The Far Infrared Dynamics of Multiferriocs RMnO3 and RMn2O5 (R=Rare Earth)

NESTOR E. MASSA

Conferencia; seminario-Université François Rabelais Tours-GREMAN; 2012

Groupement de recherche Matériaux Microélectronique Acoustique Nanotechnologies , Université François Rabelais Tours, Faculté des Sciences & Techniques, Bâtiment E, 20 avenue Monge,Parc Grandmont,37200 Tours, Francia

In this talk we will be cover experimental work carried out in perovskite and perovskite-like multiferroics. These are materials that at a certain temperature range ferroelectric and magnetic orders coexist, i.e., they have two order parameters, spontaneous polarization (antiferroelectric, ferroelectric, ferrilectric), and spontaneous magnetization (antiferromagnetism, ferromagnetism, ferrimagnetism) triggering, by magnetoelectric coupling, one order by the other. I will report on the temperature dependent lattice dynamics of orthorhombic NdMnO3 from 4 K to dissociation by combining far infrared emissivity and reflectivity techniques. We prompt by oxidation small polaron and hopping conductivity at high temperatures denoting eg electrons mobility in a Jahn-Teller distorted environment. In the far infrared a phase like-mode becomes well defined at 300 K and condenses below ~TN into two hardening modes that inelastic neutron scattering (Paih¨¨s et al, PRB 74, 134400 (2009)) allows to identify as hybridized Goldstone modes.The highest frequency ¡±phonon-like¡± soft band obeys a ¦Â=0.25 power law as for a tri-critical point similar to the soft mode in BiFeO3.(Massa et al, JAP 108, 084114 (2010)). The lower frequency ¡°spin-like¡± soft band obeys a ¦Â=0.5 power law as for a 2nd order transition expected in a Landau model for spontaneous magnetization in the ordered phase.Measurements in hexagonal TmMnO3 yield a similar picture but showing split in the phonon-like band due to lower symmetry and two superexchange interactions in the ab plane. We have also found that ¡°spin-like¡± bands depend on the Rare Earth mass.We will then review our the results for the impurity phases RMn2O5 (R=Rare Earth). Far infrared spectra of these compounds suggest that in spite of the increment in complexity both sets of compounds, RMnO3 and RMn2O5, share multiferroicity within a common framework lacking of strong structural rearrangements.