Modulated magnetic structure of Fe3PO7 as seen by Fe57 Mössbauer spectroscopy

The paper reports new results of the 57Fe M\"ossbauer measurements on Fe3PO4O3 powder sample recorded at various temperatures including the point of magnetic phase transition TN ~ 163K. The spectra measured above TN consist of quadrupole doublet with high quadrupole splitting of D300K ~ 1.10 mm/s, emphasizing that Fe3+ ions are located in crystal positions with a strong electric field gradient (EFG). In order to predict the sign and orientation of the main components of the EFG tensor we calculated monopole lattice contributions to the EFG. In the temperature range T < TN, the experimental spectra were fitted assuming that the electric hyperfine interactions are modulated when the Fe3+ spin (S) rotates with respect to the EFG axis and emergence of spatial anisotropy of the hyperfine field Hhf = S\~AI at 57Fe nuclei. These data were analyzed to estimate the components of the anisotropic hyperfine coupling tensor (\~A). The large anharmonicity parameter, m ~ 0.94, of the spiral spin structure results from easy-axis anisotropy in the plane of the iron spin rotation. The temperature evolution of the hyperfine field Hhf(T) was described by Bean-Rodbell model that takes into account that the exchange magnetic interactions are strong function of the lattice spacing. The obtained M\"ossbauer data are in qualitative agreement with previous neutron diffraction data for a modulated helical magnetic structure in strongly frustrated Fe3PO4O3.