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

We have studied the magnon and phonon spectra of RFeO₃ crystals with R=₃₉Y, ₆₅Tb, ₆₆Dy, ₆₇Ho, and ₆₉Tm. The temperature dependence of q{→}{~}0{→} magnons is examined near the respective spin-reorientation (SR) transitions of TbFeO₃, DyFeO₃, and TmFeO₃. In TbFeO₃ the lower magnon branch (M₁) exhibits an unusual stiffening on approaching the SR region, whereas TmFeO₃ exhibits an incomplete mode-softening behavior. Also, when the first-order SR transition of DyFeO₃ is approached from above, M₁ shows no significant variation in the frequency. It is suggested that the distinctly different behavior of M₁ magnon in TbFeO₃ could be related to the magnetic ordering of Tb³⁺ moments just below the SR region. The Stokes--anti-Stokes intensity ratio of the upper magnon branch (M₂) has been reexamined in YFeO₃. 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=₆₅Tb, ₆₆Dy, ₆₇Ho, and ₆₉Tm) have been obtained and compared in the (30--600)-cm^-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 cm^-1.