  We report results of 57Fe Mossbauer and 31P NMR studies of a phosphide FeP
powder sample performed in a wide temperature range including the point (TN ~
120 K) of magnetic phase transitions. The 57Fe Mossbauer spectra at low
temperatures T < TN consist of very diffuse Zeeman pattern with line
broadenings and sizeable spectral asymmetry. It was shown that the change of
the observed spectral shape is consistent with the transition into the space
modulated helicoidal magnetic structure. Analysis of the experimental spectra
was carried out assuming the anisotropy of the magnetic hyperfine field Hhf at
the 57Fe nuclei when the Fe3+ magnetic moment rotates with respect to the
principal axis of the electric field gradient (EFG) tensor. The obtained large
temperature independent anharmonicity parameter m ~ 0.96 of the helicoidal spin
structure results from easy-axis anisotropy in the plane of the iron spin
rotation. It was assumed that the very low maximal value of Hhf(11K) ~ 36 kOe
and its high anisotropy delta_Hanis(11K) ~ 30 kOe can be attributed to the
stabilization of iron cations in the low-spin state (SFe = 1/2). The 31P NMR
measurements demonstrate an extremely broad linewidth reflecting the spatial
distribution of the transferred internal magnetic fields of the Fe3+ ions onto
P sites in the magnetically ordered state.
