  The spin-phonon coupling in magnetic materials is due to the modulation of
the exchange integral by lattice vibrations. BaFe12O19 M-type hexaferrite,
which is the most used magnetic material as permanent magnet, transforms into
ferromagnet at high temperatures, but no spin-phonon coupling was previously
observed at this transition. In this letter, we investigated the
temperature-dependent Raman spectra of polycrystalline BaFe12O19 M-type
hexaferrite from room temperature up to 780 K to probe spin-phonon coupling at
the ferrimagnetic transition. An anomaly was observed in the position of the
phonon attributed to the Fe(4)O6 octahedra, evidencing the presence of a
spin-phonon coupling in BaM in the ferrimagnetic transition at 720 K. The
results also confirmed the spin-phonon coupling is different for each phonon
even when they couple with the same spin configuration.
