Mössbauer study of the magnetic structure of FeP

The compound FeP shows, at low temperatures and in zero external magnetic field, a helical spin configuration for iron with a propagation vector along the c-axis as deduced from neutron diffraction studies. We have performed ⁵⁷Fe Mössbauer measurements in the temperature interval between 295 K and 4.2 K and in an external magnetic field on both powder specimen and single crystals of FeP. The results show a nearly temperature independent electric quadrupole interaction strength eQV_zz of 1.34 mm/s and a Néel temperature of 115 K. It was, however, impossible to fit the Mössbauer spectra below T_N using a simple double spiral model. Instead a bunched spiral model with the spins crowded along the a-axis gave very good fits. The magnetic hyperfine field was furthermore found to be modulated in the ab-plane. The maximum field of +3.6 T was found along the a-direction while the overall field, presumably spread over all other directions, was 0.2 T. The measurements in external magnetic field showed that the grains become partially oriented with the basal plane parallel to the external field.