Magnetic interactions and the puzzling absence of any Raman mode in EuTiO3

Abstract: Polycrystalline ceramic samples and a single crystal of EuTiO3 have been investigated by Raman spectroscopy in the temperature range 80-300 K. Although synchrotron XRD data clearly indicated the cubic to tetragonal phase transition around 282 K, no mode from the symmetry allowed Raman active phonons was found in the tetragonal phase, contrary to the case of the homologous SrTiO3. In order to study the evolution of this unique characteristic, ceramics of EuxSr1-xTiO3 (x=0.03-1.0) characterized by synchrotron XRD for the structural phase transition have been also investigated by Raman spectroscopy, verifying the very strong influence on the Raman yield by Eu substitution. By applying an external magnetic field or alternatively hydrostatic pressure modes are activated in the Raman spectra. The temperature dependence of the main mode that is activated shows remarkable agreement with theoretical predictions. We attribute the puzzling absence of the Raman modes to a mechanism related to strong spin-lattice interaction that drives the cubic to tetragonal structural phase transition and makes the Raman tensor antisymmetric. On the contrary, the external perturbations induce a symmetric Raman tensor allowing even symmetry modes to be present in the spectra. Previous EPR, muon scattering and magnetic measurements indicated the presence of small magnetic interactions deep inside the paramagnetic phase. In order to probe those magnetic interactions in our EuTiO3 polycrystalline sample and test our hypothesis, we have performed temperature dependant XAS/XMCD, which support the existence of magnetic nanodomains even close to room temperature.