Dynamic magnetic permeability of the heterogeneous nanosystems based on (Co41Fe39B20)x(SiO2)100 – x composites

Resumo

Thin films of (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_{100 – x} nanocomposites and hybrid nanocomposite–semiconductor [(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_{100 – x}/C]₅₀ multilayers are synthesized by ion-beam deposition at various contents x of ferromagnetic metallic Co₄₁Fe₃₉B_2O nanogranules in an SiO₂ matrix and at various carbon layer thicknesses h < 2 nm. Their magnetic and electrical properties, high-frequency magnetic permeability, magnetooptical spectra, and FMR spectra are studied. It is found that both the single-layer nanocomposites and the multilayers with carbon interlayers are superparamagnetic at x < x_per, where x_per is the electric conduction percolation threshold: a hysteresis at room temperature is absent, and the blocking temperature determined in quasi-static measurements does not exceed 20–30 K and weakly depends on the carbon layer thickness. At a carbon layer thickness h = 1.2–1.8 nm, the real and imaginary parts of complex magnetic permeability at 50 MHz and room temperature are substantially higher than those of the nanocomposites without carbon layers: their values are typical of ferromagnets. This dependence points to an exchange interaction between nanogranules in layers through a carbon interlayer. The influence of a conducting layer on the static and dynamic magnetic properties of a system of interacting superparamagnetic particles is discussed.