Low-energy magnon excitations in magnetoelectric LiFePO$_4$ have been investigated by high-frequency high-field electron spin resonance spectroscopy in magnetic fields up to B = 58 T and frequencies up to f = 745 GHz. For magnetic fields applied along the easy magnetic axis, the excitation gap softens and vanishes at the spin-flop field of BSF = 32 T before hardening again at higher fields. In addition, for B smaller than BSF we observe a resonance mode assigned to excitations due to Dzyaloshinskii-Moriya (DM)-interactions, thereby evidencing sizable DM interaction of approx 150 micro eV in LiFePO4. Both the magnetisation and the excitations up to high magnetic fields are described in terms of a mean-field theory model which extends recent zero field inelastic neutron scattering results. Our results imply that magnetic interactions as well as magnetic anisotropy have a sizable quadratic field dependence which we attribute to significant magnetostriction.
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