Magnetoacoustics of rare-earth orthoferrites

Experimental and theoretical work on the magnetoacoustics of rare-earth orthoferrites near orientational phase transitions (OPT) is reviewed. The temperature and field dependences of magnetoresonant soft-mode frequencies and the sound velocity and attenuation near various OPTs, as obtained by RF and ultrasonic spectroscopy, are given. A spin-wave approximation theoretical analysis of orthoferrite magnetoacoustics is presented, accounting as fully as possible for interactions between all the subsystems involved, including the ordered ferrous, elastic, paramagnetic rare-earth, and dipole (electromagnetic) subsystems. The origin of energy gaps in the spin-wave spectrum is discussed in detail, as are the ultrasound dispersion, propagation velocity, and attenuation changes at OPT points. The energy gaps measured and the sound velocity behaviour observed are shown to result from the interaction of the orthoferrite subsystems. In most cases experimental data agree well with theoretical predictions.