Ferroelectricity in Strain-Free SrTiO$_{3}$ Thin Films

Biaxial strain is known to induce ferroelectricity in thin films of nominally non-ferroelectric materials such as SrTiO$_{3}$. However, we show that even strain-free SrTiO$_{3}$ films and the paraelectric phase of strained films exhibit bulk frequency-dependent polarization hysteresis loops on the nanoscale at room temperature, and stable switchable domains at 50 K. By a direct comparison of the strained and strain-free SrTiO$_{3}$ films using dielectric, ferroelectric, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that both strain states of SrTiO$_{3}$ films are bulk relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions. This mechanism could have broader applicability to strain-induced ferroelectricity in other incipient ferroelectric materials.