Terahertz soft mode in Ba-Pb M-type hexaferrite ceramics

An unusual ferroelectric-like soft mode is discovered in terahertz spectra of dielectric permittivity of lead-substituted barium hexaferrite. The dynamics of the mode cannot be described either by Cochran or by Curie-Weiss equations. The obtained dependence of the soft mode parameters on lead concentration x(Pb) allows to suggest quantum phase transition at x(Pb)≈0.2.