Eur. Phys. J. B 78, 367-372 (2010)
https://doi.org/10.1140/epjb/e2010-10701-0

Spin-phonon coupling in multiferroic manganites RMnO₃: comparison of pure (R = Eu, Gd, Tb) and substituted (R = Eu_1-xY_x) compounds

¹ Physikalisches Institut (EP3), Universität Würzburg, 97074 Würzburg, Germany
² Institut für Festkörperphysik, Technische Universität Wien, 1040 Wien, Austria
³ General Physics Insitute of the Russian Academy of Sciences, 119991 Moscow, Russia

Corresponding author: ^a sven.issing@physik.uni-wuerzburg.de

Received: 14 September 2010
Published online: 22 November 2010

Abstract

For rare-earth manganite RMnO₃ compounds spin-phonon coupling manifests itself as a phonon softening in the temperature range of the magnetically ordered phases. Within this class of materials, a continuous tuning of the lattice and thus also of the magnetic properties of multiferroic manganites is achieved by Y doping in substituted Eu_1-xY_xMnO₃. We compare the impact on spin-phonon coupling within this partial-substitution approach in a series of Eu_1-xY_xMnO₃ samples 0 ≤ x ≤ 0.5) with the effect of a complete exchange of the rare earth ions R³⁺ in a series of pure RMnO₃ compounds (R = Eu, Gd, Tb). For this purpose we employ polarized Raman scattering in the 10–300 K temperature range. The low-temperature results show phonon softening in all investigated compounds. For decreasing R³⁺ radius, i.e. an increasing orthorhombic distortion and magnetic frustration, we observe in both systems a weakening of the spin-phonon coupling. For known sublattice magnetization within the MnO₂-plane, quantitative results for the spin-phonon coupling constant are derived for both cases within a molecular field approximation. Our results show, that the spin-phonon coupling strength in the magnetically ordered phases of the various investigated manganites does not correlate with the magnetization pattern. Instead, the pure RMnO₃ compounds and the substituted Eu_1-xY_xMnO₃ fit excellently within a common scheme, in which the weakening of the spin-phonon coupling reflects the degree of tilting of the MnO₆ octahedra due to the orthorhombic distortion of the crystal lattice.