ИСТИНА

The existence of an incommensurate cycloid type magnetic spatial spin-modulated structure (SSMS) with a period of λ = 620±20 Å [1] in bismuth orthoferrite BiFeO3 (BFO) and BFO-based compounds leads to a deterioration in the magnetic properties and, accordingly, the prospects for its application as a multiferroic. Search for methods of controlled destruction, suppression, and modification of SSMS draws attention for several decades. Main ways of SSMS modification are: • the substitution of A-site atoms in the with atoms that differ in characteristics (magnetism, ionic radius, heterovalent substitution), • magnetic fields, • the transition to nano-scale. The existence of SSMS is possible only in the presence of a phase with a rhombohedral structure (R3c); however, the presence of a rhombohedral structure alone is insufficient to confirm and characterize the magnetic properties of the sample. There is contradictory information in the literature about the existence of SSMS with a decrease of nanocrystalline size, since the effect on the magnetic moment of nanoobjects can be modified not only by SSMS suppression, but also by the presence of uncompensated spins on the surface, size limitation, due to which there are uncompensated regions of the cycloid, and other reasons [2]. The samples under study were synthesized by the solution combustion method and characterized by X-ray diffraction, the sizes of nanocrystals were estimated from 170 to less than 40 nm. Previously, we developed a protocol for measuring NMR-spectra in a zero field on 57Fe nuclei, which makes it possible to study SSMS even in samples with a natural 57Fe isotope abundance [3]. NMR-spectroscopy revealed the presence of SSMS in all the studied samples, even in the ones with linear size less than the cycloid period. The anharmonicity parameter m, which characterizes the degree of harmonicity of SSMS, decreases monotonically with decreasing nanocrystallite size, and in the ~50 nm size region, the anisotropy type changes from “easy axis” to “easy plane”.