Influence of magnetic field on the paraelectric to ferroelectric phase transition in BaTiO₃

Abstract

Ferroelectricity in perovskites is closely related to the strong and nonlinear polarizability of the oxygen ion ^1-3. This unusual behaviour of the O^– ion is due to a dynamically enhanced dϵ/pπ hybridization between unfilled transition metal d orbitals and O 2p orbitals. A direct possibility of studying the polarizability of the oxygen ion in bulk materials is provided by a strong magnetic field which directly couples to the orbital wave functions and thereby causes a change in dϵ/pπ hybridization. This effect can be observed directly via a shift in the paraelectric to ferroelectric phase transition temperature 4. For BaTiO₃ we obtained for a field of 20 Tesla a shift of δT_c ≈ 0.35 K. A second feature of the magnetic field experiments is the slight- almost zero- increase in slope in the 1/ϵ versus T curves with increasing magnetic field. From this result we can directly conclude that the nonlinear part of the core-shell force constant of the oxygen ion changes not or only very slightly with magnetic field strength, i.e. the magnetic field mainly influences the linear polarizability of the oxygen ion which enters the harmonic soft mode frequency 4. The increase in transition temperature with magnetic field, the effect of hydrostatic pressure and of uniaxial or biaxial stress, and the effect of transitio metal ion doping ⁵ can be described qualitatively within a consistent picture which assumes that the lattice constant in the ferroelectric perovskites is smaller than twice the interionic distance which characterizes the maximum in the dϵ/pπ overlap ^4,5.