J. Fedotova

Negative capacitance in (FeCoZr)–(PZT) nanocomposite films

In this work, attention was focused on the inductive contribution to the real part of admittance G(T, f) in (Fe0.45Co0.45Zr0.10)x(PZT)(1−x) nanocomposite films deposited in a mixed argon‐oxygen atmosphere. The observed G(x, f, T ) dependences for the films on the dielectric side of the insulator‐metal transition demonstrated the negative capacitance (NC) effect that was maximal for the nanocomposites with 0.40 <x< 0.60, where the metallic FeCoZr nanoparticles were totally oxidized. The NC effect was explained by a specially developed model for the ac hopping conductance of the electrons between the fully oxidized nanoparticles embedded in the PZT matrix. In accordance with the model, under the determined conditions the observed structure of nanocomposites led to an increase in the hopping electron mean lifetime on nanoparticles and hence to the possibility of positive angles of the phase shifts θ and a proper NC (inductive-like contribution) effect. (Some figures may appear in colour only in the online journal)

Magnetoresistance in FeCoZr–Al2O3 nanocomposite films containing ‘metal core–oxide shell’ nanogranules

Temperature and magnetic field dependences of electrical conductivity are systematically studied in granular films (Fe45Co45Zr10)x(Al2O3)100?x (28???x???64) containing crystalline metallic ?-FeCo-based nanoalloy cores encapsulated in an amorphous oxide shell embedded in an amorphous Al2O3 matrix. Formation of ?metallic core?oxide shell? nanogranules is confirmed by transmission electron microscopy (TEM) and HRTEM. The structure of core and shell is governed with the difference in the oxidation states of Fe and Co ions investigated with EXAFS, XANES and M?ssbauer spectroscopy. A considerable negative magnetoresistance (MR) effect of spin-dependent nature is observed in the whole range of x values. Its increase with decreasing temperature is correlated with the magnetic saturation of superparamagnetic metallic nanogranules. The enhanced MR effect in ?core?shell? granular films is related to the percolation of oxide shells and their influence through spin filtering processes. A considerable high field MR at low temperatures and the resulting deviation of MR and squared magnetization are attributed to a magnetic randomness and/or strong magnetic anisotropy of the magnetic oxide shell.

Temperature and frequency dependences of real part of impedance in the FeCoZr-doped PZT nanogranular composites

The study of carrier transport in granular nanocomposite films consisting of ferromagnetic Fe45Co45Zr10 alloy nanoparticles embedded in a low-conductive Pb81Sr4(Na5Bi5)15(Zr57.5Ti42.5)O3 matrix has been performed. Real part of the impedance of the nanocomposites was studied as a function of composition, temperature and frequency, for the samples with 25 ≤ x ≤ 80 at.%, deposited by ion-beam sputtering in Ar + O2 gas mixture at the oxygen pressures of

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

P_{O_2 }

Growth-induced non-planar magnetic anisotropy in FeCoZr-CaF2 nanogranular films: Structural and magnetic characterization

= 2.4·10−3 and 3.7·10−3 Pa. It has been proved that approaching the threshold concentration, xC in the FeCoZr alloy resulted in the transition from high-ohmic to low-ohmic state with the x increase, without the change of hopping mechanism of carrier transport.

FeCoZr–Al2O3 Granular Nanocomposite Films with Tailored Structural, Electric, Magnetotransport and Magnetic Properties

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.

Low-Temperature DC Carrier Transport in (Fe0.45Co0.45Zr0.10)x(Al2O3)1−x Nanocomposites Manufactured by Sputtering in Pure Ar Gas Atmosphere

The relation between nanoscale structure, local atomic order and magnetic properties of (FeCoZr)x(CaF2)100−x (29 ≤ x ≤ 73 at. %) granular films is studied as a function of metal/insulator fraction ratio. The films of a thickness of 1–6 μm were deposited on Al-foils and glass-ceramic substrates, by ion sputtering of targets of different metal/insulator contents. Structural characterization with X-ray and electron diffraction as well as transmission electron microscopy revealed that the films are composed of isolated nanocrystalline bcc α-FeCo(Zr) alloy and insulating fcc CaF2 matrix. They grow in a columnar structure, where elongated metallic nanograins are arranged on top of each other within the columns almost normal to the substrate surface. Mossbauer spectroscopy and magnetometry results indicate that their easy magnetization axes are oriented at an angle of 65°–74° to the surface in films with x between 46 and 74, above the electrical percolation threshold, which is attributed to the growth-induced sh...

Influence of the short-range order in FeCoZr nanoparticles on the electrical and magnetotransport properties of FeCoZr-CaF2 nanocomposites

Unique physical properties of granular soft ferromagnetic metal-dielectric matrix nanocomposite materials (SF MMCs) make them very attractive in designing of magnetoelectronic devices. The most essential characteristics of GNMs are discussed in the chapter within the frame of percolation models, that establish correlation between microstructure and carrier transport mechanisms in MMCs.

Effect of the morphology on the mechanisms of the magnetization reversal of multilayered thin Co/Pd films

This presented work investigates the structure and temperature relationship/dependence of the DC conductivity in the (Fe0.45Co0.45Zr0.10)x(Al2O3)1−x nanocomposites deposited in Ar atmosphere with composition (  at.%) and temperature (  K). It is shown that VRH displayed crossover from Mott-like to Shklovskii-Efros regimes which occurred at temperatures of 100–120 K. It is also noted that the observed shift of the percolation threshold to higher concentrations of metallic fraction can be attributed to the disordering of the metallic nanoparticles due to the incorporation of the residual oxygen in the vacuum chamber during the deposition procedure.

57Fe Mössbauer Study of Localization and Valence Sate of Iron Ions in Granular Nanocomposites (FeCoZr)x(Al2O3)100−x for Spintronic Applications

This paper reports on the results of the 57Fe Mössbauer investigation of the short-range order in FeCoZr nanoparticles and also the studies of the electrical and magnetotransport properties of (FeCoZr)x(CaF2)100 − x granular nanocomposite films in the concentration range x = 16–75 at %. A correlation between the oxygen partial pressure during the synthesis of nanocomposites and the magnetic state of iron-containing nanoparticles has been established. The influence of the oxidation of metallic particles on the electron transport mechanisms and on the magnetoresistance of the films has been analyzed. It has been found that, in the nanocomposites with a high FeCoZr concentration (x ∼ 70–75 at %), there is a preferred orientation of the magnetic moments of α-FeCo(Zr) nanoparticles in the direction perpendicular to the film plane.