator transition (MIT), switching the
polarization of PZT induces a 10 K shift in the transition temperature
$T_{MI}$, with a maximum resistance change between the on and off states of
$\Delta$$R$/$R_{on}$ ~75%. In sharp contrast, only up to 5% resistance change
has been induced in SNNO on STO, where the MIT is second-order, with the
modulation of $T_{MI}$ negligibly small. We also observe thermally activated
retention of the off state resistance $R_{off}$ in both systems, with the
activation energy of 22 meV (28 meV) for devices on LAO (STO). The time
dynamics and thermal response of the field effect instability points to
phonon-assisted interfacical trapping of charged mobile defects, which are
attributed to strain induced oxygen vacancies. At room temperature, $R_{off}$
stabilizes at ~55% and ~19% of the initial switching levels for SNNO on LAO and
STO, respectively, reflecting the significantly different oxygen vacancy
densities in these two systems. Our results reveal the critical role of strain
in engineering and modeling the complex oxide composite structures for
nanoelectronic and spintronic applications.
