Critical current measurements

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. 

Critical current measurements have been made with a variety of techniques. The indirect technique, that of obtaining the critical current from the magnetization response is discussed in Chapter 18. Direct transport measurements, using attached current and voltage leads, and indirect measurements requiring macroscopic current circulation will be discussed. Critical currents are desired as a function of both temperature and applied magnetic field since a variety of theories discuss the functional relationship. And applications may require either or both of these data. 

Nb thin films have been deposited on PS substrates, with pores diameter and interpore distance 0 = 10 nm and A = 40 nm, respectively. The samples show well developed superconducting properties. Critical temperature and critical current measurements were performed on unpatterned samples. Future work will deal with critical current density measurements carried out on patterned samples. 

In Fig. 1 the transition temperature of the trilayers as a function of dcuNi is reported. A nonmonotonic dependence of Tc is observed. A minimum, in fact, occurs at dcuNi=4 nm, followed by a maximum at dcUNi=5 nm, until Tc decreases again. This behavior was theoretically predicted in [8] as well as experimentally observed in other superconductor/conventional ferromagnet based trilayers [9,10]. The existence of the Ji-phase was also proved in Nb/Cui xNix/Nb junction through critical current measurements [4,11]. In these cases the alloys were very diluted (x=48-52 %, TCurie 20-30 K) and, for this reason, the 0-rc crossover occurred at thicknesses of the order of dCUNi 20 nm. To check if the presence of the Ji-phase is detectable also by a change in the Nb layers coupling, the critical magnetic fields were measured. 

Critical Current Measurements. When the generator is stopped, the current remains persistent if the critical current of the circuit has not been exceeded otherwise it decays until the critical value is reached. In this way it is possible to determine quite large critical currents with reasonable accuracy. A strip of soldering tin alloy of 1.4 mm thickness and 6 mm width carried 305 amp. 

After the initial tensile data had been evaluated, additional samples were reacted for Ic measurements. Reaction times at 700°C were chosen to correspond to the regions of Figs. 7 through 10 where the ductility showed a sharp increase. Critical currents measured at 10 T and 4.2 K are listed in Table I. Because of the short sample lengths ( 67 mm), the accuracy of these measurements at thousands of amperes is necessarily limited. The values obtained, however, are reasonably consistent with each other and with the critical core current densities reported earlier [ ]. Tensile tests on samples from this series are summarized in Table I and Figs. 7 through 10. 

Figure 30 shows the Josephson critical current measured under external fields along different directions. With the field perpendicular to the twin boundary (j> 90° in the figure), the plot of the critical current exhibited the same Fraunhofer pattern as observed in an ordinary junction, except for a lower peak current. With the field parallel to the boundary = 0), a dip rather than a peak appeared at zero field B = 0), and the maximum current occurred at a field value corresponding to half-integer quantum flux, These observations indicate that the flux cancels the phase difference between the two domains and causes a current flow in the same direction. These observations indicate that Y123 favors d-wave symmetry with some s-wave admixture. 

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