ИСТИНА

57Fe Mossbauer detailed study of the spatial spin-modulated structure (SSMS) of the multiferroic BiFeO3 was carried out in a wide temperature range including the temperature of magnetic phase transition. The Mossbauer spectra have been analyzed by fitting in terms of the anharmonic spin cycloid model [1]. It is established that with the temperature increasing the anharmonicity parameter of the SSMS decreases tending to zero at ~330 K and then increases again (see Fig. 1). It is shown that at temperatures below ~330 K a magnetic anisotropy of the "easy axis" type is realized and above is the magnetic anisotropy of the "easy plane" type. An explanation for the change in the type of magnetic anisotropy is proposed, based on taking into account the different temperature dependences of the two contributions to the effective uniaxial magnetic anisotropy constant: a crystal anisotropy of net antiferromagnet and a weak ferromagnetism. In terms of the cycloid type incommensurate anharmonic SSMS model the temperature dependences of the hyperfine parameters of the Mössbauer spectrum, the shift of the Mossbauer line, quadrupole shift of the spectral components, isotropic and anisotropic hyperfine magnetic field, were obtained and analyzed. The temperature dependence of the isotropic magnetic field at low temperatures has been processed in the spin wave model, at temperatures close to the Neel temperature, within the framework of the similarity theory, and over the entire temperature range, the effective molecular field model. As a result, the parameters and critical indexes of the models are determined. The calculation of the dipole contribution to the hyperfine magnetic field produced by the localized magnetic moments of the surrounding atoms showed that the experimentally observed value of the anisotropic contribution to the hyperfine magnetic field can be explained only when anisotropy of the hyperfine magnetic interaction of the nucleus with the electrons of the own atom is taken into account. It is established that an anisotropic contribution to the hyperfine magnetic field with increasing temperature firstly (up to ~330 K) weakly increases, and then decreases, tending to zero in the temperature ~600 K. It is shown that the contribution to the tensor of the electric field gradient and, respectively, to the quadrupole shift of the resonance lines, caused by local distortion of the lattice due to the magnetoelectric interaction, is negligibly small. The calculation of the electric field gradient tensor on 57Fe nuclei has shown that the experimentally observed value of the quadrupole shift can be explained only when the dipole contribution from the oxygen anions is taken into account, as well as the electronic contribution due to the covalence effects.