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type:Journal Article, 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., source:http://link.aps.org/abstract/PRB/v72/e014464, source:http://www.aps.org

type:Journal Article, The E–J characteristics were measured for a YBCO-coated conductor by using the four probe method and the relaxation method of DC magnetization. The result on the latter measurement at extremely low electric fields was theoretically analyzed based on the flux creep-flow model. From the result on n-value the flux lines in YBCO are considered to be in the glass state even at high fields. The apparent pinning potential U0* estimated from the relaxation of DC magnetization is much larger than that of Bi-2223. The present theoretical analysis clarifies a much stronger flux pinning and a higher dimensionality of YBCO in comparison with Bi-2223., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, Size dependencies of the peak effect and irreversibility field in superconducting Sm-123 powder specimens were investigated. The peak effect was found to disappear in the two-dimensional pinning regime in which powder size is smaller than pinning correlation length. This shows that the peak effect does not originate directly from elementary pinning mechanisms, but from a cooperative phenomenon of flux lines, as proposed by the pinning-induced disorder transition model. In addition, when powder size is smaller than virtual pinning correlation length in the ideal creep-free case, irreversibility field decreases with powder size. This supports the hypothesis that longitudinal flux bundle size is given by this length, as predicted by flux creep theory. These findings indicate that pinning correlation length is a general parameter that determines various properties associated with flux pinning phenomena., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The DC susceptibility of oxide superconducting specimens in the field-cooled process has been experimentally found to depend not only on the applied DC magnetic field but also on the size of the specimens. The DC susceptibility is calculated using the critical state model in which the diamagnetism and the flux-pinning effect of superconductors are taken into account. It is shown that the saturated value of the DC susceptibility at sufficiently low temperatures, i.e., the so-called Meissner fraction, decreases with increasing DC field and/or increasing specimen size., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The third harmonic AC susceptibility is theoretically investigated by numerically solving Campbell's model in which the effect of the reversible fluxoid motion is taken into account. According to the calculated result of the AC field amplitude dependence of real and imaginary parts, their peak values decrease and the peak AC field amplitude shifts to higher values when the fluxoid motion changes from irreversible to reversible. This is quite different from the prediction of Bean's critical state model in which the peak value and the peak AC magnetic field are predicted to be constant. Since the reversible phenomenon is expected to be remarkable in superconductors with small sizes, the third harmonic AC susceptibility measurement was carried out for a superconducting powder of alkali metal doped fullerene. Experimental results can be qualitatively explained by the theoretical results, source:DOI: 10.1109/77.402822

type:Journal Article, The critical current density was measured at low temperatures for Bi-2212 single crystals with small anisotropy prepared by oxygen doping and by addition of Pb. The anisotropy parameters of these specimens were about 66. The Pb-doped specimen showed the peak effect up to T/Tc = 0.7. The critical current density at high fields of these specimens was significantly improved as well as the irreversibility field than specimens with larger anisotropy., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, An AC current source of 1000 A class for AC transport measurements was designed and fabricated with using an oxide superconducting current transformer. Two cryocoolers were installed for cooling the transformer and a sample holder separately. A parallel conductor composed of six tapes was wound for a secondary winding and a transposition was performed to make the current distribution uniform in the parallel conductor. Temperatures of the transformer and the sample holder were controlled in the range of 35–50 and 25–77 K, respectively. The peak current in the secondary winding was over 1000 A in the frequency range of 1–75 Hz, when the primary peak current was 14 A., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The critical current properties of a superconducting Bi-2223 tape produced by the over pressure processing at the final heat treatment were investigated. The critical current density increased more than the factor of densification of the superconducting region. In addition, the irreversibility field also increased. This means that the quality of flux pinning is improved by the new process. From the numerical calculation based on the flux creep and flow model, it is speculated that the fraction of weakly pinning regions decreases due to the improved connectivity of grains., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The critical current density in Bi-2212 superconductors with various anisotropies irradiated by heavy ions was investigated in the medium temperature region to understand the effects of defect size and the anisotropy of the superconductor. It was found that the critical current density and the irreversibility field were larger for the specimen with larger defect and/or with smaller anisotropy. Introduction of stronger pinning centers and the optimization of the doping condition to improve the dimensionality are desired for further improvement of the critical current properties, source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, 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., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The AC transport current loss of a meander-shaped fault current limiting device of quench melt growth (QMG) Y-based superconductor was numerically analyzed by finite element method. The magnetic field and the field-angle dependencies of the critical current density Jc were taken into account as well as a shape of a rectangular cross-section. The numerical result shows a good agreement with the experimental result. The reason why Norris’s formula fits to the experimental result seems to be because that the effect of the magnetic field dependence of Jc on the AC current loss is compensated by the geometrical effect of the superconductor and the field-angle dependence of Jc., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The magnetic field angle dependence of Jc was measured by four probe method and DC magnetization method. The latter measurement was used for the specimen before and after bending to break the intergrain current in order to investigate only the intragrain current. It was found that the intragrain critical current density could be increased by a factor 2.7 in a parallel magnetic field when the c-axis misorientation could be improved. From the field angle dependence of the intragrain critical current density, the anisotropy parameter was estimated as 20.4., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The E–J characteristics were measured for a YBCO coated-conductor by using AC inductive method of third-harmonic voltage. The results are compared with the measurements obtained by the four probe method and DC relaxation method of magnetization. The results by three kinds of measurements meet approximately on a single curve and E–J curve in a wide range of electric field is obtained. Hence, the third-harmonic voltage method is useful for a continuous measurement of Jc property of long coated tape. The obtained E–J characteristics are argued, source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, When an ac magnetic field of a suitable magnitude is applied locally normal to a wide surface of a superconducting thick film, a third harmonic voltage V3 is induced in a pick-up coil. It is predicted by the critical state model that V3 is proportional to the driving current amplitude when I0 is smaller than the threshold current Ith at which the shielding current in the film reaches the opposite side of the film. A numerical analysis using finite element method (FEM) is carried out for the V3–I0 characteristics for the film. It is found that V3 is proportional to only when I0 is smaller enough than Ith, and V3 takes a smaller value than predicted by the theory near I0 = Ith. The deviation between the theoretical prediction and the result of FEM is found to be attributed to the different distribution of the shielding current in the film., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, The AC-current loss in a quench melt growth (QMG)-processed Y-123 bulk superconductor was found to fit well with the Norris’s formula for a superconductor with an ellipsoidal cross-section based on the Bean’s model, in spite of a rectangular cross-section and the magnetic field dependence of critical current density. The reason for this coincidence is generally argued by a theoretical analysis with respect to the effects of the magnetic field dependence of Jc, the geometry of effect of superconductor and the effect of n-value on the AC loss. In this paper, finite element method is used for the numerical analysis of the AC-current loss of the QMG bulk sample with taking into account of the latter two effects. It is found that when the aspect ratio of rectangular superconductor increases, the loss becomes small. The AC loss takes on a larger value when the n-value decreases, suggesting a large effect of flux creep., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, Dependence of the irreversibility field (Bi) on the electric field criterion (Ec) was measured for a Bi-2223 silver-sheathed tape wire using various measuring methods. It was found that Bi increased monotonically with the electric field criterion. A good agreement was obtained for Bi between experiments and the theoretical analysis using the flux creep-flow model. This implies that the resistive property of superconducting wires can be well described by the mechanism of flux creep and flow. The vortex glass–liquid transition field (Bg) was also determined from E–J curves which measured in the low and high electric field regions. It was found that Bg also depends on the ranges of electric field. This is incompatible with the prediction of the vortex glass–liquid transition theory., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, 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., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, When an AC magnetic field of a suitable magnitude is applied locally to a wide surface of a superconducting film, a third harmonic voltage is induced in a pick-up coil. This measurement is used for an evaluation of the critical current density of the film. Mawatari et al. analyzed the relationship between the third harmonic voltage amplitude (V3) and the driving current amplitude for the field (I0) by using the critical state model and a simple assumption that the magnetic field is locally parallel to a flat surface. In this study, this relationship is numerically analyzed by finite element method (FEM) for a condition similar to the experiment by Mawatari et al. This result is compared with the above theoretical and experimental results. Some deviations from theory are found. However, the scaling of V3–I0 curves holds and I0 at which V3 starts to appear is proportional to the critical current density., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, From an analysis of E–J curves with the aid of the flux creep-flow model, it is found that most of the electric field is caused by the flux creep even in the range of electric field in usual resistive measurement. This result is consistent with the fact that the mechanism of flux creep generally explains various phenomena such as the critical current density, the irreversibility field and so on. On the other hand, it is much easier to use the parameters used in the percolation flow model to describe the E–J characteristics phenomenologically. The two models are compared and it is found that the two models are consistent. This gives a theoretical proof for the parameters of the percolation flow model., source:http://www.sciencedirect.com/science/journal/09214534

type:Journal Article, 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., source:http://www.sciencedirect.com/science/journal/09214534