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AbstractThis review is devoted to an analysis of the electrical resistance, the magnetoresistance, and the anomalous Hall effect in magnetic “ferromagnetic metal–insulator” nanocomposites at a metal content near the percolation threshold and the memristive properties of the capacitor structures based on these nanocomposites. A high content (up to 1022 cm–3) of dispersed atoms in intergranular gaps leads to a logarithmic temperature dependence of the electrical resistance, a positive contribution to the magnetoresistance, the appearance of tunneling anomalous Hall effect, and a multifilament mechanism of resistive switching (which causes an adaptive character of memristor nanocomposites with dispersed atoms).

AbstractThe room-temperature magnetic resonance spectra of metal–insulator (CoFeB)x(LiNbO3)100 – x and (CoFeB)x(Al2O3)100 – x nanogranular composite films with various ferromagnetic metallic phase contents x near the percolation threshold are investigated. The systems under study are characterized by a high concentration of paramagnetic ions dispersed in an insulator matrix between ferromagnetic granules. In addition to a usual ferromagnetic resonance signal, these films are found to exhibit an additional absorption peak in weak fields. In contrast to the usual ferromagnetic resonance excited by a transverse high-frequency magnetic field, the additional peak demonstrates a weak dependence of its amplitude on the resonance excitation geometry. The position of this peak depends on the composition of the system, the resonance excitation frequency (f = 7–38 GHz), and the magnetic field orientation with respect to the film plane. This behavior is associated with the paramagnetic resonance of Fe3+ ions, which are present in the insulator matrix and interact with ferromagnetic granules.

The properties of (CoFeB)x(LiNbOy)100–x nanocomposite films with a ferromagnetic alloy content x = 6–48 at % are comprehensively studied. The films are shown to consist of ensembles of CoFe granules 2–4 nm in size, which are strongly elongated (up to 10–15 nm) in the nanocomposite growth direction and are located in an LiNbOy matrix with a high content of Fe2+ and Co2+ magnetic ions (up to 3 × 1022 cm–3). At T ≤ 25 K, a paramagnetic component of the magnetization of nanocomposites is detected along with a ferromagnetic component, and the contribution of the former component is threefold that of the latter. A hysteresis of the magnetization is observed below the percolation threshold up to x ≈ 33 at %, which indicates the appearance of a superferromagnetic order in the nanocomposites. The temperature dependence of the electrical conductivity of the nanocomposites in the range T ≈ 10–200 K on the metallic side of the metal–insulator transition (44 at % < x < 48 at %) is described by a logarithmic law σ(T) ∝ lnT. This law changes into the law of “1/2” at x ≤ 40 at %. The tunneling anomalous Hall effect is strongly suppressed and the longitudinal conductivity turns out to be lower than in a (CoFeB)x(AlOy)100–x composite material by an order of magnitude. The capacitor structures based on (CoFeB)x(LiNbOy)100–x films exhibit resistive switching effects. They are related to (i) the formation of isolated chains of elongated granules and an anomalously strong decrease in the resistance in fields E > 104 V/cm because of the suppression of Coulomb blockage effects and the generation of oxygen vacancies VO and (ii) the injection (or extraction) of VO vacancies (depending on the sign of voltage) into a strongly oxidized layer in the nanocomposites, which is located near an electrode of the structure and controls its resistance. The number of stable resistive switchings exceeds 105 at a resistance ratio Roff/Ron ~ 50.

AbstractFilms of metal–insulator nanogranular (CoFeB)x(LiNbO3)100 – x and (CoFeB)x(Al2O3)100 – x composites with different content x of a ferromagnetic metallic phase have been investigated by the magnetic resonance method in a wide temperature range (4.2–360 K). The systems under study are characterized by a high concentration of paramagnetic Fe and Co ions, which are dispersed in the insulating medium between ferromagnetic CoFeB granules. The experimental spectra of these systems show a peak of ferromagnetic resonance associated with the ferromagnetic granule array and an additional less intense absorption peak associated with the electron paramagnetic resonance of Fe3+ ions in the insulating matrix. It has been found that the position and intensity of this peak depend on the composition of the system and temperature. The observed behavior is explained by existence of exchange interaction between magnetic ions and ferromagnetic granules.

An algorithm of memristor switching with high precision to a state with preset resistance has been developed based on the application of voltage pulses with smoothly increasing amplitude and the duration varying randomly within preset limits. It is shown that the proposed algorithm can be implemented in memristor structures based on (Co 40 Fe 40 B 20 ) x (LiNbO 3 ) 100– x nanocomposites with x ≈ 10 at. %. Optimum parameters are selected for the algorithm operation with a minimum number of iterations that allows the accuracy of resistance setting to be no worse than 0.5%. The obtained results can be used in the creation of neuromorphic systems.

AbstractWe present results of experimental studies of high-field magnetoresistance of Co–SiO2, Co–LiNbO3, CoNbTa–SiO2 nanocomposites with metal volume fraction close to the percolation threshold. The nanocomposite films were deposited onto a glass-ceramic substrate by ion-beam sputtering at the growth temperature not exceeding 80°C. Magnetization was measured using a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 4.2–300 K. Out-of-plane magnetoresistance was measured in a pulsed magnetic field up to 20 T in the temperature range of 4.2–300 K with the pulse duration of 11–12 ms. In addition to negative magnetoresistance, a linear positive contribution to magnetoresistance was observed in high magnetic fields for nanocomposites with the composition close to the percolation threshold. This effect was explained by the influence of the Zeeman effect on the tunnel barrier height. It is shown that the unconventional anisotropy of magnetoresistance of Co–LiNbO3 is associated with the peculiarities of its microstructure.

AbstractMicrowave magnetic impedance spectra are obtained for \\([(Co_0.52Fe_0.45B_0.2)_x + (SiO_2)_y]\\) (x = 0.46–0.88) films in a DC magnetic field of 0‒0.7 T. The modulus of microwave impedance in films with x = 0.76 is found to fall by as much as 6% upon magnetization. The maximum negative microwave magnetic impedance of the films is established along with its shift from 2.2 to 0.4 GHz as x rises. A striped magnetic structure with oppositely directed magnetizations of neighboring stripes is observed.

Neuromorphic computing networks (NCNs) with synapses based on memristors (resistors with memory) can provide a much more effective approach to device implementation of various network algorithms as compared to that using traditional elements based on complementary technologies. Effective NCN implementation requires that the memristor resistance can be changed according to local rules (e.g., spike-timing-dependent plasticity (STDP)). We have studied the possibility of this local learning according to STDP rules in memristors based on (Co 0.4 Fe 0.4 B 0.2 ) x (LiNbO 3 ) 1 – x composite. This possibility is demonstrated on the example of NCN comprising four input neurons and one output neuron. It is established that the final state of this NCN is independent of its initial state and determined entirely by the conditions of learning (sequence of spikes). Dependence of the result of learning on the threshold current of output neuron has been studied. The obtained results open prospects for creating autonomous NCNs capable of being trained to solve complex cognitive tasks.

AbstractThe effect of oxygen and water vapor in a sputtering chamber during the deposition of thin-film (Co40Fe40B20)x(LiNbO3)100 –x nanocomposites on the electrical properties of the heterogenous system is investigated. It is found that the resistivity of (Co40Fe40B20)x(LiNbO3)100 –x nanocomposites increases significantly with the partial pressure of reactive gases (oxygen and water vapor). A noticeable shift of the percolation threshold towards higher values of the metal phase volume concentration, which is observed in the plane of the film and in the perpendicular direction during the synthesis of composites with the addition of reactive gases, is attributed to the increase in the volume concentration of the dielectric phase. It is found that the percolation threshold for the measurements in the geometry perpendicular to the plane of the film is characterized by a much lower concentration of the Co40Fe40B20 alloy atoms than that for the measurements in the plane of the film, which is associated with an elongated shape of granules in the film growth direction and the effects of Coulomb blockade suppression by a high transverse electric field.

— The structural and phase changes induced by heat treatment (HT) and brief lamp processing (LP) in 70- and 1500-nm-thick Li–Nb–O films similar in elemental composition to LiNbO 3 , grown on the surface of unheated substrates (monocrystalline Si wafer and NaCl single crystal) by rf magnetron sputtering and ion-beam sputtering of a LiNbO 3 target have been studied by reflection high-energy electron diffraction and X‑ray diffraction. The as-grown films were extremely nanostructured and consisted of crystalline lithium oxides, niobium oxides, and LiNbO 3 as the product of reaction between them. We have demonstrated the conceptual feasibility of driving the synthesis of polycrystalline LiNbO 3 films to completion using HT or brief LP. In the case of LP, LiNbO 3 synthesis is three orders of magnitude faster than in the case of HT.