Light scattering from magnetic excitations in orthoferrites

The Raman spectra of YFeO3, SmFeO3, DyFeO3, HoFeO3, and ErFeO3 have been measured as a function of temperature from 8 to 650 K. For all the materials, the spectra show two features in the 5 to 20 cm-1 range. From their polariziation properties, these two modes are identified as the two predicted spin-wave modes. For several of the samples, the ratio of the intensity of the Stokes-shifted to anti-Stokes-shifted peaks is not as expected for boson excitations. In addition, rotating the sample by 90°causes an inversion of the Stokes-to-anti-Stokes intensity ratio. The effect has been attributed to an intereference between linear and quadratic magnetic contributions to the dielectric function, and this model can account for the observations. Another striking effect probed by the light scattering is the spin-reorientation transition which occurs in the vicinity of 470 and 90 K for SmFeO3 and ErFeO3, respectively. In this transition, the net magnetic moment rotates continuously by 90°in the ac plane, with a resultant change in the spin-wave symmetries. This transition occurs over a temperature interval of about 20 K, during which the frequency of one of the spin-wave modes is expected to go to zero. While the change in spin-wave symmetry is reflected in the light scattering spectra, we do not observe complete "softening," and attribute this observation to a coupling with the rare-earth electronic states.