Soft-mode spectroscopy of ferroelectrics and multiferroics: A review

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