Multiferroicity in BiFeO3 and RMn2O5 (R=Rare Earth) Similarities and Differences

NESTOR E. MASSA

Orleans

Seminario; Multiferroicity in BiFeO3 and RMn2O5 (R=Rare Earth) Similarities and Differences; 2011

CNRS-Conditions Extr¨ºmes et Mat¨¦riaux Haute Temp¨¦rature et Irradiation, 1D, Av. de la Recherche Scientifique, F-45071 Orl¨¦ans, Francia

Multiferroicity in BiFeO3 and RMn2O5 (R=Rare Earth) Similarities and Differences N¨¦stor E. Massa Laboratorio Nacional de Investigaci¨®n y  Servicios en Espectroscop¨ªa ¨®ptica Centro CEQUINOR, Universidad Nacional de La Plata, C.C. 962, 1900 La Plata, Argentina, e-mail: neemmassa@gmail.com The field of multiferroics belongs to all materials that develop ferroelectricity in addition of having magnetic order. They have two order parameters, spontaneous lattice polarization (ferroelectricity, antiferroelectricty, ferrielectricity) and spontaneous magnetization (ferromagnetism, antiferromagnetism, ferrimagnetism) triggering one order by the other through magnetoelectric coupling. In this talk I will comment on our recent measurements of compounds that are emblematic within this family. BiFeO3, has a ferroelectric phase transition at TC ~1090K becoming antiferromanegtic at TN ~640K. It has a lattice alike those found in classic perovskites. We have combined far infrared reflectivity and emissivity to elucidate phonon behavior from 4K until melting. In particular, we trace the temperature dependence of a soft- mode in the antiferromagnetic and ferroelectric phases. It ceases softening at about 400-500K obeying below these temperatures a power law ¦Â=0.25. Its behavior in the ferroelectric phase is inhibited by lattice anharmonicities and decomposition products adding to the intrinsic instability of BiFeO3. Then TN ~640K, may be considered as a cross-over temperature toward an order-disorder regime. This is not only due to thermal fluctuations but, as we verified by X-ray absorption, to chemical disorder also compromising the ferroelectric transition at TC ~1090K. On the other hand, we also searched for a ¡°ferroelectric behavior¡± in the more complex RMn2O5 (R=Y, Rare Earth, Bi) with TN ~50K and TC ~40K. We determined the non-existence of soft-phonon driven phase transition and we found, unexpectedly, a collective electronic excitation alike the infrared active phase mode of a charge density wave. We understand its origin in delocalized electrons from the dynamical distorted dimmer .sublattice made of two Mn3+ Jahn-Teller pyramids. We conclude that the detection of lattice polarizability (ferroelectricity) is due to the passage from a dynamical to static regime unscreening lattice features. There is no structural phase transition, with the loss of an inversion center at ~TC, a finding, concurrent to Raman and infrared spectra showing the same number of phonons above and below TN ~TC .