ИСТИНА

57Fe Mossbauer study of the spatial spin-modulated structure (SSMS) as well as electric and magnetic hyperfine interactions in multiferroics BiFe1−xMnxO3 (x = 0.05, 0.10, 0.25) with a rhombohedral structure were carried out. The Mossbauer spectra have been analyzed in terms of the incommensurate anharmonic SSMS of cycloid type model. All substituted ferrites contained the positions of the iron atoms in the first cation coordination sphere of which there are one, two, three, and four atoms of Mn depending on the composition of ferrites. For the studied ferrites it was determined the temperature ranges of existence of incommensurate anharmonic SSMS of the cycloid type in which iron atoms with different cationic environment participated. It was found that with increase temperature from 5.2 K up to ~150 K the anharmonicity parameter is almost constant, then it decreases to zero at ~350 K, and then it again increases. It is shown that at temperatures below the temperature at which the anharmonicity parameter is zero, a magnetic anisotropy of the "easy axis" type is realized, and above this temperature there is an magnetic anisotropy of the "easy plane" type. An increase of the anharmonicity parameter was observed for the Mn concentration increasing. A further increase in the concentration leads to the destruction of an anharmonic modulated SSMS and spin structure of the ferrite is transformed into the antiferromagnetic structure of the G-type. Based on the data obtained, a magnetic phase diagram was constructed in the "concentration" – "temperature" axes, indicating the regions of the paramagnetic state (PM), anharmonic SSMS with magnetic anisotropy of the "easy axis" and "easy plane" type, and antiferromagnetic (AFM) ordering (see Fig. 1) as well. In the framework of the model of an incommensurate anharmonic SSMS of cycloid type, the temperature dependences of the hyperfine parameters of the 57Fe Mossbauer spectrum – the isomer shift, the quadrupole shift of the spectral components, isotropic and anisotropic contributions to the hyperfine magnetic field – were obtained and analyzed. It is established that the anisotropic contributions to the hyperfine magnetic field in the investigated ferrites with increasing temperature firstly (up to ~350 K) increase slightly, and then decrease, tending to zero in the temperature range 440-580 K, depending on the degree of substitution. It was also found that the replacement of the Fe atom by the Mn atom in the nearest environment of the Fe atom decreased the isotropic contribution to the hyperfine magnetic field, which increased with increasing temperature. This replacement did not lead to a noticeable change in the anisotropic contribution to the hyperfine magnetic field, quadrupole shift and isomer shift.