THz and infrared studies of multiferroic hexagonal Y1− x Eu x MnO3 (x = 0–0.2) ceramics

We report an investigation of hexagonal Y1− x Eu x MnO3 ceramics with x = 0, 0.1, and 0.2 using infrared (IR) and terahertz (THz) spectroscopies in the temperature range between 5 and 900 K. The temperature dependence of the THz permittivity reveals a kink near the antiferromagnetic (AFM) phase transition temperature T N ≈ 70 K, giving evidence of a strong spin–lattice coupling. Below T N, two absorption peaks were revealed in the THz spectra close to 43 and 73 cm−1. While the first peak corresponds to a sharp AFM resonance exhibiting softening on heating towards T N, the second one may be attributed to an impurity mode or a multiphonon absorption peak. High-temperature THz spectra measured up to 900 K reveal only small gradual increase of the permittivity in agreement with a weak phonon softening observed in the IR reflectance spectra upon heating. This corresponds to an improper ferroelectric character of the phase transition proposed from first principle calculations by Fennie and Rabe (Phys. Rev. B 72: 100103(R), 2005).