Magnetic and vibrational excitations in rare-earth orthoferrites: A Raman scattering study

Abstract

We have studied the magnon and phonon spectra of R${\mathrm{FeO}}_{3}$ crystals with R${=\mathrm{}}_{39}$Y, $_{65}\mathrm{Tb}$, $_{66}\mathrm{Dy}$, $_{67}\mathrm{Ho}$, and $_{69}\mathrm{Tm}$. The temperature dependence of q\ensuremath{\rightarrow}\ensuremath{\sim}0\ensuremath{\rightarrow} magnons is examined near the respective spin-reorientation (SR) transitions of ${\mathrm{TbFeO}}_{3}$, ${\mathrm{DyFeO}}_{3}$, and ${\mathrm{TmFeO}}_{3}$. In ${\mathrm{TbFeO}}_{3}$ the lower magnon branch (${M}_{1}$) exhibits an unusual stiffening on approaching the SR region, whereas ${\mathrm{TmFeO}}_{3}$ exhibits an incomplete mode-softening behavior. Also, when the first-order SR transition of ${\mathrm{DyFeO}}_{3}$ is approached from above, ${M}_{1}$ shows no significant variation in the frequency. It is suggested that the distinctly different behavior of ${M}_{1}$ magnon in ${\mathrm{TbFeO}}_{3}$ could be related to the magnetic ordering of ${\mathrm{Tb}}^{3+}$ moments just below the SR region. The Stokes--anti-Stokes intensity ratio of the upper magnon branch (${M}_{2}$) has been reexamined in ${\mathrm{YFeO}}_{3}$. The nonthermal aspect of this ratio, due to interference effects, occurs only for crossed polarizations of incident and scattered photons; however, no such anomaly is evident for the parallel-polarized case. The latter behavior is consistent with the antisymmetric property of the linear magnetic contribution to the dielectric function. Polarized, first-order phonon spectra of different crystals (R${=\mathrm{}}_{65}$Tb, $_{66}\mathrm{Dy}$, $_{67}\mathrm{Ho}$, and $_{69}\mathrm{Tm}$) have been obtained and compared in the (30--600)-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range. These results indicate that the changes in the rare-earth ion mass do not affect the lower-frequency modes which lie below 300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.