Intriguing spin-lattice interactions in EuTiO.

During the last decade the cubic perovskite oxide EuTiO (ETO) has attracted enormous novel research activities due to possible multiferroicity, hidden magnetism far above its Néel temperature at T = 5.5 K, structural instability at T = 282 K, possible application as magneto-electric optic device, and strong spin-lattice coupling. Here we address a novel highlight of this compound by showing that well below T a further structural phase transition occurs below 210 K without the application of an external magnetic field, and by questioning the assumed tetragonal symmetry of the structure below T where tiny deviations from true tetragonality are observed by birefringence and XRD measurements. It is suggested that the competition in the second nearest neighbor spin-spin interaction modulated by the lattice dynamics is at the origin of these new observations.