Accurate THz measurements of permittivity and permeability of BiFeO3 thin films

The electrodynamic properties of BiFeO 3 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.