A 57Fe Mössbauer study of Fe2As: a magnetically induced electric-field-gradient asymmetry

The 57Fe Mössbauer spectrum of Fe2As has been measured in the temperature range 25–456 K. Evidence is found for two factors that affect the spectrum and that had been neglected in earlier investigations. These are (i) spin relaxation in a time period that is short compared with the characteristic Mössbauer measurement time and (ii) an anisotropy induced in the electric field gradient at the 57Fe nuclei, by the Weiss molecular field, via the spin orbit coupling. Taking these factors into account, we find that the Mössbauer spectra give consistent and physically reasonable parameters over the whole temperature range. There is also agreement with previous neutron-diffraction studies in that the spins in the antiferromagnetic phase are perpendicular to the c axis. The disagreements reported in earlier studies are seen to arise from a neglect of these factors.