The results of Mössbauer studies of the magnetic structure and composition of Fe3O4 / γ-Fe2O3 nanoparticles at temperatures of 300 and 80 K are presented. It was found that particles Fe3O4 /γ-Fe2O3 is a core / shell nanocomposite (NC) in which magnetite (Fe3O4) is a coated by shell of maghemite (γ-Fe2O3). It was shown that the thickness of the maghemite (γ-Fe2O3) shell depends on the synthesis technology. It was found that, in the Fe3O4 /γ-Fe2O3 NC, on the surface of the maghemite (γ-Fe2O3) shell, a layer whose magnetic structure differs from the structure of the inner part of the shell (γ-Fe2O3). An intermediate layer is formed between the core and the shell in a spin-glass state. The obtained data on the structure of core / shell nanocomposites open up prospects for explaining the properties of such particles, which are of great interest for applications in various fields, including biomedicine.

The results of Mössbauer studies of the composition and magnetic structure of Fe3O4 / -Fe2O3 nanoparticles placed in an external magnetic field with a strength of 1.8 kOe, which is a continuation of the work [A.S. Kamzin, I.M. Obaidat, A.A. Valliulin, V.G. Semenov, I.A. Al-Omari. FTT No. 10/2020]. It is shown that the thickness of the maghemite (-Fe2O3) shell can be changed by the synthesis conditions. It was found that on the surface of the maghemite (-Fe2O3) shell in the Fe3O4 / -Fe2O3 nanocomposites there is a layer in which the magnetic moments are not oriented collinearly to the moments located in the depth of the shell, i.e., there is a canted spin structure. An intermediate layer in the spin-glass state is formed between the core and the shell. The data obtained on the structure of core / shell particles are important for understanding the properties of nanocomposites, which are of great interest for applications in various fields, including biomedicine.

The phase composition, the structure of cores and shells, and the dependences of the shell thickness on the fabrication technique were determined by Mössbauer spectroscopy for core/shell Fe_3O_4/γ-Fe_2O_3 magnetic nanoparticles (MNPs) with Fe_3O_4 cores of a fixed size (8 nm) and γ-Fe_2O_3 shells of a varying thickness (1, 3, or 5 nm). A layer on the MNP surface with a magnetic state differing from that in the bulk was found. It was also found that a spin-glass state probably exists between the core and the shell. The studied core/shell Fe_3O_4/γ-Fe_2O_3 MNPs have promising applications in various fields (e.g., biomedicine).