The parameters of the hyperfine interaction of 57Fe nuclei in single crystals of iron borate FeBO3 and its isostructural solid solution Fe0.91Ga0.09BO3 have been determined by Mössbauer spectroscopy. A theoretical model has been developed to describe resonant transitions of iron nuclei in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction, taking into account the statistical distribution of Ga and Fe over octahedral positions in the Fe0.91Ga0.09BO3 crystal. It has been established that even a low Ga concentration leads to a significant change in the hyperfine structure of the 57Fe nuclei inFeBO3, which manifests itself in the appearance of additional components in the Mössbauer spectra of the Fe0.91Ga0.09BO3 single crystal.

The parameters of the hyperfine interaction of 57 Fe nuclei in single crystals of iron borate FeBO 3 and its isostructural solid solution Fe 0.91 Ga 0.09 BO 3 have been determined by Mössbauer spectroscopy. A theoretical model has been developed to describe resonant transitions of iron nuclei in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction, taking into account the statistical distribution of Ga and Fe over octahedral positions in the Fe 0.91 Ga 0.09 BO 3 crystal. It has been established that even a low Ga concentration leads to a significant change in the hyperfine structure of the 57 Fe nuclei in FeBO 3 , which manifests itself in the appearance of additional components in the Mössbauer spectra of the Fe 0.91 Ga 0.09 BO 3 single crystal.

The parameters of the hyperfine interaction of 57Fe nuclei in single crystals of iron borate FeBO3 and its isostructural solid solution Fe0.91Ga0.09BO3 have been determined by Mössbauer spectroscopy. A theoretical model has been developed to describe resonant transitions of iron nuclei in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction, taking into account the statistical distribution of Ga and Fe over octahedral positions in the Fe0.91Ga0.09BO3 crystal. It has been established that even a low Ga concentration leads to a significant change in the hyperfine structure of the 57Fe nuclei in FeBO3, which manifests itself in the appearance of additional components in the Mössbauer spectra of the Fe0.91Ga0.09BO3 single crystal.

The Faraday effect in the rhombohedral weak ferromagnet FeBO 3 , which is due to the magnetization component parallel to the С 3 axis of the crystal, is predicted and experimentally observed for the first time. This magnetization component is almost three and a half orders of magnitude smaller than the magnetization in the basal plane. The measured effect is six orders of magnitude smaller than the Faraday effect caused by the magnetization in the basal plane. The origin of the significant difference in the magnitudes of the Faraday effect due to the magnetization in the basal plane and to the magnetization parallel to the С 3 axis is discussed.

The Faraday effect in the rhombohedral weak ferromagnet FeBO3, which is due to the magnetization component parallel to the С3 axis of the crystal, is predicted and experimentally observed for the first time. This magnetization component is almost three and a half orders of magnitude smaller than the magnetization in the basal plane. The measured effect is six orders of magnitude smaller than the Faraday effect caused by the magnetization in the basal plane. The origin of the significant difference in the magnitudes of the Faraday effect due to the magnetization in the basal plane and to the magnetization parallel to the С3 axis is discussed.

Surface morphology and crystal structure of iron borate, FeBO3, annealed at different temperatures, have been studied by scanning electron microscopy and X-ray diffraction analysis. The temperature range of structurally stability of iron borate has been determined. It has been established that in the range of temperatures 800–900°C recrystallization in the iron orthoborate Fe3BO6 phase, and more than 900°C − in α-Fe2O3 phase, occurs.

A change in the quasistatic magnetic susceptibility in thin plates of iron borate (FeBO3), which is a weak ferromagnet, has been revealed at adsorption of water molecules. The measurements have been performed at room temperature with the use of the magneto-optical Faraday effect. The change of the susceptibility in saturated water vapors is about 30%. The observed effect is reversible. The time of establishing the susceptibility after the introduction of water vapors is 1.5 min, which is twice as large as the time of establishing the susceptibility after the evacuation. The effect is explained by the appearance of uniaxial surface magnetic anisotropy in the basal plane because of the adsorption of water molecules.

For the first time, solid solutions in the entire spinel region in the system Ni-Mn-Fe-O have been synthesized by firing in air and oxygen ambients. The cubic unit cell parameter а, Curie point Тс, saturation magnetic moment μ at 0 К were determined. Using earlier developed calculation method, Gibbs energies of ionic transformations and dependences of а, Тс, μ on composition were obtained. They are found to be in good agreement with experimental data demonstrating the correctness of the proposed calculation method.