Spin-phonon coupling in multiferroic manganites RMnO3: comparison of pure (R = Eu, Gd, Tb) and substituted (R = Eu1-xYx) compounds

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- x} Y _x MnO₃. We compare the impact on spin-phonon coupling within this partial-substitution approach in a series of Eu_{1- x} Y _x MnO₃ 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- x} Y _x MnO₃ 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.