Structural transformations of Ni$_{1-x}$Cu$_{x}$Fe$_{2}$O$_{4}$ nanoparticles depending on the number of Cu ions

Changes in the structure and properties of the Ni$_{1-x}$Cu$_{x}$Fe$_{2}$O$_{4}$ ferrites-spinel magnetic nanoparticles depending on the Cu ion concentration (0 $\le x \le$ 1.0) have been studied using Mossbauer spectroscopy. It has been found that, with the increasing of the Cu ion concentration, the structure of nanoparticles changes from the structure of reverse spinel (NiFe$_2$O$_4$) to that of mixed spinel (Cu$_{x}$Fe$_{2}$O$_{4}$). It has been shown that the hydrothermal synthesis method makes it possible to obtain single-phase nanoscale particles with a very narrow size distribution and ideal magnetic ordering, which are promising for biomedical applications. The relationship between the distribution of cations on sublattices, the value of the inversion parameter, and the Cu ion concentration has been established.