Vol 61, No 9 (2019)

Methods for controlling states of interacting superconducting flux qubits using power-efficient devices of fast single-quantum logic (Josephson nonlinearity cavities) are studied. One- and two-qubit quantum logical operations performed within the conventional control technique using Rabi pulses and using picosecond single unipolar magnetic field pulses are comparatively analyzed. It is shown that all main operations can be implemented with an accuracy of better than 97% due to optimization of the shape and parameters of unipolar control pulses (associated with, e.g., propagation of fluxons in transmission lines). The efficiency of the developed technique for programming a two-qubit quantum processor implementing the simplest Deutsch–Jozsa algorithm is demonstrated.

Possible causes of magnetic oscillations with a low frequency of about 2% of the Brillouin zone, observed in high-temperature superconductors of the YBCO family, are studied. A scenario is proposed in which the observed frequency is proportional to the difference of the bilayer-split closed pockets in the Fermi surface rebuilt due to scattering to charge-density-wave vectors. A method is proposed for experimental determination of which of the four possible scenarios for the rise of these magnetic oscillations is realized in YBCO.

Using the near-field microwave microscopy method, the temperature dependence of third harmonics power P_3ω(T) of the MoN/Al superconductor–normal metal structures with the proximity effect is studied. The presence of a secondary (low-temperature) maximum and zeroings is detected in the P_3ω(T) dependence of the MoN/Al structure, which disappear upon applying a weak magnetic field perpendicular to the plane of the structure.

The theoretical model of conductivity of a layered anisotropic normal metal containing small superconducting ellipsoidal granules with an arbitrary ratio of semiaxes is developed. Calculation data obtained under two simple approximations (self-consistent and Maxwell) are compared. The results may be applied in the analysis of the observed temperature dependence of the conductivity anisotropy in various anisotropic superconductors with the superconducting phase emerging in the form of isolated superconducting granules. The temperature dependence of the electric resistance along and across the conducting layers above and near the superconducting transition temperature is studied experimentally for bridge structures of a varying thickness. It is demonstrated that this resistance and even the effective superconducting transition temperature depend strongly on the bridge thickness (i.e., the number of layers through which the electric current flows). Note that significant differences were observed only for the resistance across the layers.

High-quality aluminum films on GaAs substrates are studied experimentally. The critical temperature of superconductivity is found to increase markedly with decreasing the film thickness. The observed phenomenon is considered as a manifestation of the quantum confinement effect, which affects both the density of states and the electron–phonon interaction.

The effect of deformations on magnetic nanoparticles of elliptic, square, and triangular shapes is simulated. The distribution of the magnetization of such particles is studied as functions of their shapes and applied deformations. A deformation is shown to possibly lead to the formation of new metastable states and also a change in the type of the ground state of a particle. The deformation substantially changes the distribution of the particle magnetization, which can be detected by their MFM contrast.

The magnetic properties of CoPt, CoPd, and FePd thin films with different contents of a ferromagnetic material fabricated by electron beam evaporation have been compared. The micromagnetic structure investigations have shown a decrease in the magnetic domain size with an increase in the Co content in the CoPt and CoPd films, which has been attributed to the presence of two magnetic phases in the films. The magnetooptical studies have shown anomalously large Faraday angles and the absence of Kerr effect in the CoPt films, which make them attractive for application in optics. In the FePd films, the minimum coercivity values have been obtained from the magnetic-field dependences of the magnetization, which has also been related to the magnetic structure features.

The report presents a model of the magnetic structure of diluted iron garnets with nonmagnetic ions in a dodecahedral sublattice. The dilution of magnetic iron sublattices is assumed to be selective: in the limiting case, the substitution of nonmagnetic ions for iron takes place only in a tetrahedral sublattice. In this case, iron ions in the octahedral environment have a variable number of the nearest magnetic neighbors; thus, octahedral sublattice are introduced in the dependence on the number of magnetic neighbors. This model is shown to describe well the magnetic properties of diluted iron garnets with a compensation point.

Using micromagnetic simulation, the dependence of the structure and energy of a domain wall localized in the region of a magnetic crossbar between ferromagnetic films on the geometric parameters of the system has been investigated. The effect of displacement and change in the shape of the magnetization curve of the magnetically soft layer, caused by the exchange interaction with the magnetically hard layer through the magnetic filament, is demonstrated.

Film structures of the Gd–Co/Co/Cu/Co type having the properties of heat-sensitive spin valves have been developed, fabricated, and investigated. The results of magnetic and magnetoresistive measurements in a wide range of fields and temperatures for these structures are presented. It is shown that the occurrence of noncollinear magnetic structure in the Gd–Co/Co system significantly affects the shape of the magnetoresistive hysteresis loops. The magnitude of the critical fields of magnetization reversal depends on the temperature and can vary due to a change in the ratio of thicknesses of the film system layers.

Comparative analysis of Hall effect in substitutional solid solutions Ho_0.5Lu_0.5B₁₂, Mn_1 – xFe_xSi (0 ≤ x ≤ 1) and Eu_1 – xGd_xB₆ (x < 0.04) is carried out at temperatures 2–300 K in magnetic fields up to 8 T. Anomalous contribution to the Hall effect \(\rho _{{xy}}^{{\text{A}}}\) ~ \({{\rho }_{{xx}}}M\) has been identified for systems with various types of magnetic frustration. The linear scaling \(\rho _{{xy}}^{{\text{A}}}\) ~ ρ_xx is detected in the resistivity range ρ_xx ~ 0.01–1 mΩ cm lying outside the range of applicability of the classical model of asymmetric scattering. It is associated with the increase in the amplitude of spin fluctuations in the paramagnetic phase of the investigated compounds with noncollinear magnetic structure. The topological contribution to the Hall effect is extracted for Ho_0.5Lu_0.5B₁₂ and Eu_1 – xGd_xB₆. Its amplitude is found to vary from 80 nΩ  cm (Ho_0.5Lu_0.5B₁₂) to 7.5 μΩ  cm (Eu_0.97Gd_0.03B₆).

Nanowires (NWs) consisting of Ni/Cu and Co/Cu alternating layers with a diameter of 100 nm and layer thicknesses varying between 10 and 500 nm are prepared by template synthesis in pores of polymer track-etched membranes. Bath compositions and different regimes for pulsed electrodeposition of NWs are explored. A procedure for electrodeposition of NWs using pulses of equal charge is developed. By diminishing the amount of charge per pulse, initially we manage to lower the layer thickness to 10–15 nm, but further diminishing of charge in pulses leads to the blending of elemental composition of adjacent layers and/or formation of rod–shell nanostructures within the NWs. The coercive force (15–30 mT) and residual magnetization of our layered NWs are determined from magnetization measurements. For NWs with a layer thickness of 50–100 nm, the magnetization curves recorded in the out-of-plane and in-plane geometries are similar in shape and have similar parameters. For NWs with thicker layers (250 and 500 nm), magnetization curves are markedly different due to magnetic anisotropy (an easy magnetization axis emerges longitudinally to NWs) and interference between neighboring NWs. Magnetic force microscopy of isolated NWs identifies that the NWs comprise magnetic regions extending over ~100–150 nm. The NW can be partially remagnetized by applying an external magnetic field (+16 mT) longitudinally.

The magnetic properties of heterostructures consisting of two films are studied. The upper layer involves rare-earth intermetallic nanostructured superlattices consisting of exchange-coupled layers (TbCo₂/FeCo)_n (TCFC), and the lower layer includes either epitaxial manganite La_0.7Sr_0.3MnO₃ (LSMO) with optimum strontium doping or an epitaxial film of an yttrium–iron garnet Y₃Fe₅O₁₂ (YIG) with a Bi additive. TCFC is a ferromagnet having high Curie temperature and provides controllable induced magnetic anisotropy. Experimental studies showed that the interlayer interaction of the TCFC/LSMO heterostructure is antiferromagnetic. There was an increase in the FMR line width in the structures due to the flow of a spin current through the interface between two films. There was electric voltage in the TCFC/YIG heterostructure induced in the TCFC intermetallide film, due to an inverse spin Hall effect under ferromagnetic resonance conditions.

A method of manufacturing double-lattice magnetoplasmonic crystals with the structure (Au/BIG)², in which the plasmon gold lattices are displaced relative to each other by half a period, has been presented. Gold films with thicknesses of about 40 nm have been formed by the ion-beam sputtering–deposition method, while the adhesive properties of the films allow the dimensional etching by a sharp-focused ion beam. It has been shown that the formation of the second plasmon lattice located on top of the 100 nm thick garnet layer allows the preserving of the periodicity of the first Au lattice. However, there is a significant influence of diffusion spreading on the lower lattice material, which leads to a decrease in its density. The dependence of the intensity magnetooptical effect in the geometry on the transmission as a function of the thickness of the upper lattice and presence of an additional layer of Ta₂O₅ has been studied.

The electron transport properties of strained thin La_0.7Ba_0.3MnO₃ (LBMO) epitaxial films are studied. Films 40–100 nm in thickness were prepared by laser ablation at a temperature T = 700–800°C in pure oxygen atmosphere of 0.3–1 mBar. Ferroelectric crystal substrates (011)0.79PbMg_1/3Nb_2/3O₃–0.21-PbTiO₃ (PMN–PT) with a Curie temperature of 150°C and high piezoelectric constants were used to create a mechanical stress. The ferroelectric polarization and piezoelectric effects on the electrical parameters of LBMO films are studied.

The temperature dependence of the linear electrodynamic response of thin-film superconductor (MoN)–normal metal (Al) hybrid structures with a high conductivity ratio in the normal state has been theoretically and experimentally investigated. Low-frequency measurements of the coefficient of mutual induction of two coils with a sample placed between them indicate an increase in the magnetic screening of the superconductor–normal metal (SN) structures with an increase in the Al layer thickness d_Al near liquid-helium temperatures. Measurements of the frequency shift δf of a microwave dielectric resonator, brought into contact with the sample, as a function of temperature and d_Al showed that (i) the character of the dependence δf(T) depends strongly on d_Al and (ii) the resonance frequency shift of SN structures at temperatures close to the critical temperature T_c is not described by dependence const/(1 – T/T_c), which is typical of thin superconducting films. Numerical calculations performed within the Usadel model well describe the observed effects. Thus, these anomalies of the electrodynamic properties of SN structures can be explained by the presence of a minigap in the spectrum of quasiparticles due to the proximity effect in a normal-metal layer, which depends on d_Al, and by the high conductivity of the Al layer.