V. V. Rylkov

Ferromagnetism of low-dimensional Mn-doped III-V semiconductor structures in the vicinity of the insulator-metal transition

The structural and transport properties of GaAs/Mn/GaAs/InxGa1−xAs/GaAs quantum wells (x≈0.2) with Mn δ-layer (4–10 at. %), separated from the well by a GaAs spacer, have been studied. The hole mobility in the investigated structures has exceeded the values known for magnetic III-V heterostructures by two orders of magnitude. For structures with the conductivity of the metal type, we have succeeded to observe at low temperatures Shubnikov–de Haas oscillations just confirming the two dimensionality (2D) of the hole energy spectrum. Exactly those 2D holes promote the ferromagnetic ordering of the Mn layer. That has been proven by (i) observing maxima (at 25–40 K) in temperature dependencies of the resistance, which positions agree with calculated values of Curie temperatures (for structures with the indirect interaction of Mn atoms via 2D holes), and (ii) revealing the negative spin-dependent magnetoresistance (NMR) as well as the anomalous Hall effect (AHE), which values are also in good agreement with cal...

High-temperature ferromagnetism in Si1 − xMnx (x ≈ 0.5) nonstoichiometric alloys

It has been found that the Curie temperature (TC ≈ 300 K) in nonstoichiometric Si1 − xMnx alloys slightly enriched in Mn (x ≈ 0.52–0.55) in comparison to the stoichiometric manganese monosilicide MnSi becomes about an order of magnitude higher than that in MnSi (TC ∼ 30 K). Deviations from stoichiometry lead to a drastic decrease in the density of charge carries (holes), whereas their mobility at about 100 K becomes an order of magnitude higher than the value characteristic of MnSi. The high-temperature ferromagnetism is ascribed to the formation of defects with the localized magnetic moments and by their indirect exchange interaction mediated by the paramagnetic fluctuations of the hole spin density. The existence of defects with the localized magnetic moments in Si1 − xMnx alloys with x ≈ 0.52–0.55 is supported by the results of numerical calculations performed within the framework of the local-density-functional approximation. The increase in the hole mobility in the nonstoichiometric material is attributed to the decay of the Kondo (or spin-polaron) resonances presumably existing in MnSi.

Quantum size effect transition in percolating nanocomposite films

We report on unique electronic properties in Fe-SiO2 nanocomposite thin films in the vicinity of the percolation threshold. The electronic transport is dominated by quantum corrections to the metallic conduction of the Infinite Cluster (IC). At low temperature, mesoscopic effects revealed on the conductivity, Hall effect experiments and low frequency electrical noise (random telegraph noise and 1/f noise) strongly support the existence of a temperature-induced Quantum Size Effect (QSE) transition in the metallic conduction path. Below a critical temperature related to the geometrical constriction sizes of the IC, the electronic conductivity is mainly governed by active tunnel conductance across barriers in the metallic network. The high 1/f noise level and the random telegraph noise are consistently explained by random potential modulation of the barriers transmittance due to local Coulomb charges. Our results provide evidence that a lowering of the temperature is somehow equivalent to a decrease of the metal fraction in the vicinity of the percolation limit.

Ferromagnetism of MnxSi1-x(x ∼ 0.5) films grown in the shadow geometry by pulsed laser deposition method

The results of a comprehensive study of magnetic, magneto-transport and structural properties of nonstoichiometric MnxSi1-x (x ≈ 0.51-0.52) films grown by the Pulsed Laser Deposition (PLD) technique onto Al2O3(0001) single crystal substrates at T = 340°C are present. A highlight of used PLD method is the non-conventional (“shadow”) geometry with Kr as a scattering gas during the sample growth. It is found that the films exhibit high-temperature (HT) ferromagnetism (FM) with the Curie temperature TC ∼ 370 K accompanied by positive sign anomalous Hall effect (AHE); they also reveal the polycrystalline structure with unusual distribution of grains in size and shape. It is established that HT FM order is originated from the bottom interfacial self-organizing nanocrystalline layer. The upper layer adopted columnar structure with the lateral grain size ≥50 nm, possesses low temperature (LT) type of FM order with Tc ≈ 46 K and contributes essentially to the magnetization at T ≤ 50 K. Under these conditions, AHE ...

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: temperature dependence

We present our last results on anomalous Hall effect (AHE) in (Co41Fe39B20)x(Al–O)100-x nanocomposites focusing on the possible correlation between temperature dependence of AHE and resistivity. It is shown that the temperature dependence of conductivity G=1/Rxx, where Rxx is resistivity, for compositions with x=49-56% at 10K<T<Tk follows the relation, where the parameters A, , Tk depend on x. For x=47% this relation changes to the exponential law “1/2” Rxx ∝ exp (Т0/T)1/2. The correlation between AHE resistivity RH (T) and resistivity Rxx (T) can be described as RH ∝ (Rxx)m , where m increases from 0.38 to 0.58 with an increase of x from 49 to 56 %.

Pecularities of Hall effect in GaAs/δ〈Mn〉/GaAs/InxGa1−xAs/ GaAs (x ≈ 0.2) heterostructures with high Mn content

The transport properties of GaAs/δ〈Mn〉/GaAs/InxGa1−xAs/GaAs structures containing an InxGa1−xAs (x ≈ 0.2) quantum well (QW) and a Mn delta layer (DL) with relatively high content, about one Mn monolayer (ML), are studied. In these structures the DL is separated from the QW by GaAs spacer with thickness ds = 2–5 nm. All structures possess a non-metallic character of conductivity and display a maximum in the resistance temperature dependence Rxx(T) at the temperature ≈ 46 K, which is usually associated with the Curie temperature TC of ferromagnetic (FM) transition in DL. However, it is found that the Hall effect concentration of holes pH in the QW does not decrease below TC as one ordinary expects in similar systems. On the contrary, the dependence pH(T) experiences a minimum at T = 80–100 K depending on the spacer thickness, then increases at low temperatures more strongly when ds is smaller, and reaches a giant value pH = (1–2) × 1013 cm−2. The obtained results are interpreted in the terms of magnetic proximity effect of the DL on the QW, inducing spin polarization of the holes in the QW. Strong structural and magnetic disorder in the DL and in the QW, leading to phase segregation in them is taken into consideration. The high pH value is explained as a result of the compensation of the positive normal Hall effect component by the negative anomalous Hall effect component.

Symmetry of the Conduction‐band Minima in AlP

We investigate the properties of quasi‐two‐dimensional electrons in AlP quantum wells by measuring cyclotron resonance, quantum Hall effect, and Shubnikov de Haas oscillations in modulation‐doped AlP‐GaP type‐II quantum wells. We find that in wide AlP wells, the lowest conduction band states are in the Xt valleys transverse to the growth direction, that the valley degeneracy of this state is gν=2, and that the cyclotron effective mass mtml = (0.52 ± 0.01) × m0. These results indicate that the biaxial strain resulting from the lattice mismatch of AlP quantum well with respect to the GaP substrate and barrier layers causes the longitudinal Xl valley to be lifted above the transverse Xt valley. Further, the two‐fold degeneracy of the Xt valley indicates that the conduction band minimum in AlP is located exactly at the X‐point of the Brillouin zone.We investigate the properties of quasi‐two‐dimensional electrons in AlP quantum wells by measuring cyclotron resonance, quantum Hall effect, and Shubnikov de Haas oscillations in modulation‐doped AlP‐GaP type‐II quantum wells. We find that in wide AlP wells, the lowest conduction band states are in the Xt valleys transverse to the growth direction, that the valley degeneracy of this state is gν=2, and that the cyclotron effective mass mtml = (0.52 ± 0.01) × m0. These results indicate that the biaxial strain resulting from the lattice mismatch of AlP quantum well with respect to the GaP substrate and barrier layers causes the longitudinal Xl valley to be lifted above the transverse Xt valley. Further, the two‐fold degeneracy of the Xt valley indicates that the conduction band minimum in AlP is located exactly at the X‐point of the Brillouin zone.

A Precise Algorithm of Memristor Switching to a State with Preset Resistance

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 (Co40Fe40B20)x(LiNbO3)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.

Magnetic Metal-Nonstoichiometric Oxide Nanocomposites: Structure, Transport, and Memristive Properties

Abstract Magnetic metal-insulator nanocomposites (NCs) consist of an ensemble of ultrafine ferromagnetic nanogranules chaotically distributed in a nonmagnetic dielectric matrix. Depending on chemical compositions of constituent materials, a volume fraction of metal close or far from the percolation threshold, technological parameters during fabrication these materials exhibit a wide diversity of magnetic, transport, high frequency, optical and magneto-optical properties important for fundamental research and promising for various applications. In this chapter structural, magnetic, magneto-transport, and memristive properties of NCs thin films (Co40Fe40B20)х(Al2O3–z)100–х and (Co40Fe40B20)х(LiNbO3–z)100–х fabricated by ion-beam sputtering are discussed focusing on novel features such as presence of a large amount of dispersed metal ions in the matrix, logarithmic temperature dependence of electrical resistivity, tunneling anomalous Hall effect, resistive switching.