*3.1. Morphological Properties*

The morphologies related to surface and structures of Co0.5Ni0.2Mg0.3Fe2O4 (A1) and Co0.5Ni0.1Mg0.4Fe2O4 (A2) at a voltage of 2.0 kV with a magnification of 50,000× were observed in Figure 3.

 **Figure 3.** *Cont.*

**Figure 3.** Field emission scanning electron microscopy (FESEM) images for (**a**) A1 and (**b**) A2 at 50,000× magnification at a voltage of 2.0 kV.

As shown in Figure 3a,b, the A1 and A2 samples exhibit spherical shapes and the nanoparticles are clearly aggregated, which is likely due to strong Van der Waals or magnetostatic interactions between bigger nanoparticles (promoted by the sintering process), a phenomenon which has been reported in previous research by Rosnan et al. [37]. In addition, by using the J-image software, the mean particle sizes observed were in the range of 35 to 80 nm.

Meanwhile, as for the MRE with nanosized Ni-Mg cobalt ferrite, the results of the micrographic analysis for one of the samples (MRE + A2) at different magnifications (1000× and 2500× magnification) are shown below.

Figure 4a displayed the dispersion of particles at 1000× magnification, while Figure 4b showed the dispersion behaviour of particles in an MRE at the higher magnification of 2500×. As demonstrated in Figure 4a,b, the CIPs and nanosized Ni-Mg cobalt-ferrites are randomly dispersed in the silicone rubber matrix. The SEM image also shows that no large aggregation or agglomeration of particles occurred while fabricating the MRE samples, although a small void is detected upon observation of the micrographic analysis.

**Figure 4.** Scanning electron microscopy (SEM) images for MRE + A2 at (**a**) 1000× and (**b**) 2500× magnification.
