High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe₁₂O₁₉

High quality single crystals of BaFe₁₂O₁₉ were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe₁₂O₁₉ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe³⁺ 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 to 300 K. The inverse dielectric permittivity, 1/ε, along the c-axis shows a T² 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/ε. These features resemble those of classic quantum paraelectrics such as SrTiO₃. The presence of the upturn in 1/ε indicates that BaFe₁₂O₁₉ is a critical quantum paraelectric system with Fe³⁺ ions involved in both magnetic and electric dipole formation.