Chuanbing Cai

Formation and pinning properties of growth-controlled nanoscale precipitates in YBa2Cu3O7-δ/transition metal quasi-multilayers

To study the possibility of enhancing the pinning forces in YBa2Cu3O7?? films through the introduction of nanosized precipitates, quasi-multilayers of YBa2Cu3O7?? and a transition metal (TM = Ti,Zr,Hf) were deposited on single-crystal SrTiO3 substrates using pulsed laser deposition. The transition metal layer thickness was chosen to be less than one unit cell, resulting in separated nanoscale islands that form inclusions with perovskite structure BaTM O3 during film growth. These inclusions grow biaxially textured within the film. Whereas the Ti-doped films show a very strong decrease in the critical temperature Tc and, hence, a strong decrease in the critical current density Jc with increasing TM amount for all temperatures, Hf and Zr doping show an increase in Jc for the smallest amounts of doping. An irreversibility field as high as 10.3?T at 77?K was observed in the case of low Hf content.

Structural and magnetotransport properties of YBa2Cu3O7−δ∕Y2O3 quasimultilayers

A series of quasimultilayers of YBa2Cu3O7−δ(YBCO)∕Y2O3, namely, 70×[YBCO(m)∕Y2O3(n)] (m=40 pulse, and n=2, 5, 10, and 20 pulse) were prepared on single-crystal SrTiO3 using pulsed-laser deposition. X-ray diffraction measurements revealed that both in-plane and out-of-plane textures of YBCO in the present quasimultilayers are as good as in pure YBCO films. Nanoscale Y2O3 precipitates grow epitaxially inside YBCO. With increasing Y2O3 pulse number, Tc decreases slightly, while ΔTc remains less than 1.5K. As well, the YBCO lattice parameter c has an increasing trend with increasing n. This can be attributed to the effect of epitaxial strain induced by lattice mismatch. The flux pinning force density in films with lower Y2O3 content (such as n=2), is improved in large ranges of field and temperature. In contrast, films with high Y2O3 content show enhanced flux pinning only at low temperature, which is understandable from the temperature dependence of irreversibility fields.

Thickness effect of La2Zr2O7 single buffers on metallic substrates using pulsed laser deposition for YBa2Cu3O7−δ-coated conductors

In the present work, the single buffer layers of La2Zr2O7 (LZO) with different thicknesses were prepared using pulsed laser deposition (PLD) on highly textured Ni?5?at.%W tapes for YBa2Cu3O7??-coated conductors. The highly textured LZO layers are obtained on the metallic substrates, making the subsequent epitaxial growth of superconducting YBa2Cu3O7?? layers feasible. The superconducting transition temperature increases as the LZO thickness increases up to 240?nm, achieving a value of Tc of more than 92?K with a narrow transition width less than 1?K. Inductive measurement reveals that the critical current density (Jc) reaches 0.77?MA?cm?2 at 77?K in self-field, showing that the single buffer of LZO is comparable to standard buffer architectures such as CeO2/YSZ/Y2O3 or CeO2/LZO. This implies that the LZO as a single buffer layer is promising for the simplification of the preparation process, leading to cost-effective coated conductors.

Flux pinning of stress-induced magnetic inhomogeneity in the bilayers of YBa2Cu3O7−δ/La0.67Sr0.33MnO3−δ

Elaborately designed bilayers consisting of epitaxial YBa2Cu3O7−δ (YBCO) and La0.67Sr0.33MnO3−δ (LSMO) films were fabricated by pulsed laser deposition with respect to the investigation into magnetic-dependent vortex pinning effect. The improvement in the critical current density and a pronounced upward shift in the superconducting irreversibility line based on magnetotransport measurements are observed when compared to the pure YBCO film, suggesting the enhancement in flux pinning in the studied bilayer. It is believed that the improved flux pinning for YBCO arises from the magnetic inhomogeneity of the underlying LSMO. Magnetization measurements show a nonuniform magnetic state in the LSMO film, most probably being macroscopically phase-separated clusters with ferromagnetic (FM) and anti-FM domains, which can be caused by the epitaxial strain due to the lattice mismatch. Such a magnetic disorder is hardly affected by the external magnetic field, unlike the magnetic disorder induced by the domain structu...

Surface texture and interior residual stress variation induced by thickness of YBa2Cu3O7-δ thin films

YBa2Cu3O7-δ (YBCO) thin films with various thicknesses from 80 nm to 2000 nm are prepared on single crystal SrTiO3 by means of pulsed laser deposition technique. While it is hard for the x-ray diffraction to observe the evolutions in epitaxial orientation and interior lattice structure, the atomic force microscope shows the degraded surface morphologies and coalesced particles which arise from more misoriented grains with increasing thin film thickness. A detailed Raman spectrum investigation reveals that the a-axis grains exist predominately at the top surface of the films as the total thickness of the film increases up to 2000 nm. It is also evident that the Raman peak corresponding to the O2+/O3− mode emerges with the redshift first and then the blueshift as the film thickness increases. These Raman shifts suggest that the tensile stress in the (a, b) plane arising from the lattice mismatch between the epitaxial film and substrate may release gradually with increasing the film thickness, while the addi...

Angular-dependent vortex pinning mechanism in YBa2Cu3O7−δ/YSZ quasi-multilayer

The temperature and magnetic field dependences of anisotropic pinning contributions of YBa2Cu3O7−δ/YSZ quasi-multilayer have been investigated by the measurement of angular-dependent critical current density (Jc). With the isotropic and anisotropic pinning contributions in a wide range of temperature, we identify the possible sources of the pinning centers and classify them into the weak and the strong flux pinning model. Angular-dependent Jc(H,T) measurements have demonstrated that the growth control strategy is very effective in preventing the vortex motion at high fields and high temperatures. It is suggested that at high applied fields, such as 7 T, the pinning contribution of the nanostructured quasi-multilayers is dominated by the anisotropic disorders, while at intermediate-low fields (such as 1 T) the pinning contribution is determined by both isotropic and anisotropic disorders, suggesting the coexistence of isotropic and anisotropic pinning.

Robust high-temperature magnetic pinning induced by proximity in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 hybrids

An elaborately designed bilayer consisting of superconducting YBa2Cu3O7−δ (YBCO) and ferromagnetic La0.67Sr0.33MnO3−δ (LSMO) was prepared on a single crystal LaAlO3 substrate by pulsed laser deposition (PLD), with a view to understanding the mechanism behind the influence of superconductor/ferromagnet proximity on the critical current density, Jc. The present bilayer system shows significant modifications in Jc, as evidenced by the suppressed decay of its temperature dependence, as well as the crossing behavior of the magnetic field dependence of Jc at high temperatures. This indicates that enhanced flux pinning emerges at high temperatures, and it is believed to arise from the special magnetic inhomogeneity, i.e., the ferromagnet/antiferromagnet clusters caused by phase separation due to the epitaxial stress between LSMO and the substrate.

Formation of nanosized BaZrO3 and the magnetotransport properties in YBa2Cu3O7−δ/YSZ quasi-multilayers

A series of quasi-multilayer films of YBCO/YSZ is prepared by means of pulsed laser deposition (PLD). X-ray diffraction measurements reveal that a chemical reaction takes place in YSZ-doped films, leading to a heterogeneous BaZrO3 perovskite phase. The presence of the second Jc peak observed from the angular dependence of Jc at H∥c implies that the c-axis correlated defects increase due to a lattice mismatch induced by YSZ doping. The quasi-multilayers with a low doping content may show an increased flux pinning force density and then enhanced Jc in high magnetic fields, leading to a crossover behaviour in Jc–H curves compared with the pure YBCO thin film. Furthermore, a strong temperature dependence of such a crossover behaviour is observed. As the temperature increases, the artificial tailoring of flux pinning may appear clearer since the mixed vortex state moves towards the region of weak vortex glass and vortex liquid.

Heteroperovskite phase formation and magnetotransport properties of YBa2Cu3O7−x/SrRuO3 quasimultilayers

A series of quasimultilayers consisting of YBa2Cu3O7−δ (YBCO) and SrRuO3 (SRO), namely, p×(YBCO(m)/SRO(n)) (p: repetition periods; m: YBCO pulse number; n: SRO pulse number), is prepared on single crystal SrTiO3 by pulsed laser deposition. X-ray diffraction measurements reveal that a heterogeneous phase of Ba2YRuO6 is formed due to the chemical reaction, while both the in-plane and out-of-plane textures of the present quasimultilayers are as good as in the pure YBCO films. The content of Ba2YRuO6 increases as the pulse number of SRO increases, leading to the decrease in superconducting transition temperature (Tc). There is no obvious difference in Tc between the samples with the different deposition sequence orders of YBCO and SRO. The magnetic field dependence of critical current density (Jc) shows an enhanced flux pinning at relatively high fields for all the present quasimultilayers, which occurs at different ranges of fields depending on the applied temperatures. The Jc of the quasimultilayers can rea...

Proximity and reversal magnetoresistance behaviour in the oxide heterostructure of cuprate and manganite

In this paper, the oxide bilayer and trilayer consisting of superconducting cuprate and antiferromagnetic manganite are prepared using the technique of pulsed-laser deposition. Magnetic hysteresis loops at different temperatures and magnetic fields show distinct magnetic performances due to the proximity and interplay between Meissner currents in the YBCO layer as well as the magnetic fields emerging in the antiferromagnetic manganite layer. A giant positive magnetoresistance (MR) for the trilayers is clearly demonstrated in the region of mixed vortex states near the onset temperature of the superconducting transition. It is believed that the observed positive MR should be associated with the thermally activated flux dissipation, while the spin-dependent interface scattering due to the existing proximity effect may play a more significant role.