Structural and magnetic properties of Co$_{1-x}$Zn$_x$Fe$_2$O$_4$ (0 $\le x\le$ 1) nanoparticles for biomedical applications

Magnetic nanoparticles of Zn-substituted CoFe$_2$O$_4$ spinel ferrites Co$_{1-x}$Zn$_x$Fe$_2$O$_4$ (at $x$ = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were successfully synthesized by chemical co-precipitation. The structural, morphological and magnetic properties of the obtained particles were studied and characterized by X-ray diffraction (XRD), vibrating-sample magnetometry (VSM), Raman and Mossbauer spectroscopy. The introduction of zinc ions causes noticeable changes in the structural and magnetic properties of spinel ferrite. The sizes of particles of Co$_{1-x}$Zn$_x$Fe$_2$O$_4$ change from 10 to 3 nm with an increase of the number of Zn ions according to X-ray data and their sizes change from 15 to 4 nm according to Mossbauer data. It was found that the saturation magnetization increases with an increase of the amount of Zn to $x$ = 0.4 and gradually decreases with a further increase of the concentration of Zn. The important information about the difference between the magnetic structures of the surface layer and the volume of particles was obtained for the first time using Mossbauer spectroscopy without external magnetic fields. A collinear ordering of spin moments is observed in the volume of magnetic nanoparticles of ferrite Co$_{1-x}$Zn$_x$Fe$_2$O$_4$, whereas a canting spin structure is observed on the surface of particles because of the impact of the surface. The mechanism of transition of spinel ferrite MNPs from a magnetically ordered to a paramagnetic state with the introduction of paramagnetic ions is described. Studies have shown that the obtained nanoparticles are perspective in view of biomedical applications.