Title: Modulated magnetic structure of Fe3PO7 as seen by 57Fe Mössbauer spectroscopy,
Abstract: The paper reports new results of the 57Fe Mössbauer 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 = SÃI at 57Fe
nuclei. These data were analyzed to estimate the components of the anisotropic
hyperfine coupling tensor (Ã). 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
Mössbauer data are in qualitative agreement with previous neutron diffraction
data for a modulated helical magnetic structure in strongly frustrated
Fe3PO4O3.