A. S. Vedeneev

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.

High-frequency reactive diode sputtering of magnetite films on the sapphire surface

The conditions (oxygen partial pressure and growth rate) for the deposition of magnetite (iron oxide) Fe3O4 on the r plane of the single-crystalline sapphire using the high-frequency reactive diode sputtering of the Fe target are determined. The resulting ferromagnetic layers exhibit polycrystalline structure with a typical block size of 100–200 nm. The X-ray analysis is used to demonstrate that the textured phase of magnetite that is normally oriented with respect to the substrate dominates in the blocks and Fe and Fe2O3 impurities are almost absent.

Formation of the ordered ensemble of cobalt nanocylinders in pores of anodic aluminum oxide on the surface of GaAs structures

A method for creation of cobalt nanocylinder quasi-regular ensembles with controlled height in the pores of anodic aluminum oxide on the surface of GaAs structures with a thin n-GaAs surface layer is developed. The pores of aluminum oxide are filled using galvanic deposition of cobalt from the CoCl2 solution, so that hydrogen emission is minimized and the uniform growth of cobalt in the pores is provided. The method is promising for development of the matrix hybrid magnetic-semiconductor structures for spintronics.

Transport, Magnetic, and Memristive Properties of a Nanogranular (CoFeB) x (LiNbO y )100–x Composite Material

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.

Effects of electron drag of gold in pores of anodic aluminum oxide: Reversible resistive switching in a chain of point contacts

The current–voltage characteristics of Au/AAO(Au)/probe structures based on anodic aluminum oxide with pores incompletely filled with gold have been studied. It has been found that an electric field initiates the mass transfer of a rear Au electrode and the subsequent growth of the metal in unfilled parts of pores of the oxide matrix in the form of chains of gold islands. It has been established that this transfer, which appears at a positive potential of the probe, is due primarily to the effect of electron drag of the metal (Au). Estimates have been obtained for the effective radius of Au islands (2 nm), the width of a gap between islands (0.5 nm), the height of potential barriers (100 meV), and the characteristic resistance of tunnel junctions (30 kΩ ~ h/e2), which is typical of point quantum contacts. The structures demonstrate reversible resistive switching between low- (~1 MΩ) and high-resistance (>100 GΩ) states.

Preparation of thin anodic alumina membranes and their utilization for template electrodeposition

A method for the preparation of arrays of anodic aluminium-oxide porous films on a metal frame is proposed. The suggested approach makes it possible to obtain very thin porous-oxide films possessing a high mechanical strength, which are suitable for various practical applications such as templates for the formation of one-dimensional nanostructures or membranes for gas separation and liquid filtration. To demonstrate the possibility of the utilization of thin anodic alumina films as the templates for nanowire formation, cobalt nanowires with 50 nm diameter are prepared using 5 μm thick alumina membranes.

Synthesis of spatially ordered ensemble of Co nanocylinders in porous alumina matrix on surface of GaAs structures

A method for producing quasi-regular ensembles of Co nanocylinders with controllable height in anodic alumina pores on the surface of GaAs structures is improved. Alumina pores are filled by galvanic deposition of cobalt from CoCl2 solution, which ensures uniform growth of Co in pores. It is established that, even at a small ratio of length of Co nanocylinders to their diameter l/d ≈ 2, the preferred direction of magnetic anisotropy coincides with the axis of nanocylinders.

Nanostructured layers of anodic aluminum oxide on insulating substrates

A method for creation of porous alumina layers on insulating substrates is proposed. The anodic oxidation of thin aluminum films is performed in two stages: formation of a thin dense oxide layer and its local breakdown followed by the final oxidation of the film with a propagation of the oxidation front from a breakdown region to periphery. Aluminum oxide layers exhibit quasi-ordered structure with the pore diameter ranging from 10 to 100 nm and depending on conditions of electrochemical oxidation. The method provides the aluminum film oxidation right to the insulating substrate (i.e., the absence of residual metal phase inclusions on the interface) and can be used for the creation of matrix and composite structures for semiconductor microelectronics.

Mesoscopic fluctuations of conductance in a depleted built-in channel of a MOSFET

Mesoscopic fluctuations of the off-diagonal component Rxy of the resistance tensor have been observed in macroscopic Si-MOS structures with a built-in p-channel at T = 77 K under the conditions in which the channel is depleted of free holes. It was found that the fluctuations δRxy are related to transition from the 3D conduction by free holes to 2D-percolation in the fluctuation potential of ionized impurities of the p-doped surface layer depleted due to the field effect. From the analysis of data on δRxy, the correlation length Lc of the percolation cluster, which describes the spatial scale of electrical nonuniformity in the structure, is obtained as a function of the gate potential Vg. The dependence of Lc on Vg is well described in terms of the concepts of nonlinear screening of the fluctuation potential by holes and of the percolation nature of the hole transport for Lc varying from ∼10 nm to ∼1 μm.

Conductance of quasi-two-dimensional semiconductor systems with electrostatic disorder in the region of the percolation metal-insulator transition

Special features of the percolation transition in quasi-two-dimensional (quasi-2D) electron systems (metal-nitride-oxide-semiconductor structures with n-type inversion channels) with a strong fluctuation potential (FP) and a gate length smaller than the correlation radius of a percolation cluster (in which case the structure conductance is controlled by isolated saddle-point regions of the FP) are considered. Experimentally measured dependences of the conductance on the field-electrode potential and the temperature are analyzed in the context of the Landauer-Büttiker formalism. Energy parameters of the FP saddle-point regions and effective density of electron states (NSS ∝ m/πℏ2) near the percolation level are determined from the experimental data. Consistency between the experimental results and the proposed statistical model of the formation of FP saddle-point regions in quasi-2D systems is demonstrated. It is shown that saddles transform into potential troughs extended in the direction of the percolation route as the percolation trough is approached.