Helical magnetic structure and hyperfine interactions in FeP studied by 57Fe Mössbauer spectroscopy and 31P NMR

• The 57Fe Mössbauer and 31P NMR studies of FeP in a wide temperature range.
• Anharmonicity of helicoidal magnetic structure results from easy-axis anisotropy.
• The spatial distribution of the transferred magnetic fields of the Fe3+ ions onto P sites.
• Mechanisms of the anisotropy of the magnetic hyperfine field Hhf at the 57Fe nuclei.

We report results of 57Fe Mössbauer 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 Mössbauer spectra at low temperatures T < TN present a very complex 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 a space-modulated helicoidal magnetic structure. Analysis of the experimental spectra was carried out assuming an 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 a very low maximal value of Hhf(11 K) ≈ 36 kOe and its high anisotropy ΔHanis(11 K) ≈ 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.