High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe${}_{12}$O${}_{19}$

Abstract

High quality single crystals of BaFe${}_{12}$O${}_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe${}_{12}$O${}_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe${}^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\epsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\epsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO${}_3$. The presence of the upturn in $1/\epsilon$ indicates that BaFe${}_{12}$O${}_{19}$ is a critical quantum paraelectric system with Fe${}^{3+}$ ions involved in both magnetic and electric dipole formation.

9 pages, 9 figures, submitted to APL Materials