Finite-size effect on Néel temperature in antiferromagnetic nanoparticles

Muon spin relaxation/rotation (µSR) and magnetic susceptibility measurements were carried out on antiferromagnetic nanoparticles of CuO. Nanoparticles with center size of around 5 nm were prepared by ball-milling from single crystals of CuO and investigated using µSR measurements. In the ~5 nm assembly, the TN was reduced drastically to ~30 K, compared with the bulk TN=229 K. A similar effect was observed in a system of 2 to 3 nm diameter nanorods, which was synthesized by a direct solution reaction method, where TN was suppressed further to 13 K. The present work reports direct evidence of a dramatic finite-size effect on the magnetic transition temperature in antiferromagnetic systems

Evaluation of Critical Current in Junction of Superconductors with Crack Using Finite Element Method

In the present study, in order to clarify the electrical and mechanical properties of the junction of superconducting wires, a junction was numerically simulated using the finite element method, and the electrical and mechanical properties were evaluated. We used a 1 µm-thick YBa2Cu3O7−δ superconducting coated conductor to act as a superconducting wire and joined two films together for current density simulation. The magnetic field dependence of the critical current density is based on the experimental results for YBa2Cu3O7−δ. We introduced a crack in the superconducting junction and confirm that the critical current is drastically reduced by increasing the width of crack. We then attempted to reinforce the crack using solder and superconducting film. The effectiveness of reinforcement of Ic was clarified using very thin solder or a silver diffusion junction

Flux-pinning property in melt-processed Sm-123 superconductor

The flux-pinning property in a melt-processed Sm-123 bulk superconductor was investigated and the observed irreversibility field was compared with the theoretical model of flux creep and flow assuming that the usual condensation energy interaction by 211 particles is dominant in flux pinning. The good agreement suggests that this hypothesis is valid. The pinning mechanism around the peak effect is discussed based on the results on scaling of the pinning force density

Magnetic characterization of superconducting MgB2

The critical current density Jc of pressed polycrystalline MgB2 specimen was measured using a DC magnetization and Campbell’s method. It was found that the two Jcs are coincident and this result confirms that the intergrain Jc is not deteriorated by grain boundaries. The upper critical field, lower critical field, thermodynamic critical field and Ginzburg–Landau parameter were estimated to characterize the magnetic properties. Observed Jc and irreversibility field could be approximately explained by the flux creep-flow model. To foresee the potential of application of this material, Jc was calculated when strong pins could be introduced

Enhancement of critical current density and flux pinning in Bi-2212 thick films due to MgO addition

In order to substitute cheaper sheath materials for Ag, Bi-2212 superconducting thick films grown on oxidized Ni substrates were prepared by using a normal partial melt process. 0-5 vol% of fine MgO particles were doped in Bi-2212 phase during the fabrication for the purpose of enhancement of the critical current density (J/sub c/) in Bi-2212. The samples were analyzed with the assistances of X-ray diffractometer (XRD) and electron probe microanalyzer (EPMA). The critical temperature and J/sub c/ were measured by using the conventional resistive method (4-probe method). An apparent improvement in J/sub c/ characteristic was observed in the samples with fine MgO particles doped. The J/sub c/ value in the 5 vol% MgO doped sample reached to the level comparable with that in other Ag-sheathed samples. Furthermore, the irreversibility field was confirmed to be largely enhanced by the addition of MgO particles. The pin parameters derived from the scaling behavior of pinning force density turned out to be similar to those predicted in the case of normal precipitate flux pinning, indicating that MgO particles in Bi-2212 act as effective pinning centers

Standard measurement method of irreversibility field of bulk RE-123 superconductors

We propose a standard method of measurement of the irreversibility field parallel to the c-axis for bulk oxide superconductors at 77.3 K. It is proposed that commercial magnetometers, such as SQUID magnetometers or VSMs (vibrating sample magnetometers), are used for the measurement. Since the irreversibility field depends appreciably on the electric field criterion, the size of the specimen and the time to starting the measurement after setting the external magnetic field are prescribed to fix the electric field criterion. The number of measurement points is proposed as 5–10 within the field range ± 5% around the irreversibility field. It is confirmed that the irreversibility fields of two (Nd, Eu, Gd)-123 (NEG-123) specimens were successfully measured with the proposed standard measurement method. The target COV (coefficient of variation) is 5%

Flux pinning property and peak effect in Y-123 bulk superconductor with changed oxygen content

It is known that the peak effect in melt-processed Y-123 bulk superconductors is caused by the disorder transition of flux lines pinned by oxygen deficient regions with lower Tc. In order to study the effect of dimensionality of superconductor on the peak effect, a change in the pinning characteristics by heat treatment at 550 °C was measured. Jc decreased and the peak effect became less pronounced. At the same time, the peak field and the dip field reduced. This shows that the disorder transition of flux lines took place much easier than before the heat treatment. It is speculated that the pinning energy and the transverse flux bundle size reduced due to a change of specimens to more two-dimensional underdoped state