Feature of Nonlinear Electromagnetic Properties and Local Atomic Structure of Metals in two Systems of Nanocomposites Cox(MgF2)100-x and (CoFeZr)x(MgF2)100-x

The article presents a comparative analysis of the results of complex studies of two nanocomposite systems: metal-dielectric Cox(MgF2)100-x and alloy-dielectric (CoFeZr)x(MgF2)100-x, obtained by ion-beam sputtering of composite targets in a wide range of compositions. Section 3 of this paper provides a mini-review of the investigations results about the features of the influence of the atomic composition and structural-phase state in the two systems on their nonlinear electroresistive, magnetoresistive, magneto-optical properties, published by us separately in each of the two systems in 2019-2023. In the metal-dielectric system Cox(MgF2)100−x, the formation of the hexagonal Co nanocrystals coincides with the achievement of the electric and magnetic percolation thresholds at xper=37 at.% and is accompanied by a transition from the superparamagnetic to the ferromagnetic state, which, with an increase of the metal content (>42 at.%), acquires a hard magnetic character with a coercive force of up to 95 Oe. In the alloy-dielectric system (CoFeZr)x(MgF2)100−x, the formation of the hexagonal CoFeZr nanocrystals coincides with the values of the electric and magnetic percolation thresholds at xper=34 at.%. Below this value, the nanocomposites exhibit superparamagnetic properties, and at high values of x > 34 at.%, they become soft magnets and remain so far beyond the percolation threshold and after the phase transition of nanocrystals from the hexagonal hcp to the cubical bcc structure at x = 43 at.%, with a maximum value of the coercive force Hc ≤30 Oe. One of the reasons for the significant decrease in Hc compared to the first system may be the formation of the paramagnetic phase FeF2 at the interphase boundaries of CoFeZr nanocrystals with the dielectric matrix MgF2, detected by the Mössbauer spectroscopy. Further in Section 4 for the first time, the concentration dependences of the oscillating fine structure of XANES K-absorption edges of Co atoms in the first system and Co and Fe atoms in the second system are presented, which undergoes changes at the percolation thresholds in each of the two systems and thus confirms the nonlinear nature of the electromagnetic properties changes in each of the two systems at the atomic level.