PHONON-SPECTRUM FOR BARIUM FERRITE

We have shown in previous work that the calculation of the intrinsic ferrimagnetic (FMR) linewidth in barium ferrite on the basis of the Kasuya-LeCraw relaxation mechanism requires a knowledge of the magnon and phonon dispersion curves far from the center of the Brillouin zone. Therefore, in this article, we have calculated the full phonon spectrum for this crystal on the basis of the rigid-ion model in anticipation of future FMR-linewidth calculations. The short-range forces were approximated by the Born-Mayer potential, with parameters derived from compressibilities and effective radii of the constituent ions in binary divalent compounds exhibiting the NaCl structure and composed of Ba2+, O2-, Mn2+, and Fe2+. Our results are in good agreement with recent infrared-reflectivity data on barium ferrite in the frequency range of the magnon spectrum (the range needed for FMR-linewidth calculations). The extreme sensitivity of the lowest transverse-optic frequency to variation of one of the short-range repulsive parameters is indicative of possible soft-mode behavior. Experimental verification of this prediction would be possible by measuring the reflectivity of this material at various temperatures with the incident electric field parallel to the c axis of the crystal. We propose an explanation of the dynamics of the trigonal-site iron in barium ferrite based on our calculation.