Dielectric spectra of Bi$_{0.98}$Nd$_{0.02}$FeO$_3$ multiferroic thin films in the terahertz frequency range

Transmission and reflection spectra of Bi$_{0.98}$Nd$_{0.02}$FeO$_3$ multiferroic thin films on MgO single-crystal substrates have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 8 to 1000 cm$^{-1}$ at room temperature. The complex permittivity spectra of the films have been calculated in terms of the layered medium model. It has been revealed that a decrease in the film thickness leads to a considerable increase in the losses in a range of 30 cm$^{-1}$ and the corresponding fivefold increase in the static permittivity (to 500 for a film 32 nm thick). This phenomenon has been discussed in the framework of the phenomenological theory of phase transitions.