  Optical spectroscopy has been combined with computational and theoretical
techniques to show how the spin dynamics in the model multiferroic BiFeO3
responds to the application of hydrostatic pressure and its corresponding
series of structural phase transitions from R3c to the Pnma phases. As pressure
increases, multiple spin excitations associated with non-collinear cycloidal
magnetism collapse into two excitations, which show jump discontinuities at
some of the ensuing crystal phase transitions. Effective Hamiltonian approach
provides information on the electrical polarization and structural changes of
the oxygen octahedra through the successive structural phases. The extracted
parameters are then used in a Ginzburg-Landau model to reproduce the evolution
with pressure of the spin waves excitations observed at low energy and we
demonstrate that the structural phases and the magnetic anisotropy drive and
control the spin excitations.
