Magnetically frustrated behavior in multiferroics RMn2O5 (R=Bi, Eu, and Dy): A Raman scattering study.

A temperature dependent Raman scattering study in multiferroic single crystals RMn2O5 (R=Bi, Eu, and Dy) was performed. The Raman spectra were measured in the range from 150 to 450 cm-1 involving mostly Mn–O–Mn bending vibrations, complementing our previous work in a higher frequency range involving Mn–O stretching modes. A number of studied phonons present anomalous frequency behavior below a characteristic temperature, T*∼60–65 K, such as that found for the stretching modes. The sign and magnitude of such anomalous behavior appear to be correlated with the ionic radius of R, being softening for R=Bi and hardening for R=Eu and Dy in the range between TC/TN and T*. The anomalous phonon behaviors in both bending and stretching modes are consistent with an interpretation in terms of the spin-phonon coupling in a scenario of strong magnetic correlations. [ABSTRACT FROM AUTHOR]