## 3 Results
### Concentration Dependences of Electrical Resistance and Magnetoresistive Effects
Magneto-Optical and Magnetic Properties of Co\({}_{x}\)(MgF\({}_{2}\))\({}_{100\times x}\) Nanocomposites  
The magneto-optical Kerr effect changes the intensity of light reflected by a sample magnetized perpendicular to the plane of incidence of light [12]. The value and sign of  
Figure 3: Impedance diagrams (hodographs) for nanocomposites Co\({}_{x}\)(MgF\({}_{2}\))\({}_{100\times x}\) of different compositions (32 \(\leq\) x \(\leq\) 59). TKE are defined as the ratio of the difference between the intensities of the light reflected by the sample in the magnetized (I) and demagnetized (I\({}_{0}\)) states to the light intensity I\({}_{0}\):  
\[\delta=\frac{\text{I}-\text{I}_{0}}{\text{I}_{0}}=\frac{\Delta\text{I}}{\text{I} _{0}} \tag{3}\]  
The absolute value of TKE depends on the amount of the metal phase in the composite. Its increase leads to an increase in the TKE modulus, which reaches the maximum at a concentration corresponding to the percolation threshold. Figure 4 shows the concentration dependences of the transversal Kerr effect (TKE) for Co\({}_{\text{x}}\)(MgF\({}_{2}\))\({}_{100\text{-x}}\) nanocomposites at three values of the incident light photon energy of E = 1.14 eV, 1.97 eV, and 3.17 eV. The maximum absolute value in the concentration dependences of TKE is observed at the energy values of E = 1.97 eV and 3.28 eV. This maximum in the concentration dependence of TKE coincides with the electrical resistive percolation threshold x\({}_{\text{per}}\) = 37 at.%. In contrast, the concentration dependences of TKE in the NC (CoFeZr)\({}_{\text{x}}\)(MgF\({}_{2}\))\({}_{100\text{-x}}\) system, which we obtained earlier from [29; 30], showed two maxima, one of which corresponded to the formation of hexagonal nanocrystals of the CoFeZr alloy based on \(\alpha\)-Co (x = 30 at.%). The second at x = 45 at. % corresponded to the transition of the hexagonal structure to the cubic structure \(\alpha\)-Fe. During the MO study of our Co\({}_{\text{x}}\)(MgF\({}_{2}\))\({}_{100\text{-x}}\) system, we obtained two x values with maxima of the TKE concentration dependence at the lowest incident light energy E = 1.14 eV: the first value, x = 27 at.%, corresponds to the maximum of the magnetoresistive effect (Figure 2b), and the second value, x = 45 at.%, is close to the threshold of impedance; the hodograph changes from a capacitive to an ohmic character at x = 42 at.% in Figure 3.  
Thus, magneto-optical spectra respond to changes in the composition, atomic structure, and magnetic order of complex heterophase systems. In Figure 5, we present the dependences of the magnetizations on the magnetic field arising parallel and perpendicular to the sample plane, which show the first appearance of a hysteresis loop with a coercive force of H\({}_{\text{c}}\approx\) 18 Oe in a Co\({}_{32}\)(MgF\({}_{2}\))\({}_{68}\) sample with a cobalt content of x = 32 at.%. In this case, saturation magnetization has different values. Figure 4: Concentration dependencies of the transversal Kerr effect (TKE) for nanocomposites Co\({}_{\text{x}}\)(MgF\({}_{2}\))\({}_{100\text{-x}}\) at different values of incident light energy. We assume that there is magnetic anisotropy in granular nanocomposites, which is not related to structural properties but is determined by internal stresses in the nanocomposite. During the deposition of nanocomposites on a substrate, both internal and microscopic stresses in the sample volume and macroscopic stresses at the nanocomposite-substrate interface are allowed to appear. It can also be assumed that micro- and macrostresses in nanocomposites should affect the magnitude and direction of the anisotropy fields of both individual ferromagnetic granules and the material as a whole. Parallel tensile microstress increases the in-plane anisotropy field in the direction perpendicular to the sample axis. Compressive microstress reduces the magnitude of the anisotropy field in the perpendicular direction. We do not exclude the influence of the texture of nanocomposites on their magnetic properties.