Weak Intergranular Coupling Research Articles

Superconducting properties and intergranular coupling of high-strength Sr1-xKxFe2As2/Ag/Nb/Cu composite conductors

The mechanical properties of composite metal sheaths have large difference, resulting in the easy breakage of superconducting wires. Here, a roll-die drawing method is proposed to fabricate 19-filamentary Sr1-xKxFe2As2/Ag/Nb/Cu composite tapes with long length and uniform shape. It is found that there is no reaction layer between Ag matrix and superconducting core; Nb sheath provides high mechanical strength and prevents the diffusion of K element; and outer Cu sheath has good mechanical ductility. Consequently, the magnetic critical current density Jc of multi-core Sr1-xKxFe2As2 tape reaches up to 4.7 × 104 A/cm2 at 5 K and 5 T. Meanwhile, the magnetic Jc still remains 1.1 × 104 A/cm2 even at 20 K and 5 T. These remarkable magnetic Jc values are probably attributed to both high mass density of superconducting core and high magnetic onset Tc (∼35.2 K). While, the transport Jc of multi-core tape is greatly decreased into about 2.0 × 103 A/cm2 at 4.2 K and 10 T. Besides the Tp1 peak associated with the intragranular critical current, an additional broad Tp2 peak in the imaginary χ''(T) parts of AC susceptibility indicates the weak intergranular coupling in this tape. These results can provide a prime reference for the structure design and coupling performance of multi-core pnictide conductors.

Magnetization reversal in Co/Pt multilayer with weak intergranular coupling

A [Co 0.4 nm/Pt 1 nm]8 multilayer with weak intergranular coupling was fabricated. The magnetization states after applying various negative fields along the easy axis were observed using a magnetic force microscope to reveal magnetization reversal behavior. Measurement results showed the magnetization reversal area separated into many small spots consisting of one or more grains. These spots reversed independently and the reversed spots did not expand at the boundary but remained stable while the negative magnetic field increased. A coherent rotation was further deduced by calculating the thermal activation volume by a dynamic coercivity method.

Perpendicular magnetic recording thin film media using Co/Pd superlattice on ultrathin indium–tin–oxide seed layers

Co/Pd superlattices (13 bilayers) have been fabricated on ultrathin (2 nm) indium–tin–oxide (ITO) seed layers for use as perpendicular magnetic recording media. The ITO seed layers have significantly increased the coercivity from 1 to 5 kOe as well as the crystalline orientation texture. It was also found that excellent magnetic properties could be obtained by using Ar instead of Kr as sputtering gas. The sheared hysteresis loop and high squareness suggest weak intergranular coupling and adequate perpendicular magnetic anisotropy which make Co/Pd supelattice a promising candidate for ultrahigh recording densities with thermal stability.

Critical-state scaling and weak links in Ag-sheathed Bi2Sr2Ca2Cu3Oz.

Transport and magnetization measurements are compared for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{z}}$/Ag tapes with critical current densities approaching ${\mathit{J}}_{\mathit{c}}$=${10}^{4}$ A/${\mathrm{cm}}^{2}$ at 77 K, and at zero field. Upon subdivision of the sample, the magnetic hysteresis is found to scale approximately with the sample dimension, indicating that persistent currents flow throughout the superconducting core. The ${\mathit{J}}_{\mathit{c}}$ determined using the critical-state analysis is a factor of \ensuremath{\sim}2--3 higher than that obtained in transport. A weak magnetic-field dependence of ${\mathit{J}}_{\mathit{c}}$ is demonstrated at low temperatures, and yet, a comparison with data for powder samples indicates that the transport ${\mathit{J}}_{\mathit{c}}$ is limited by weak intergranular coupling. At low temperature, the transport ${\mathit{J}}_{\mathit{c}}$ is 2 orders of magnitude below the intragranular ${\mathit{J}}_{\mathit{c}}$. At high temperatures, weak intragranular flux pinning appears to be the dominant limiting mechanism. The degree to which transport properties might be enhanced through the introduction of pinning centers and better intergranular coupling is explored.

Transverse field penetration paradox in thin films and magnetic properties of Nd 1.85Ce 0.15CuO 4− x epitaxial films

Peculiarities of magnetic measurements on thin films in a transverse field are discussed. The shielding current distribution is calculated and a well-known few orders discrepancy between calculated and measured penetration field H ∗ is believed to be cleared up. The results for epitaxial (001) and (110) electron-doped HTSC Nd 1.85Ce 0.15CuO 4− x films are presented. The critical current of 0.84 × 10 6 A/cm 2 in a (100) film at 4.2 K from remanent magnetization is derived which indicates a homogeneous behaviour and strong pinning. On the contrary poor shielding and low magnetization in (110) shows its granularity with a weak intergranular coupling.

Resistivity of Sintered YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>x</i></sub> at Large Current Density

Resistivity of a sintered YBa2Cu3O7-x oxide superconductor was measured at large current densities (j=-100A/cm2) above liquid nitrogen temperature to clarify the poor critical current property of a sintered specimen. Three steps of resistivity transition were observed as the temperature lowered; (1) A sharp drop was found at T=93K, which corresponds to the superconducting transition of the material. (2) A broad shoulder appeared at T=92K to 82K. A gradual decrease in resistivity may be due to the weak intergranular coupling at grain boundaries. (3) The resitivity became almost constant and a finite value was detected even at 77K, suggesting that extremely weakly-coupled grain boundaries are present in a sintered specimen.