Mössbauer studies of spatial spin-modulated structure and hyperfine interactions in multiferroic Bi$^{57}$Fe$_{0.10}$Fe$_{0.85}$Cr$_{0.05}$O$_{3}$

Results of Mössbauer investigations on $^{57}$Fe nuclei in multiferroic material Bi$^{57}$Fe$_{0.10}$Fe$_{0.85}$Cr$_{0.05}$O$_{3}$ in the temperature range from 5.2 to 300 K are presented. Bulk rhombohedral samples were obtained by solidstate synthesis at high pressure. Mössbauer spectra were analyzed using the model of spatial incommensurate spin-modulated structure of the cycloidal type. Information on the influence of substituting Cr cations for Fe cations on hyperfine spectral parameters was obtained: the shift and quadrupolar shift of a Mössbauer line, and isotropic and anisotropic contributions into the hyperfine magnetic field. The anharmonicity parameter m of the spatial spin-modulated structure increases almost 1.7 times at 5.2 K when BiFeO$_{3}$ is doped with chromium. The data on m were used for calculation of the uniaxial magnetic anisotropy constants and their temperature dependences for pure and chromium-doped BiFeO$_{3}$.