Abstract
The 57Fe Mössbauer spectra of FeMnP show a remarkable magnetic field distribution for T ≲ 175K, even though all iron atoms are crystallographically equivalent. It seems possible to explain this distribution by a spin modulation described by the coexistence of an antiferromagnetic and a helimagnetic mode. The mechanism that couples these modes is taken to be the so-called forced degeneracy. The signs and approximate values of the different exchange integrals consistent with forced degeneracy are calculated by the matrix method. The results show that superexchange between two of the neighbouring Fe atoms occurs and that the exchange couplings between the Fe-Fe atoms and between the Mn-Mn atoms are larger than between the Fe-Mn atoms. For 175K ≲ T < TN (265 K) the magnetic structure can be described by a single, probably antiferromagnetic, mode.