Soft Mode Behavior in Transition Metal Doped SrTiO3 Thin Films on MgO Substrates

Abstract

The ferroelectric soft mode in polycrystalline pristine SrTiO₃ and weakly doped SrTiO₃:M (M = 2 at % Fe, Ni, Mn, Co) thin films on (001) MgO substrates has been studied using time-domain terahertz spectroscopy. Spectra of real and imaginary parts of film permittivity were determined in the frequency range of 5–100 cm^–1 at temperatures between 5 and 300 K. Central frequency and dielectric contribution of the ferroelectric soft mode show Barrett-like temperature dependencies similar to crystalline SrTiO₃. Large negative values of Curie temperature and enhanced positive values of Barrett quantum temperatures are discovered indicating that doped SrTiO₃ thin films are farther from ferroelectric phase transition than SrTiO₃ crystals.