Complex symmetry of phases and hidden magnetic properties of EuTiO3

Some novel puzzling and unexplained features of the phase transitions in EuTiO₃ (ETO) have now been reanalyzed and an unexpected unambiguous explanation presented. Since ETO is almost multiferroic it has recently attracted much experimental and theoretical interest. In addition, its proximity and similarity to SrTiO₃ (STO) have raised hope that it could serve as a substrate material with magnetic properties. Here it is shown that considerable and distinct features differentiate it from STO and all other perovksite oxides.

The experimental data are obtained from birefringence measurements and high precision high quality synchrotron XRD data. Both have been studied as a function of temperatures and a small magnetic field. Theoretically a spin-lattice coupling approach is used from which the magnetic exchange interactions, the ground state energies and the Néel temperature are derived as a function of temperature and magnetic field.

Opposite to hitherto knowledge we demonstrate that the cubic - "tetragonal" phase transition at 282K leads to a much lower symmetry than tetragonal and can only be explained by a monoclinic symmetry. This transition is followed by another one around 210K, where the symmetry is further lowered but remains monoclinic. While most likely a coexistence of two monoclinic phases dominates the regime below 282K, a single one survives below 210K. A marked influence of the magnetic field is apparent in all experimental results where the lattice constants shrink with field and the domain structure becomes directional dependent. The latter may find applications in magneto-optical devices.

The theoretical analysis suggests competing magnetic ground states and frustration which suppress the formation of macroscopic magnetism but are visible dynamically.