Accurate THz measurements of permittivity and permeability of BiFeO3 thin films

The electrodynamic properties of BiFeO3 films in the THz region are investigated via time domain spectroscopy. Combining the use of transmission (T˜) and reflection (R˜) measurements under normal incidence, the refractive index and impedance of the samples under test are evaluated using a retrieval method. From T˜ (R˜) data two complex functions describing the refractive index n˜T (n˜R) and impedance z˜T (z˜R) are extracted from the independent minimization of the error functions given by the difference between the theoretical model and measurements. Knowledge of the pairs (n˜T, n˜R) and (z˜T, z˜R) enables to calculate with a high accuracy both complex permittivity ε˜ and permeability μ˜ of the sample. Signatures of magnetoelectric effects and phononic resonances are observed in the permittivity and permeability functions and discussed in detail.