A Mössbauer Study of Helimagnetic FeAs

The 57 Fe Mössbauer spectra of FeAs above the Néel temperature, 77 K, display an electric-quadrupole split doublet with a total splitting of 0.55 mm/s at 295 K. This value increases almost linearly (0.05 mm/s per 100 K) with decreasing temperature, a change which can be correlated to the decreasing average Fe-As nearest-neighbour distance. Below 77 K a complex, partly nonresolved Mössbauer pattern develops. The magnetic hyperfine field varies strongly with the rotation of the magnetic moment in the ( a, b )-plane, giving values between 4.8 T and 1.6 T. Using the conversion factor 10 T /μ B the average magnetic moment would be 0.30μ B . Whether these highly anisotropic hyperfine fields also correspond to an anisotropic distribution of magnetic moments is still an open question.