Structure and properties of Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ nanoparticles depending on the amount of Co ions (0 $\le x\le$ 1.0)

The properties of magnetic nanoparticles (MNPs) of spinel ferrites Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ (at $x$ = 0.0; 0.2; 0.3; 0.4; 0.5; 0.6; 0.8; 1.0) synthesized by chemical co-precipitation method have been studied. The studies of the synthesized Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ MNPs were carried out using X-ray diffraction (XRD), Raman scattering and Mössbauer spectroscopy. The results of XRD, Raman and Mössbauer studies indicate that the obtained Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ MNPs are single-phase. It was established from XRD measurements that the average size of Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ crystallites is 34.86 nm for MnFe$_2$O$_4$($x$ = 0) and decreases to 14.99 nm for CoFe$_2$O$_4$ ($x$ = 1.0) with increasing Co ions concentration. An analysis of the Mössbauer spectra showed that the average crystallite size varies from 25 nm for MnFe$_2$O$_4$ ($x$ = 0) to 12 nm for CoFe$_2$O$_4$ ($x$ = 1.0). On the Raman spectra of Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ MNPs, in the region of $\sim$620 cm$^{-1}$, splitting of the A1g line is observed, which means that the studied MNPs have a reverse spinel structure. The intensity ratio of the A1g (1) and A1g (2) peaks indicates a significant redistribution of the Co$^{2+}$ and Fe$^{3+}$ cations between tetra- and octahedralpositions in MNPs of the Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ ferrite, which is confirmed by Mössbauer data. Mössbauer spectroscopy data indicate that the synthesized Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ MNPs consist of large particles with magnetic ordering and small particles in the paramagnetic phase. With an increase in the concentration of Mn ions, the proportion of fine particles increases, which leads to a decrease in the magnetic blocking temperature. The saturation magnetization of MNPs at $x$ = 0.2 (Co$_{0.2}$Mn$_{0.8}$Fe$_2$O$_4$) is 57.41 emu/g and this sample, as was found in [V. Narayanaswamy, I.A. Al-Omari, A.S. Kamzin, B. Issa, H.O. Tekin, H. Khourshid, H. Kumar, A. Mallya, S. Sambasivam, I.M. Obaidat. Nanomaterials $\mathrm{11}$, 1231 (2021)] has the highest specific absorption rate. As shown by Mössbauer studies, this is due to the fact that these particles are in a superparamagnetic state and the magnetic blocking temperature of these MNPs is in the region of $\sim$ 315 K, which is most suitable for the treatment of malignant tumors by magnetic hyperthermia. Thus, the synthesized Co$_x$Mn$_{1-x}$Fe$_2$O$_4$ MNPs are promising for biomedical applications.