Magnetic ordering effects in the phonon spectra of orthorhombic RMnO(3) compounds

The phonon properties of orthorhombic manganites RMnO(3) are studied using a Green's function technique taking into account anharmonic spin-phonon and phonon-phonon interaction terms. The strong spin-phonon interaction leads to anomalies in the phonon energy and the damping around the magnetic phase transition. The phonon spectrum is discussed for different exchange interaction J(1) and spin-phonon interaction R(sp) constants. In dependence on the sign of R(sp) we obtain softening or hardening of the phonon modes with decreasing temperature below the phase transition temperature T(N). This is associated with phonon modulation of the exchange interaction. It is shown that the phonon energy and the phonon damping depend on the radius of the rare earth ion r(R) and on the ion doping. The influence of the external magnetic field is discussed, too.