Spin-order-dependent Raman scattering in RuSr2GdCu2O8

A detailed study of the magnetic ordering effects on the Raman spectra of the ferromagnetic superconductor RuSr2GdCu2O8 is presented. As the temperature is lowered, additional phonon peaks are observed and assigned to folded phonons rendered active by the doubling of the unit cell consequent with rotations of the RuO6 octahedra. The intensity of these modes strengthens very rapidly as the sample is cooled below the Curie temperature, evidencing the importance of spin-dependent phonon Raman scattering in this material. In addition, all phonon fines display either an "anomalous" softening or hardening below Tc, also indicative of a strong spin-phonon interaction, Most interestingly, a different low energy peak which is nor seen in the related ferromagnet SrRuO3 develops and shifts to higher energies below Tc. A tentative explanation is proposed in that this latter feature reflects the two-dimensional density of RuO2-layer magnon states, observable by Rarnan scattering through an "exchange-scattering" mechanism that involves the Gd3+ paramagnetic spins. This conclusion and some of the phonon peak assignments are partly supported by complementary measurements on a RuSr2EuCu2O8 sample where Gd3+(S=7/2) has been fully replaced by the J=0 ground-state Eu3+ ion.