From this, it is clear that D xy and that the effect is stronger if the uncoupled excitation energies of phonon and spin excitations are closer to resonance. For the near-resonant phonon, a strongly positive signal is observed in field, i.e., the transition becomes more allowed upon application of a field. This may be understood as a consequence of a significant mixing of spin and phonon transition at zero field, which transfers some of the transition strength of the phonon to the spin-transitions, leading to a decreased phonon transition probability. As the magnetic field is switched on, the spin-transitions move to different energies, and the phonon transition gradually assumes its full, non-perturbed intensity. At frequencies further away from the zero-field spin transition in either direction, the differential signal is predominantly negative, as now the coupling to the spin system perturbs the phonon transition at finite fields, whereas it was unperturbed at zero field. xz ) to the spin system were assumed for each of the vibrational modes. The resulting calculated FIR spectrum (Fig. ) shares many salient features with the experimental spectrum (Fig. ): A strong positive feature, flanked by two weaker negative features. Note that the experimental spectrum is plotted as the zero-field transmission