Uniaxial ferroelectric quantum criticality in multiferroic hexaferritesBaFe12O19 and SrFe12O19

摘要

BaFe12O19 is a popular M-type hexaferrite with a Néel temperature of 720 K and is of enormous commercial value ($3 billion/year). It is an incipient ferroelectric with an expected ferroelectric phase transition extrapolated to lie at 6 K but suppressed due to quantum fluctuations. The theory of quantum criticality for such uniaxial ferroelectrics predicts that the temperature dependence of the electric susceptibility χ diverges as 1/T³, in contrast to the 1/T² dependence found in pseudo-cubic materials such as SrTiO3 or KTaO3. In this paper we present evidence of the susceptibility varying as 1/T³, i.e. with a critical exponent γ = 3. In general γ = (d + z – 2)/z, where the dynamical exponent for a ferroelectric z = 1 and the dimension is increased by 1 from deff = 3 + z to deff = 4 + zdue to the effect of long-range dipole interactions in uniaxial as opposed tomultiaxial ferroelectrics. The electric susceptibility of the incipientferroelectric SrFe12O19, which is slightly further from thequantum phase transition is also found to vary as 1/T³.