Figure (2) shows the diffraction patterns of the NiFe2O4 prepared by both methods: sol-gel and dc reactive magnetron sputtering. Distinguished peaks ascribed to the crystal planes (220), (222), (311), (400), (422), (440) and (511) were observed and they indicate the presence of cubic spinel NiFe2O4 structures in the prepared samples [14]. It is clear from these patterns that the nanoparticles prepared by the sol-gel method (Fig. 2a) include sharp peaks, which reveal better crystallization when compared to the broadening observed in the patterns of the samples prepared by DCRMS [15].
The morphology of the prepared nanostructures was investigated by FE-SEM and those prepared by the sol-gel method reasonably include agglomeration while those prepared by DCRMS approximately have a spherical shape and uniform distribution. The nanoparticle size was determined to be within 33–62 and 22–40 nm for the samples prepared by sol-gel and dc reactive magnetron sputtering, respectively.
The elemental composition of the prepared NiFe2O4 samples was analyzed using EDS, as shown in Fig. (4). It was observed that the main components of the sample prepared by sol-gel are Fe, O, Ni, and C with 38.3, 19.3, 18.3, and 24 wt.%, respectively, as shown in Fig. (4a). On the other side, the elemental composition of the sample prepared by dc reactive magnetron sputtering showed the same elements Fe, O, Ni and C with the weight ratios of 37.5, 24.9, 23.2, and 13.5 wt.%, respectively, in addition to Mg of 0.9 wt.%. The increase of elemental contents of oxygen and nickel and decrease of carbon content in the second sample is highly preferred as the total content of NiFe2O4 is accordingly increased. The existence of carbon in the final product is unavoidable as the precursors in the sol-gel method include the availability of citric acid, which is the source of carbon while in the sputtering system, the possible source of carbon is using Teflon parts inside the deposition chamber. However, the content of carbon in the second sample is about half that in the first sample.
Figure (5) indicates the hysteresis loop of the synthesized NiFe2O4 nanostructures. The saturation magnetization (Ms) value in both methods is far lower from the recorded values for bulk nickel ferrite. The decrease in saturation magnetization is mainly attributed to the decrease in particle size [16]. The lower values of Ms associated with the crystalline nanoparticles (NiFe2O4) are due to the structural deformity of the surface as opposed to the bulk particles. This can be explained by the existence of transition metal ions to contain pure magnetic moment, which reacts through the oxygen atoms in the spinel lattice leading to a situation in which the magnetic moments of ions in the metal oxides forming the ferrite are aligned. The moments between the same metal oxide line up parallel, while the moments between different metal oxides are anti-parallel to result in a ferromagnetic order. By distributing metal ions to these sites, it is possible to calculate the net magnetic moment for each formula unit. Because the particles are very small, as the specimen has a large ratio of surface-to-volume, the net magnetic moment is decreased. Thus, on the surface of structural deformation particles, there is an availability of metal ions. The lengths and angles of the bonds are different when compared to the bulk to give low magnetic moments [17]. The saturation magnetization (Ms) and coercivity (Hc) of NiFe2O4 prepared by the sol-gel method are 30.02 emu/g and 187.8 Oe, respectively. Figures (5b) explains the typical behavior of the superparamagnetic behavior of the nanoparticles synthesized by DCRMS. It can be noticed that the saturation magnetization is 27.05 emu/g and the area of hysteresis is absent. This means that the coercivity is approximately zero. Because of the quantum size effect, the superparamagnetic behavior of the NiFe2O4 nanoparticles indicates that they have a single domain structure [18]. The superparamagnetic material in MRI in the absence of the applied magnetic field lose their magnetism, as well as become highly dispersed in the liquid, making them suitable for cancer treatment in clinical treatments [19].