Flux creep

Resistivity measurements are also routinely made with an ac four probe technique. The wiring would follow according to Figure 1 and the measuring currents used would be in the range 0.1 - 10 mA with frequencies of 100 Hz (9). For flux creep now known to modify susceptibility and critical current measurements care must taken with ac measurements of resistivity although for the low current densities involved the effect will not likely be observed except very close to Tc (10) or in a magnetic field. 

The power law approximation of the voltage current characteristic for superconductors above Ic has been known for some time (64). Such studies have been made in Y-Ba-Cu-O (65) with results similar to those shown in Figure 16. The value of n in V In has been found to decrease as the magnetic field is increased, and of course becomes ohmic above Hc2. Another representation of the current voltage data is shown in Figure 17 from Enpuku et al. (66), log V vs 1/T for increasing currents (above critical). The expected near straight line arises from the flux creep model of Tinkham for T/Tc 1. 

Enpuku, K., Kisu, T., Sako, R., Yoshida, K., Takeo, M. and Yamafuji, K., Effect of Flux Creep on Current-Voltage Characteristics of Superconducting Y-Ba-Cu-O Thin Films, Jpn. J. Appl. Phys. 28 L991 (1989). 

The basic measurement of flux creep is magnetization versus time (relaxation) in a constant field at fixed temperature (73). The... 

Figure 11 Magnetization relaxation versus log(time) at 10 K in fields applied normal to a 0.7 /un-thick T Ca BaoCujOjQ superconducting film (Tc is 108 K). The solid triangles show relaxation due to flux creep out of the film in zero applied field after cooling in a 10 kOe field and removal of the field. The open triangles represent flux creep into the film in a S00 Oe field applied following zero field cooling. The dashed lines show the fit to Eq. (13).

Figure 12 Ratio of flux creep rate S(T) to the magnetization at 1 second MQ(T) versus temperature. S and MQ are obtained by fitting data such as those in Figure 11 using Eq. (13). The slope of the dashed line corresponds to an effective flux pinning potential U = 83 meV according to a simple thermally activated flux creep model which yields Eq. (14).

For strong type-II superconductors a difference may exist between B 2 obtained from ac-susceptibility data and the thermodynamically relevant upper critical field, 5c2, for which (2.13) and (2.14) were derived originally. Prom extensive work on high-Tc cuprates it is known that B 2 is related to the so-called irreversibility line [196]. The basic idea of a semiquantitative flux-creep theory [197] is that pinned vortices can be activated thermally over an energy barrier Uq resulting in a reduced critical current of the form [196]... 

Direct transport measurements are generally employed on thin films, tapes and coated conductors. They quickly reach their natural limits as the currents go up and/or the cross-section of the conductor increases. They are fast, i.e., can be started immediately after the magnetic field is set, and trace the I-V characteristics, which is of course most valuable if flux creep or thermally assisted flux flow (Kes et al. 1989) prevails, because the electric field is directly determined. On the other hand, must be defined by a criterion e.g., 1 [xV/cm. It will be affected by the presence of stabilising materials, the silver tube or conducting substrates, and thus generally refers to the overall Jc, wfrich can be converted to the critical current density of the superconductor if the shape and the volume fraction of the superconducting material are known. 

Nature of the driving force on flux creep phenomena (with J. Friedel and J. Matricon), AppI. Phvs. Lett.. 2, 199 (1963). 

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