T. Nurgaliev

Magnetization reversal of thin La0.7Sr0.3MnO3 manganite films grown on LaAlO3

The kinetics of magnetization reversal and the dynamics of domain walls in thin La0.7Sr0.3MnO3 manganite films grown on a LaAlO3 substrate are studied. The micro-domains with out-of-plane spontaneous magnetization are shown to be metastable. These domains are replaced by wide macro-domains with an in-plane spontaneous magnetization after saturation by both out-of-plane and in-plane magnetic fields. The subsequent magnetization reversal is found to occur via the nucleation and thermoactivated motion of the 180° domain walls, the dynamic parameters of which have been determined.

Microwave properties of YBCO thin films on YSZ substrates in the coupled grain model approach

Abstract Microstrip resonators with fundamental resonance frequency of 3 GHz were fabricated from YBCO thin films deposited by RF magnetron sputtering on 5 × 5 mm 2 YSZ substrates. Microwave experimental data were treated using the Phenomenological Loss Equivalence Method and film resistivity, film characteristic surface resistance and superconducting penetration depth were obtained. In spite of the high quality of the films (highly c -axis oriented, epitaxial growth in a - b -plane, high critical temperature T c and critical current density J c ) it was not possible to describe the obtained results in the framework of the two-fluid model. The interpretation was performed using the coupled grain model. The grain sizes were assumed to be equal to the mean distance between the microdefects observed by Atomic Force Microscopy. In this approach the intergrain penetration depth and the intergrain critical current density J ig were calculated. The relatively low film resistivity was explained as due to the residual microwave loss contributed from the grain boundaries and the film microdefects. The decrease of the Q -factor with the RF current density J observed at J J c , J c ig was interpreted taking into account the spatial inhomogeneous distribution of the current density in the strip.

Magnetron sputtering of polycrystalline LSMO/YBCO structures on sapphire substrates

La0.7Sr0.3MnO3 (LSMO) films with different thickness were deposited directly on r-cut sapphire substrates by RF off-axis single magnetron sputtering with a view to produce a suitable buffer for growing HTS YBa2Cu3O7-δ (YBCO) films on top. These LSMO layers were polycrystalline and showed a ferromagnetic behavior at room temperatures. The top YBCO films were sputtered in a DC off-axis double magnetron system. The polycrystalline structures thus produced were characterized and discussed.

Electrical characteristics of HTS/manganite double layers ∗

Superconductor/ferromagnetic (SC/FM) Y1Ba2Cu3O7−δ/La0.7Sr0.3MnO3 (YBCO/LSMO) double layers were prepared on LaAlO3 substrates by magnetron sputtering and their electrical and microwave parameters were investigated at 77 K. In the theoretical plan, simple formulas for estimation of the sensitivity of the SC surface impedance to the concentration changes of normal charge carriers were proposed and the surface resistance RS peculiarities of both SC and FM surfaces were described. Thinner YBCO/LSMO structure was characterized by lower SC parameters and higher surface resistance RS at ∼ 4 GHz. The difference of RS of sample SC surfaces was interpreted as due to a difference between the normal charge carrier densities in these samples. RS of the FM surface was higher than that of the SC surface due to the microwave losses of the magnetic subsystem. A peak of the microwave losses, observed in the thicker double layer, was assumed to be caused by uniform FMR in the LSMO film.

Characterization of YBCO thin films for microwave applications

Abstract The results of a systematic study of YBCO films for microwave applications deposited on LaAlO 3 substrates using 2-opposed-magnetron sputtering technique in the off-axis geometry are reported. The samples were characterized in quasi-static, low-frequency and microwave electromagnetic fields. The films with high values of the critical temperature and the critical current density demonstrated, as a rule, good microwave properties, irrespective of their surface roughness. The quality factor of an elliptic microstrip disk resonator, fabricated from such films was 9600 at 4.9 GHz and 77 K.

Transformations of head-to-head domain walls in (La,Sr)MnO3 thin films

Kinetics of the in-plane magnetization reversal in a (La,Sr)MnO3 thin film has been studied at different temperatures using the magneto-optic visualization technique. In addition to changes of the domain wall coercivity and mobility, dramatic transformations of the domain wall structure have been observed. Motion of a single zig-zag domain wall is typical for high temperature magnetization reversal, whereas complicated diffuse transient structure between the two principal domains is observed at low temperature. Generation of secondary bubble domains in front of a moving wall has been observed for the first time in a quasistatic regime, contrary to previous superfast experiments.

Theoretical study of superconducting film response to AC magnetic field

Nonlinear electromagnetic properties of thin superconducting films with respect to a time-harmonic external magnetic field of different intensities are theoretically studied in the framework of the approximation, which supposes a dependence of the film resistivity on the local shielding current density. Peculiarities of the spatial distribution of the induced electric field and shielding current and their effect on the response signal behavior are analyzed. On the basis of this, the methods suitable for contactless measurement of the fundamental and higher harmonics of the local electric field and shielding current in the superconducting film are proposed.

Na‐doping effect on the magnetic properties of the YBCO ceramics

The ac magnetization and the microwave surface resistance (at 12 GHz) were measured on a series of Na‐doped YBCO ceramic samples at 77 K. Their magnetic behavior was explained on the basis of the modified critical state model by taking into account the existence of a field‐dependent component Jc1 (due to weakly linked grains) and a field‐independent component Jc0 (due to perfectly linked grains) in the bulk critical current density of the samples. The Na and Cu remaining after the heat treatment of the samples changed the intergranular medium parameters and impeded the correlation between the grains. As a result, an increase of the Na concentration led to a decrease of the specimens critical current density and an increase of their surface resistance. At small Na concentrations, a certain increase of Jc1 was observed, which can be explained by taking into account the possibility of partial pinning of Josephson vortices in the ‘‘weakly seeded’’ places in the intergranular media.

Properties of SFS heterostructures prepared by a focused-ion-beam technique

We present a study of superconductor-ferromagnet (SF) bilayers and superconductor-ferromagnet-superconductor (SFS) heterostructures of nanometer dimensions prepared by a gallium focused-ion-beam (FIB) technique. The SFS heterostructures were implemented on the basis of high-Tc superconducting YBa2Cu3Ox (YBCO) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) thin films deposited by magnetron sputtering on SrTiO3 (100) single crystal substrates. SFS weak link junctions require weak link dimensions in the range of nanometer size realizable by FIB patterning. On the other hand, the focused gallium ion beam might cause unacceptable degradation of the superconducting and ferromagnetic thin film properties. Our results confirm such influence of the FIB technology. However, protection of the structures by a gold thin film may effectively solve this problem, as is presented in the paper.

Fabrication of hybrid thin film structures from HTS and CMR materials

We present the preparation of bilayers from high-temperature superconductors (HTS) and half-metallic ferromagnetic (FM) manganite with a colossal magnetoresistance (CMR). We used YBa2Cu3O7-x (YBCO) and Tl2Ba2CaCu2O8 (TBCCO) thin films as a HTS material and La0.67Sr0.33MnO3 (LSMO) film as a CMR material. In the case of YBCO/LSMO, we prepared FM/HTS heterostructure for studying the spin-polarized current injection effect on the electrical properties of the YBCO strip in dc or low-frequency regimes and on the microwave characteristics of the strip. For the first time, we report the preparation of a TBCCO/LSMO bilayer. In some applications, the TBCCO offers better parameters (higher working temperature, lower surface resistance, lower 1/f noise) than YBCO.