Spin-order-dependent Raman scattering inRuSr2GdCu2O8

Abstract

A detailed study of the magnetic ordering effects on the Raman spectra of the ferromagnetic superconductor ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ 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 ${\mathrm{RuO}}_{6}$ 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 lines display either an ``anomalous'' softening or hardening below ${T}_{C},$ also indicative of a strong spin-phonon interaction. Most interestingly, a different low energy peak which is not seen in the related ferromagnet ${\mathrm{SrRuO}}_{3}$ develops and shifts to higher energies below ${T}_{C}.$ A tentative explanation is proposed in that this latter feature reflects the two-dimensional density of ${\mathrm{RuO}}_{2}$-layer magnon states, observable by Raman scattering through an ``exchange-scattering'' mechanism that involves the ${\mathrm{Gd}}^{3+}$ paramagnetic spins. This conclusion and some of the phonon peak assignments are partly supported by complementary measurements on a ${\mathrm{RuSr}}_{2}{\mathrm{EuCu}}_{2}{\mathrm{O}}_{8}$ sample where ${\mathrm{Gd}}^{3+}(S=7/2)$ has been fully replaced by the $J=0$ ground-state ${\mathrm{Eu}}^{3+}$ ion.