Gennady A. Ovsyannikov

Magnetotransport characteristics of strained La0.7Sr0.3MnO3 epitaxial manganite films

The electrical and magnetic characteristics of La0.7Sr0.3MnO3 (LSMO) epitaxial manganite films are investigated by different methods under conditions when the crystal structure is strongly strained as a result of mismatch between the lattice parameters of the LSMO crystal and the substrate. Substrates with lattice parameters larger and smaller than the nominal lattice parameter of the LSMO crystal are used in experiments. It is shown that the behavior of the temperature dependence of the electrical resistance for the films in the low-temperature range does not depend on the strain of the film and agrees well with the results obtained from the calculations with allowance made for the interaction of electrons with magnetic excitations in the framework of the double-exchange model for systems with strongly correlated electronic states. Investigations of the magneto- optical Kerr effect have revealed that an insignificant (0.3%) orthorhombic distortion of the cubic lattice in the plane of the NdGaO3(110) substrate leads to uniaxial anisotropy of the magnetization of the film, with the easy-magnetization axis lying in the substrate plane. However, LSMO films on substrates (((LaAlO3)0.3+(Sr2AlTaO6)0.7)(001)) ensuring minimum strain of the films exhibit a biaxial anisotropy typical of cubic crystals. The study of the ferromagnetic resonance lines at a frequency of 9.76 GHz confirms the results of magnetooptical investigations and indicates that the ferromagnetic phase in the LSMO films is weakly inhomogeneous.

Magnetic anisotropy in strained manganite films and bicrystal junctions

Transport and magnetic properties of LSMO manganite thin films and bicrystal junctions were investigated. Manganite films were epitaxially grown on STO, LAO, NGO and LSAT substrates and their magnetic anisotropy were determined by two techniques of magnetic resonance spectroscopy. Compare with cubic substrates a small (about 0.3 persentage), the anisotropy of the orthorhombic NGO substrate leads to a uniaxial anisotropy of the magnetic properties of the films in the plane of the substrate. Samples with different tilt of crystallographic basal planes of manganite as well as bicrystal junctions with rotation of the crystallographic axes (RB - junction) and with tilting of basal planes (TB - junction) were investigated. It was found that on vicinal NGO substrates the value of magnetic anisotropy could be varied by changing the substrate inclination angle from 0 to 25 degrees. Measurement of magnetic anisotropy of manganite bicrystal junction demonstrated the presence of two ferromagnetically ordered spin subsystems for both types of bicrystal boundaries RB and TB. The magnitude of the magnetoresistance for TB - junctions increased with decreasing temperature and with the misorientation angle even misorientation of easy axes in the parts of junction does not change. Analysis of the voltage dependencies of bicrystal junction conductivity show that the low value of the magnetoresistance for the LSMO bicrystal junctions can be caused by two scattering mechanisms with the spin- flip of spin - polarized carriers due to the strong electron - electron interactions in a disordered layer at the bicrystal boundary at low temperatures and the spin-flip by anti ferromagnetic magnons at high temperatures.

YBa2Cu3Ox Josephson junctions on a bicrystal sapphire substrate for devices in the millimeter and submillimeter wavelength ranges

High-temperature superconducting Josephson junctions on bicrystal sapphire substrates were fabricated and investigated experimentally. The critical parameters of the junctions satisfy the constraints for the design of devices in the millimeter and submillimeter wavelength ranges. The results of dynamic measurements show that in these junctions the superconducting current exhibits a sinusoidal dependence on the phase difference of the superconducting wave functions of the electrodes, and a simple resistive model is used to analyze their microwave properties. At the same time, the temperature dependence of the critical current of the junctions differs appreciably from that typical of tunnel junctions with s-superconductors and may be explained as d-type pairing of the superconducting electrodes and an SNS junction.

Triplet superconducting correlations in oxide heterostructures with a composite ferromagnetic interlayer

The superconducting current induced by the penetration of the long-range triplet component of superconducting correlations into a composite ferromagnetic interlayer has been detected in mesa-heterostructures based on oxide cuprate superconductors YBa2Cu3O7 − δ and Au/Nb bilayer films with the composite oxide interlayer that is made of ferromagnetic films of manganite La0.7Sr0.3MnO3 and ruthenate SrRuO3 and has a thickness much larger than the length of correlations determined by the exchange field. The deviation of the superconducting current in the mesa-heterostructure with the fraction of the second harmonic of 13% from a sinusoidal current-phase relation has been detected; this deviation can also be due to the generation of the triplet component of superconducting correlations in the ferromagnet.

Temperature Behavior of Electron Transport in Normal-Metal-HTSC Heterojunctions

Current transport in micron-sized normal-metal-high-temperature superconductor heterojunctions (Au/YBa2Cu3O6+x) was studied for two crystallographic orientations of YBCO films. It is shown that depending on the transport-current flow direction relative to the crystallographic axes of the YBCO film, the electronic transport properties of Au/YBCO heterojunctions with highly transparent boundaries change from quasi-tunneling (along the YBCO c axis) to close-to-Ohmic (in the directions lying in the YBCO basal plane).

Particle formation on the YBCO thin film surface: effect of stoichiometry and substrate material

Abstract Particle formation on the YBa 2 Cu 3 O 7− x (YBCO) thin film surface deposited by laser ablation was studied. Ablation of a stoichiometric YBCO target results in Ba-deficient films, probably due to Ba scattering in oxygen atmosphere. Partial melting of the target surface during ablation provided additional Ba flow and resulted in smooth films. Surface quality of the YBCO film with improved element composition depends on lattice mismatch between the growing film and the substrate. Increase of deposition temperature over some threshold level, specific for each tested substrate material, results in particle appearance on the film surface even in the films with improved element composition. Chemical interaction, caused by oxygen depletion of the substrate surface, can be the reason for this effect.

Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with noncollinear magnetization

Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic SrRuO3, and superconducting YBa2Cu3O6+x were studied experimentally. Structures of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3O6+x were prepared by pulsed laser deposition, and their high quality was confirmed by x-ray diffraction and reflectometry. Anoncollinear magnetic state of the heterostructureswas revealed by means of superconducting quantum interference device magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

Magnetic anisotropy of strained epitaxial manganite films

The in-plane magnetic anisotropy of epitaxial La0.7Sr0.3MnO3 (LSMO) films is studied at room temperature by the following three independent techniques: magnetooptical Kerr effect, ferromagnetic resonance at a frequency of 9.61 GHz, and recording of absorption spectra of electromagnetic radiation at a frequency of 290.6 MHz. The films are deposited onto NdGaO3 (NGO) substrates in which the (110)NGO plane is tilted at an angle of 0–25.7° to the substrate plane. The uniaxial magnetic anisotropy induced by the strain of the film is found to increase with the tilt angle of the (110)NGO plane. A model is proposed to describe the change in the magnetic anisotropy energy with the tilt angle. A sharp increase in the radio-frequency absorption in a narrow angular range of a dc magnetic field near a hard magnetization axis is detected The anisotropy parameters of the LSMO films grown on (110)NGO, (001)SrTiO3, and (001)[(LaAlO3)0.3 + (Sr2AlTaO6)0.7] substrates are compared.

Electron Transport in Hybrid Superconductor Heterostructures with Manganite Interlayers

Hybrid herostructures comprising an YBa2Cu3Ox (YBCO) high-temperature superconductor (HTS) layer and Nb/Au low-temperature superconductor (LTS) bilayer (with critical HTS and LTS temperatures Tc and T′c, respectively), separated by a thin (dM = 5–20 nm) interlayer of LaMnO3, La0.7Ca0.3MnO3, or La0.7Sr0.3MnO3 manganite have been studied. The electric resistance and magnetic properties of individual (evaporated directly onto the substrate) manganite films and related hybrid herostructures have been measured. Based on quasi-classical equations, analytical expressions for the conductivity of herostructures at T ≤ T′c are obtained in the case of a low-transparency superconductor/manganite interface. It is established that the conductivity of heterostructures is determined by the proximity effect (related to the penetration of a condensate wavefunction from the Nb/Au bilayer to manganite) and depends strongly on interface transparency. At low temperatures (T ≪ Tc′), the conductivity peaks are found at voltages determined by the exchange field of the manganite interlayer. At Tc′ < T < Tc, conductivity features at nearly zero bias voltages are observed, which are related to the superconductivity of the YBCO electrode.

Metal-insulator transition in epitaxial films of LaMnO3 manganites grown by magnetron sputtering

We have studied thin films of LaMnO3 manganite grown by RF magnetron sputtering at high pressure on crystalline substrates with cubic symmetry. It is established that these films exhibit a metal-insulator transition, whereas LaMnO3 grown on orthorhombic substrates remains in a dielectric state. The parameters of the metal-insulator transition have been studied as dependent on the level and symmetry of mechanical stresses that arise during the epitaxial growth of LaMnO3 films on various substrates. The resistance of LaMnO3 films grown on SrTiO3 substrates has been studied as a function of the film thickness. It is found that the presence of excess oxygen due to substitution in the cation system can significantly influence the Mn4+/Mn3+ ion ratio in the film and thus lead to the appearance of the metal-insulator transition.