NMR study of magnetic structure and hyperfine interactions in the binary helimagnet FeP

We report a detailed study of the ground-state helical magnetic structure in monophosphide FeP by means of ³¹P NMR spectroscopy. We show that the zero-field NMR spectrum of the polycrystalline sample provides strong evidence of an anisotropic distribution of local magnetic fields at the P site with substantially lower anharmonicity than that found at the Fe site by Mössbauer spectroscopy. From field-sweep ³¹P NMR spectra we conclude that a continuous spin-reorientation transition occurs in an external magnetic field range of 4-7 T, which is also confirmed by specific-heat measurements. We observe two pairs of magnetically inequivalent phosphorus positions resulting in a pronounced four-peak structure of the single crystal ³¹P NMR spectra characteristic of an incommensurate helimagnetic ground state. We revealed a spatial redistribution of local fields at the P sites caused by Fe spin-reorientation transition in high fields and developed an effective approach to account for it. We demonstrate that all observed ³¹P spectra can be treated within a model of an isotropic helix of Fe magnetic moments in the (a b )-plane with a phase shift of 36^∘ and 176^∘ between Fe1-Fe3 (Fe2-Fe4) and Fe1-Fe2 (Fe3-Fe4) sites, respectively, in accordance with the neutron scattering data.