Phonon and magnon scattering of antiferromagnetic Bi2Fe4O9

The phonon structure of antiferromagnetic Bi2Fe4O9 (space group Pbnm No. 55, T-N approximate to 240 K) was studied theoretically by calculations of lattice dynamics and experimentally between 10 and 300 K by polarized Raman spectroscopy. Most of the 12A(g) + 12B(1g) + 9B(2g) + 9B(3g) Raman modes were unambiguously identified. Strong second-order scattering was observed for ab-plane-confined incident and scattered light polarizations. In addition to the phonon-scattering, broad Raman bands with typical characteristics of magnon scattering appear below TN. The magnon bands are analyzed on the basis of magnetic structure of Bi2Fe4O9 and attributed to two-magnon excitations.