A. V. Sitnikov

Electrical properties of amorphous (Co45Fe45Zr10)x(Al2O3)1−x nanocomposites

The electrical properties of (Co45Fe45Zr10)x(Al2O3)1−x granular nanocomposites have been studied. The concentration dependences of electrical resistivity are S-shaped (in accordance with the percolation theory of conduction) with a threshold at a metallic component concentration of ∼41 at. %. An analysis of the temperature behavior carried out in the range 300–973 K revealed that structural relaxation and crystallization of the amorphous phase are accompanied by a decrease in the electrical resistivity of the composites above the percolation threshold and by its increase below the percolation threshold. For metallic phase concentrations x<41 at. %, variable range hopping conduction over localized states near the Fermi level was found to be dominant at low temperatures (77–180 K). A further increase in temperature brings about a crossover of the conduction mechanism from Mott’s law ln(σ) ∝ (1/T)1/4 to ln(σ) ∝ (1/T)1/2. A model of inelastic resonance tunneling over a chain of localized states of the dielectric matrix was used to find the average number of localized states involved in the charge transport between metallic grains.

Multilayer nanogranular films (Co40Fe40B20)50(SiO2)50/α-Si:H and (Co40Fe40B20)50(SiO2)50/SiO2: Magnetic properties

Magnetic properties of multilayers, consisting of nanogranular (Co40Fe40B20)50(SiO2)50 layers as thin as magnetic granule diameter alternating the α-Si:H or SiO2 layers and the single layer film (Co40Fe40B20)50(SiO2)50 with the thickness much larger than the magnetic granule diameter are reported and compared. The thick single layer film is ferromagnetic but the multilayer film with the ultrathin granular layers and SiO2 spacer is superparamagnetic. This is interpreted as the result of increasing percolation threshold in the 2D granular media above 50% concentration of magnetic granules in the multilayer with the nonmagnetic and dielectric SiO2 spacer. The multilayer with the α-Si:H spacer is superparamagnetic at 300 K but it becomes ferromagnetic, when temperature is below 250 K. It is assumed to be resulted from the exchange interaction of magnetic granules through the semiconductor α-Si:H layers. The value of exchange interaction through the semiconductor spacer is estimated.

Effect of oxygen pressure on phase composition and magnetic structure of FeCoZr-Pb(ZrTi)O3 nanocomposites

The elemental and phase compositions and the magnetic state of metal particles in FeCoZr-Pb(ZrTi)O3 granular nanocomposites (GNCs) synthesized by ion-beam sputtering of composite targets in oxygen-containing media at different oxygen partial pressures have been studied. The phase transformations in GNCs have been monitored by the Raman and Mössbauer spectroscopy techniques. Correlation between the oxygen pressure during GNC synthesis and the valence state of iron ions in metal granules has been established. It has been confirmed that (i) the degree of metal oxidation increases with increasing oxygen pressure and (ii) the degree of crystallinity of oxides increases as a result of annealing. It has been established that the percolation threshold in GNCs can also be varied by changing the oxygen pressure during GNC synthesis.

Fractal magnetic microstructure in the (Co41Fe39B20)x(SiO2)1−x nanocomposite films

Magnetostructural methods are applied to determine the exchange bond percolation limit in (Co41Fe39B20)x(SiO2)1−x nanocomposites (xc = 0.30 ± 0.02), which separates the phase plane along the metal concentration axis into a superparamagnetic region and a ferromagnetic region. It is shown that, with respect to the singularities of the magnetization up to the magnetization saturation curves, the ferromagnetic region is further subdivided into three regions differing in the character of the spatial propagation of the magnetization ripples or in the magnetic correlation function characteristics. The fractal dimension of the nanocomposite magnetic microstructure near the percolation threshold is determined.

Electrical and magnetic properties of [(CoFeZr)x(Al2O3)1 − x/(α-SiH)]n multilayer structures

The concentration dependences of the electrical resistivity and complex permeability of [“(Co45Fe45Zr10)x(Al2O3)100 − x”/“α-Si: H”]n multilayer structures and (Co45Fe45Zr10)x(Al2O3)100 − x composites have been studied. It has been established that introduction of a semiconductor interlayer into the (Co45Fe45Zr10)x(Al2O3)100 − x composites substantially decreases the electrical resistivity of [“(Co45Fe45Zr10)x(Al2O3)100 − x”/“α-Si: H”]n multilayer structures. The concentration dependences of the real and imaginary parts of the complex permeability of the [“(Co45Fe45Zr10)x(Al2O3)100 − x”/“α-Si: H”]n nanomultilayer structures substantially differ from those of the (Co45Fe45Zr10)x(Al2O3)100 − x composites. The real part of the complex permeability of the [“(Co45Fe45Zr10)x(Al2O3)100 − x”/“α-Si: H”]n nanomultilayer structures follows the curve with a minimum near the percolation threshold of the composite, and the imaginary part smoothly decreases as the ferromagnetic phase concentration increases. The results obtained are explained by the increase in the bifurcation temperature due to the conduction electrons of the semiconductor interlayer, which favor magnetic ordering of ferromagnetic grains.

Isotropic positive magnetoresistance in Co-Al2On nanocomposites

The magnetotransport properties of Cox(Al2On)100 − x nanocomposites were studied in a wide concentration range (34 ≤ x ≤ 74 at %). Negative tunnel magnetoresistance reaching 6.5% in a field of 10 kOe was established. In addition to the negative magnetoresistance, the Cox(Al2On)100 − x composites were found to exhibit positive magnetoresistance reaching 1.5% in fields of 10 kOe over the concentration range corresponding to the percolation threshold (54 ≤ x ≤ 67 at %). The positive magnetoresistance is assumed to be due to the simultaneous existence in the composite structure of clusters and individual nanoparticles characterized by different values of the magnetic anisotropy and due to the dipole-dipole interaction between the clusters and nearest neighbor particles.

Evolution of the optical and magnetooptical properties of amorphous metal-insulator nanocomposites

Magnetic, optical, and magnetooptical (MO) properties of (Co45Fe45Zr10)x(SiO2)100−x and (Co41Fe39B20)x(SiO2)100−x granular nanocomposites of the amorphous ferromagnetic metal-insulator type were studied in a broad range of the magnetic component concentrations x. The MO response of nanocomposites increases in the vicinity of the percolation transition. Using the experimentally determined values of optical constants and the equatorial Kerr effect, the diagonal and nondiagonal components of the permittivity tensor of nanocomposites were calculated for the first time. The nondiagonal components of this tensor are nonlinear functions of x, the most pronounced variations being observed near the percolation threshold. Experimental data on the MO effect and the permittivity tensor were theoretically modeled within the framework of the effective medium approximation and the Maxwell-Garnett approximation. The most adequate description was obtained with the symmetrized Maxwell-Garnett approximation, which provides for a good (semiquantitative) agreement between theory and experiment under certain assumptions about the microstructure of nanocomposites.

Electrical and magnetic performance of multilayer structures based on (Co40Fe40B20)33.9(SiO2)66.1 composite

The dependences of the resistivity and complex magnetic permeability of (Co40Fe40B20)33.9(SiO2)66.1 composite-based multilayer films on the thickness of different metallic and semiconducting spacers are studied. It is found that in the presence of a continuous spacer with a resistivity on the order of 0.01 Ω m, the superparamagnetic state of these heterogeneous systems changes to ferromagnetic ordering.

Ferromagnetic resonance and magnetic microstructure in nanocomposite films of Cox(SiO2)1 − x and (CoFeB)x(SiO2)1 − x

This paper reports on the results of the investigation of the relation between the magnetic microstructure and ferromagnetic resonance (FMR) in ferromagnetic metal-insulator composites by using granular alloys (Co41Fe39B20)x(SiO2)1 − x and Cox(SiO2)1 − x as an example. A comparative analysis of the properties of FMR spectra and parameters of random magnetic anisotropy leads to correlations between these quantities. It has been found that the main mechanism that determines the FMR line width in the films under investigation is the exchange narrowing mechanism.

Planar Hall effect and anisotropic magnetoresistance in layered structures Co0.45Fe0.45Zr0.1/a-Si with percolation conduction

Magnetic and magnetotransport properties of multilayered nanostructures Co0.45Fe0.45Zr0.1/a-Si obtained by ion-beam sputtering are investigated. The temperature dependence of the resistance obeys a law of the form Rxx ∝-logT, which is typical of metal-insulator nanocomposites on the metal side of the percolation transition. The magnetoresistance anisotropy effect, as well as the planar Hall effect, is observed for the first time for this type of nanocomposites in the vicinity of the percolation transition. The correlation of these two effects with the transverse (between Hall probes) magnetoresistive effect, which may reach 6–9%, is revealed. A weak negative magnetoresistance of the order of 0.15%, which is observed for subnanometer amorphous silicon layer thicknesses, is attributed to spin-dependent electron transitions between adjacent ferromagnetic layers in the case when the exchange interaction between these layers is of the antiferromagnetic type.