Quantum Critical Point study in Multiferroic Hexaferrites: BaFe12O19, SrFe12O19, and PbFe3Ga9O19 -- Verification of the Khmelnitskii Theory

BaFe12O19 is a popular M-type hexaferrite with T(Neel) = 720 K of enormous commercial value (3 billion dollars/year). It exhibits an incipient ferroelectric phase transition (in violation of the Spaldin-Hill rule) extrapolated to lie at 6.0 K Kelvin but suppressed due to quantum fluctuations (as in SrTiO3). The QCP theory of Khmelnitskii for such uniaxial ferroelectrics predicts that the inverse isothermal electric susceptibility varies as T cubed, in contrast to that for pseudo-cubic materials such as SrTiO3 or KTaO3, a hypothesis we verify.