Yu. E. Kalinin

Magnetorefractive effect in magnetic nanocomposites

The magnetorefractive effect in ferromagnetic metal-insulator granular nanostructures (CoFeZr)-SiOn, Co-Al-O, FeSiOn, and (CoFe)-(Mg-F) is investigated in the infrared spectral region in a wavelength range from 5 to 20 μm. The magnitude of the effect varies from 0.1 to 1.5% for different nanocomposites and strongly depends on the frequency of light and magnetoresistance. It is shown that the reflection coefficient changes in a magnetic field not only due to the magnetorefractive effect, but also due to the even magnetooptical effect. Simple relations describing this effect are given for the case when the reflection from the substrate is insignificant and in the case of a three-layer (insulator-film-substrate) system. The expression for the frequency dependence of the magnetorefractive effect in nanocomposites is derived and its features in the case of high-frequency spin-dependent tunneling are analyzed.

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.

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.

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.

Morphology and the magnetic and conducting properties of heterogeneous layered magnetic structures [(Co45Fe45Zr10)35(Al2O3)65/a-Si:H]36

The morphology and the magnetic and conducting properties of an amorphous multilayer nanosystem [(Co45Fe45Zr10)35(Al2O3)65/a-Si:H]36 consisting of (Co45Fe45Zr10)35(Al2O3)65 magnetic layers and semiconducting hydrogenated amorphous silicon (a-Si:H) layers of various thicknesses have been studied. Using a combination of methods (including polarized neutron reflectometry and grazing incidence small-angle X-ray scattering), it is shown that the magnetic and electrical properties of these multilayer structures are determined by their morphology. It is established that the magnetization and electric resistance of a sample is a nonmonotonic function of the a-Si:H layer thickness. Both characteristics are at a minimum for a structure with a semiconductor layer thickness of 0.4 nm. Samples with silicon layer thicknesses below 0.4 nm represent a three-dimensional structure of Co45Fe45Zr10 grains weakly ordered in space, while in samples with silicon layer thicknesses above 0.4 nm, these grains are packed in layers alternating in the vertical direction. The average lateral distance between nanoparticles in the layer plane has been determined, from which the dimensions of metal grains in each sample have been estimated.

Tunneling anomalous Hall effect in nanogranular CoFe-B-Al-O films near the metal-insulator transition

We present results of experimental studies of structural, magneto-transport and magnetic properties of CoFe-B-Al-O films deposited onto a glass ceramic substrate by the ion-beam sputtering of the target composed of Co40Fe40B20 and Al2O3 plates. The system consists on the strained crystalline CoFe metallic nanogranules with the size 2-5 nm which are embedded into the B-Al-O oxide insulating matrix. Our investigations are focused on the anomalous Hall effect (AHE) resistivity Rh and longitudinal resistivity R at T=5-200 K on the metallic side of metal-insulator transition in samples with the metal content x=49-56 at.%, that nominally corresponds to (Co40Fe40B20)x(Al2O3)100-x in the formula approximation. The conductivity at T > 15 K follows the lnT behavior that matches a strong tunnel coupling between nanogranules. It is shown that the scaling power-laws between AHE resistivity and longitudinal resistivity strongly differ, if temperature T or metal content x are variable parameters: Rh(T)~R(T)^0.4-0.5 obtained from the temperature variation of R and Rh at fixed x, while Rh(x)/x~R(x)^0.24, obtained from measurements at the fixed low temperature region (10-40 K) for samples with different x. We qualitatively describe our experimental data in the frame of phenomenological model of two sources of AHE e.m.f. arising from metallic nanogranules and insulating tunneling regions, respectively, at that the tunneling AHE (TAHE) source is strongly shunted due to generation of local circular Hall currents. We consider our experimental results as the first experimental proof of the TAHE manifestation.

Anomalous Hall effect in (Co41Fe39B20)x(Al-O)100 − x nanocomposites

The concentration dependence of the coefficient Rs characterizing the anomalous Hall effect (AHE) has been studied by measuring the electrical resistivity ρ, magnetoresistance, and the magnetic field dependence of magnetization and Hall resistivity of (Co41Fe39B20)x(Al-O)100 − x nanocomposite thin films. It has been demonstrated that the AHE coefficient increases by more than an order of magnitude with a decrease in the percentage x of the amorphous ferromagnetic metal from 60 to 30 and its behavior is described by the relation Rs ∼ ρm, where m = 0.46 ± 0.1. At the same time, the coefficient characterizing the normal Hall effect grows by a factor of less than 10. The mechanisms underlying the giant Hall effect in nanocomposites have been discussed.

Effect of pulsed photon irradiation on the formation of a nanocrystalline structure in Fe-Pb-Nb amorphous alloys

X-ray diffraction, transmission electron microscopy, and microhardness and internal friction measurements are used to study the formation of a nanocrystalline structure in Fe-Pb-Nb amorphous alloys subjected to pulsed photon irradiation. The threshold light energies that are incident on a sample and cause nanocrystallization and hardening of amorphous alloys are found, and a model of crystal phase nucleation in amorphous alloys is developed.

Enhancement of magneto-optical response in nanocomposite-hydrogenated amorphous silicon multilayers

The dependence of the magnetic and magneto-optical properties on the semiconductor layer thickness has been studied for a [(Co45Fe45Zr10)35(Al2O3)65(X)/α-Si:H(Y)]30 multilayer. It is found that an increase in the Si layer thickness to 1.3–1.7 nm leads to an increase in the transverse Kerr effect, magnetization, and coercive force. The changes in the properties of the nanomultilayer system are related to the percolation transition between CoFeZr granules through Si streaks. This percolation leads to effective exchange interaction between isolated ferromagnetic granules of Co45Fe45Zr10 alloy and increase in magneto-optical response.

Nanocrystallization in amorphous Al83Ni10La7 and Al83.5Ni9.5La5.6 Si1.4 alloys during thermal annealing and flash lamp processing

Nanocrystallization in amorphous Al83Ni10La7 and Al83.5Ni9.5La5.6Si1.4 alloys during heat treatment and flash lamp processing has been studied using X-ray diffraction, transmission electron microscopy, scanning calorimetry, and internal friction, modulus of elasticity, and microhardness measurements. The results demonstrate that thermal annealing and flash lamp processing have identical effects on the formation of a nanocrystalline structure in the amorphous alloys and their phase transformations.