Anomalous phonon shifts in the paramagnetic phase of multiferroic RMn2O5 (R=Bi, Eu, Dy): Possible manifestations of unconventional magnetic correlations

Abstract

A Raman spectroscopic study of the high-frequency optical phonons in single crystals of the multiferroic system RMn2O5 (R=Bi, Eu, Dy) was performed. All studied materials show anomalous phonon shifts, below a new characteristic temperature for these materials, T(*)similar to 60-65 K. The sign and magnitude of such shifts appear to be correlated with the ionic radius of R, envolving from softenings for R=Bi to hardenings for R=Dy and showing an intermediary behavior for R=Eu. Additional phonon anomalies were identified below similar to T(N)similar to 40-43 K, reflecting the onset of long-range ferroelectric and/or magnetic order of the Mn sublattice. Complementary dc-magnetic susceptibility [chi(T)] measurements for BiMn2O5 up to 800 K yield a Curie-Weiss temperature theta(CW)=-253(3) K, revealing a fairly large frustration ratio (parallel to theta(CW)parallel to/T-N=6.3). Deviations of chi(T) from a Curie-Weiss paramagnetic behavior due to magnetic correlations were observed below temperatures of the order of parallel to theta(CW)parallel to, with the inverse susceptibility showing inflection points at similar to 160 K and similar to T-*. Supported by chi(T) data, the anomalous Raman phonon shifts below T-* are interpreted in terms of the spin-phonon coupling, in a scenario of strong magnetic correlations. Overall, these results support significant magnetic frustration, introduce a new characteristic temperature (T-*), and suggest a surprisingly rich behavior for the magnetic correlations in the paramagnetic phase of this system.