Zn$_x$Fe$_{3-x}$O$_4$ (0 $\le x\le$ 1.0) magnetic nanoparticles functionalized with polyacrylic acid (PAA)

Studies of the properties of Zn$_x$Fe$_{3-x}$O$_4$ ($x$ = 0, 0.25, 0.5, 0.75, 1.0) magnetic nanoparticles synthesized by a modified hydrothermal method are presented in comparison with the properties of the same nanoparticles stabilized with polyacrylic acid Zn$_x$Fe$_{3-x}$O$_4$@PAA. The structure, size, morphology, and magnetic properties of the samples were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT IR), physical properties measurements (PPMS), and Mössbauer spectroscopy. The synthesized nanoparticles are single-phase, without additional impurities, have a narrow size distribution and are in the superparamagnetic phase. From the (XRD) measurements, it was found that with an increase in the Zn content from $x$ = 0 to $x$ = 1.0, the sizes of the nanoparticles were increasing from 17 to 33 nm. Analysis of the Mössbauer spectroscopy data showed that when doped with Zn ions from $x$ = 0 to $x$ = 1.0, the sizes of the nanoparticles were decreasing from 15 nm to 5 nm. The results of the Mössbauer studies showed that both Zn$_x$Fe$_{3-x}$O$_4$ and Zn$_x$Fe$_{3-x}$O$_4$@PAA has a core/shell type structure in which the core is magnetically ordered, whereas the shell does not have magnetic ordering. Mössbauer studies indicate that the coating of citric acid particles leads to their isolation from each other, reducing or eliminating interactions between particles, reducing the thickness of the paramagnetic shell, and thereby increasing the diameter of the core, which is in a magnetically ordered state.