Granular Nanocomposites Research Articles

Studying the internal structure of granular magnetic nanocomposites by ferromagnetic resonance

A method for estimating the form of magnetic nanoparticles in composite film structures based on the observation of ferromagnetic resonance phenomenon is offered. Within the model of the effective medium, an explanation is given for experimentally observed concentration and temperature dependences of resonant fields for composite nanosystem (Co45Fe45Z10) f +(Al2O3)100−f .

On electrocontact oxidation of iron nanocondensates

The transformation (under the effect of current passed) of a nanostructured metal-oxide coating produced by vacuum deposition of iron in an oxygen atmosphere at a pressure of 10−4 mm Hg on an amorphous glass substrate is studied by measuring voltammetric characteristics. In the freshly deposited film, a linear characteristic is observed that may correspond to direct quantum mechanical tunneling in thin oxide interlayers between metal nanograins. With an increase in the applied current density, when a certain threshold is reached, the conductivity of the conductor rapidly drops because of the thermo- and electrooxidation. In a degraded conductor at a prethreshold current density, a nonlinear voltammetric characteristic is observed, which may be related to the change in the mechanism of current flow restricted by the volume charge, as well as to the negative temperature resistance coefficient typical of fine-crystalline granular nanocomposites.

Synthesis and characterization of (NiZnFe 2O 4) 0.5/(SiO 2) 0.5 granular nanocomposites

(NiZnFe 2O 4) 0.5/(SiO 2) 0.5 nanocomposites were synthesized by three different methods: sol–gel, wet impregnation of nitrates in powders of amorphous silica and wet dispersion of previously ball-milled powders into amorphous silica. The as-prepared samples were then annealed for 1 h at 1273 K, in air atmosphere, to obtain the desired nanocomposites. Crystallographic phases, particle sizes and lattice parameters were determined from X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Specific surface area of the powders was estimated by the BET method and Fourier transform infrared spectroscopy was used to detect the ferrite phase. Hysteresis properties were characterized in the temperature range from 5 to 300 K. The samples synthesized by the sol–gel technique consisted of NiZn ferrite nanoparticles embedded in the amorphous SiO 2 matrix. The same phases were observed in samples obtained by wet mixture of previously milled powders, but in this case a quite broad crystal size distribution results and the ferrite particles are dispersed in a fine silica sintered powder rather than embedded in the SiO 2 matrix. The sample synthesized by wet impregnation of nitrates presented a large mean ferrite crystallite size and small quantities of other phases, as hematite and willemite. All samples displayed ferromagnetic hysteresis in the temperature range studied and it was found that the saturation magnetization of the nanostructured NiZn ferrite was lower than that of the bulk material, in the three composites analyzed.

The real part of AC conductance in amorphous nanocomposites ferromagnetic alloy–dielectric

Results of AC conductivity of the granular nanocomposites consisting of amorphous ferromagnetic alloy nanoparticles (Fe0.45Co0.45Zr0.10) embedded into amorphous dielectric matrix (Al2O3) are presented and analyzed here. Conductivity measurements were made for the samples of different metal-to-dielectric ratio x (25 < x < 65 at.%) in the frequency range of 0.1–1000 kHz at temperature of 80–340 K. Real part of AC conductance at low frequencies (f ≤ 5 kHz) have shown temperature dependencies σreal(T) corresponding to Mott hopping regime at x below the percolation threshold and metallic one beyond the percolation threshold. It was shown that σreal(T) dependencies satisfactorily follow the known relations of 3D percolate models with critical indexes t ≈ 1.6, q ≈ 0.9, and s = 0.62. The numerical estimations of the density of localized states N(EF) displayed a tendency to be decreased with x increase and the electron wave-function localization length a was about 11 nm.

Electromagnetism and magnetization in granular two-phase nanocomposites: A comparative microwave study

Cold-pressed powder compacts in our experiments were prepared from commercial nanopowders of ZnO, Ni, Co and γ-Fe2O3. A systematic study of the room temperature effective permeability tensor of composite samples made of these nanophases is performed and provides a signature for the nonreciprocity of wave propagation in these nanostructures. Our measurements which cover a broad range of frequency in the microwave region provide a wealth of information leading to a much better understanding of the electromagnetic wave transport in nanogranular materials throughout this frequency range. We report our observations on the frequency and composition dependences of the permeability tensor components of a large set of nanocomposites (NCs) at different magnetic fields. It is found that mixing Ni nanoparticles with ZnO nanoparticles results in a smaller linewidth of the gyromagnetic resonance and an increased coercivity compared to a sample consisting solely of Ni nanoparticles. On the contrary, mixing of Co nanoparticles with ZnO nanoparticles resulted in the disappearance of the off-diagonal component of the permeability tensor and an increase in coercivity. Deviations of the saturation magnetization of Ni and Co in the Ni∕ZnO and Co∕ZnO NCs from bulklike values were observed. It is believed that the different microwave magnetic behaviors of the Ni∕ZnO and Co∕ZnO NCs are related to the difference in magnetic anisotropy of the Ni and Co particles. It is argued that surface and boundaries in the samples can play a significant role in the microwave magnetic response of these nanostructures. These NCs are promising for implementing the nonreciprocal functionality employed in many microwave devices, including isolators and circulators.

The effect of the sputtering process ambient on the magnetic state and phase composition of the film nanocomposites (Fe0.45Co0.45Zr0.10) x (Al2O3)1−x

The effect of oxygen-containing ambient arising at sputtering of granular nanocomposites (Fe0.45Co0.45Zr0.10) x (Al2O3)1−x (30 at.%≤x≤65 at.%) on their magnetic state and phase composition has been investigated. It was shown that the presence of oxygen resulted in the formation of oxide shells preventing the ferromagnetic interaction between Co0.45Fe0.45Zr0.10 nanoparticles and also the formation of metallic percolative net beyond the percolation threshold (as opposed to the films prepared in pure argon atmosphere).

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.

Optical and magnetooptical properties of CoFeB/SiO2 and CoFeZr/Al2O3 granular magnetic nanostructures

The optical and magnetooptical properties of the new granular nanocomposites (CoFeB)/(SiO2) and (CoFeZr)/(Al2O3), which are grains of amorphous ferromagnetic alloys embedded in dielectric matrices, have been studied. The dependence of the optical, magnetooptical, and magnetic properties of the nanocomposites on their qualitative and quantitative composition, as well as on the conditions of their preparation, was investigated. Spectra of the dielectric functions ε = ε1 − iε2 were obtained by the ellipsometric method in the range 0.6–5.4 eV. Above 4.2 eV, the absorption coefficient of the (CoFeB)/(SiO2) composites was found to be close to zero for all magnetic-grain concentrations. The polar Kerr effect measured at a photon energy of 1.96 eV in dc magnetic fields of up to 15 kOe reaches values as high as 0.25°–0.3° for these nanocomposites and depends only weakly on the conditions of preparation. On the other hand, the (CoFeZr)/(Al2O3) nanostructures reveal a considerable difference in the concentration dependences of the Kerr effect between samples prepared in a dc magnetic field and in zero field.

Novel nanostructures in microalloyed Pr-Fe-B nanocomposite permanent magnets

Microalloying of Pr–Fe–B nanocomposite permanent magnets with refractory metals have resulted in the development of very fine two-phase nanostructures. Energy-filtered transmission electron microscopy has been used to characterize the phase and chemistry distributions in the nanostructures. With Hf and Ti additions, α-Fe and Pr2Fe14B form as discrete phases, forming a granular nanocomposite. However, with Nb additions grains with a discrete chemical segregation pattern were formed. The grain interiors were found to be poor in Fe and rich in Pr, while the grain shells were found to be poor in Pr and rich in Fe, Co and B. An increased coercivity was attributed to this unique structure.

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.

Tunneling magnetoresistance in granular cermet films with particle size distribution

Abstract The correlation between tunneling magnetoresistance (TMR) and field sensitivity (dMR/d H ) for granular films (Co 50 Fe 50 ) x -(Al 2 O 3 ) 1− x was studied. The position of TMR maximum is shifted towards the lower x in the higher applied magnetic fields. Such a behavior was observed for metal granular nanocomposites but is first reported for granular cermets. However the highest dMR/d H was found for the compositions just below the percolation threshold.

Giant Hall effect in superparamagnetic granular films

A comprehensive review of the giant Hall effect (GHE) is presented, with emphasis on novel experimental data obtained in Ni–SiO2 and Co–SiO2 films prepared by co-sputtering. GHE is observed close to and on both sides of the metal–insulator transition. From the point of view of microscopic conduction mechanisms, this means a crossover from metallic conductivity with weak localization to tunneling, or hopping, between separate granules across insulating barriers. Magnetic percolation is also interrupted at this concentration of metal, leading to superparamagnetic behavior of the composite and blocking phenomena. Temperature dependencies of magnetization and extraordinary Hall coefficient in the composites near the critical concentration are compared. In single phase magnetic metals and alloys, the extraordinary Hall is believed to be directly proportional to the total magnetization, due to side jumps or skew scattering. In a metal–insulator composite, only those electrons traveling in conduction critical paths can contribute to the Hall signal, thus only magnetization of the material belonging to these paths is important in the Hall measurements. Comparison with the magnetic results leads to new possibilities in understanding both the electronic and magnetic properties of granular nanocomposites.

Influence of particle size distribution in cermet nanocomposites on magnetoresistance sensitivity

Summary form only given. Granular (in particular -cermet) nanocomposites and thin films attract much attention due to their unique physical properties and potential applications for magnetic sensors, magnetic recording media etc. Easy preparation procedure and high stability are the striking features of the granular structures. At the same time low magnetoresistance (MR) sensitivity comparing layered structures can be considered as an imperfection. Recently it has been shown that formation of specific granular structures (layered granular films or discontinuous multilayers) lead to increase in MR sensitivity. However the influence of the topology on magnetic and MR properties of granular nanocomposites is not completely clear. We report the influence of particle size distribution (PSD) on magneto-transport properties of (Co/sub 50/Fe/sub 50/)/sub x/-(Al/sub 2/O/sub 3/)/sub 1-x/ granular films prepared by dual electron beam evaporation (x -is the volume fraction). Magnetic susceptibility, magnetization hysteresis loops and magnetotransport properties were studied in the temperature range 5-300K and in the fields (H) up to 50 kOe. PSD was obtained from the approximation of M(H) curves at different temperatures. The average particle size, also estimated from ZFC-FC high temperature susceptibility, varies from 1 nm (x=0.07) to 3 nm (x=0.24) and the PSD become broader. The correlation between magnetization and MR was studied. It has been found that MR(M) curves does not follow (M/M/sub g/)/sup 2/ dependence (M/sub g/ is saturation magnetization), predicted for uniform size spherical particles with uncorrelated magnetic moments. The derivation from square-law is determined by PSD and is maximal for the films with the broadest distribution of superparamagnetic particles. In this region the highest values of MR were also observed. The effect of PSD could be used for enhancement of MR sensitivity in granular nanocomposites.

Magnetotransport and magnetism in granular EuS-Co and EuS-Ag nanocomposites prepared by mechanical alloying

Magnetic and magnetotransport properties have been studied in nanocomposites (EuS)-Co and (EuS)-Ag prepared by mechanical alloying. For (EuS)/sub 70/Co/sub 30/, the large negative magnetoresistance (/spl sim/50%) found at 20 K, just above the Curie temperature (T/sub c/) of EuS, arises from the suppression of spin fluctuation in EuS. Below T/sub c/, the positive magnetoresistance (/spl sim/15%), which is uncharacteristic of ferromagnetic semiconductors, is probably due to defect-induced magnetic fluctuation. On the other hand, (EuS)/sub 70/Ag/sub 30/, where nonmagnetic silver replaces cobalt, exhibits negative magnetoresistance below T/sub c/, which is possibly related to the spin dependent scattering commonly seen in GMR materials. While (EuS)/sub 70/Co/sub 30/ shows semiconducting conductivity, (EuS)/sub 70/Ag/sub 30/ is metallic. Reentrant spin glass behavior has been observed in EuS-Ag system, where the rise of antiferromagnetic coupling is due to the defects and disorder caused by mechanical milling.