Soft-mode spectroscopy of ferroelectrics and multiferroics: A review

This article summarizes the results of the investigations of the dynamics of ferroelectric (FE) phase transitions (PTs) obtained in Prague during the last 25 years. After a short introduction, explaining differences between displacive and order-disorder types of FE PTs, the results of the broadband dielectric, THz, and mainly IR spectroscopic investigations of hydrogen-bonded FEs, BaTiO3, relaxor FEs, strained incipient FEs, and various multiferroics are reviewed. The high sensitivity of the IR spectroscopy to polar phonons was demonstrated in ultrathin films, which allowed us to reveal strain-induced FE PTs. Electrically active magnons (i.e., electromagnons) can be observed in the IR and Raman spectra of multiferroics. Their frequencies soften on heating toward temperatures of magnetic PTs similarly as phonons in displacive FEs. As expected, the electromagnons can be dependent on the external magnetic field. This behavior has been demonstrated in BiFeO3, SrMn7O12, and multiferroics with Y- and Z-type hexaferrite crystal structures.