Flux flow and critical currents in an amorphous superconductor Zr85Si15

IA 6

Physical 0 7B (1981} 465-466 North-Holland Publishing Company

FLUX FLOW AND CRITICAL CURRENTS IN AN AMORPHOUS SUPERCONDUCTOR Zr85Si15

Naoki Toyota, Tetsuo Fukase, Akihisa Inoue, Yoshimi Takahashi, Tsuyoshi Masumoto

The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan, 980

Studies have been performed on the resistive states in the superconducting mixed state of the amorphous binary alloy Zr85SiI5(Tc=2.71K) prepared by the improved liquid quenching technique. Observed critical currents are extremely low so that the flux flow resistivity can be well defined in the whole range of the magnetic field except just below Hc2 where a sharp peak ~ffect appears. We obtained the GL parameter <-130 and GL coherence lengths ~*(0)=70 A that can be estimated from H~2(O), the extrapolated value using the Kim's law. It is the amorphous state that brings about a high < and extremely "soft" type II superconductivity.

I.

INTRODUCTION

Since the discovery of amorphous superconductors prepared by liquid quenching technique,(1) there have been considerable interests in their superconducting properties. Recently Masumoto et al. succeeded in obtaining continuous amorphous ribbons by improving the techniques.(2) A homogeneous amorphous material can be expected to be one of the best objects in studying the high < (Ginzburg Landau parameter) superconductivity in dirty limit with extremely weak pinning force. To date, however, there have been few reports on the resistive states in the superconducting mixed state of amorphous materials.(3) We report studies of flux flow phenomena and critical currents of a binary amorphous alloy Zr85S15. 2.

TECHNIQUES AND RESULTS

The detailed techniques for preparing specimens will be described elsewhere.(4) The electrical resistance was measured by the four probes method and the specimen of a ribbon form (Ix 0.03x15 n~n3) was immersed directly into a He bath. The temperature detection was done by the calibrated Ge thermometer and temperature was stabilized within i0 mK over 2 hours by the manostat.(5) The magnetic field was applied transversely to the current through a specimen. In Figure 1 are shown the transition curves from superconducting mixed state to normal state at 1.51 K under currents ranging 0.3 to 300 A/cm 2. Resistive states are seen in the wide range of fields, following the sharp dips (cases, 3-6) as the field approaches Hc2. On the contrary, there appears resistance over the whole field under the high currents (cases, I and 2). Figure 2 illustrated the current voltage characteristic under various fields at the same temperature with Figure i. The slope in the linear regime corresponds to the flux flow resistivity and the critical current is defined as the threshold current at which non-zero vol-

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rage (>I ~V) is first detected. As is shown ( cases 3 and 4) any linear regime is not observed

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at the field values that give the resistance dip in Figure i. We interprete it as a sharp "peak effect" which is often observed in many other superconductors.(6)

465

466

Figure 3 shows the critical current density as a function of the field at several temperatures. The arrow indicates the upper critical field defined from the flow data.(Figure 4) Jc decreases rapidly in the low field range and a sharp peak appears below Hc2 , which corresponds to the above mentioned peak effect. It must be strongly stressed that values of in our specimen are extremely small (i A/cm2J tc H/Hc2=0.5~, compared with crystalline high K 10 Amorphous

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DISCUSSIONS

The values of Hc2*(O) and pn(~3xl0-4~'cm) permit one to analyze the foundamental parameters such as K and ~*(0)(GL coherence length). Using the Gor'kov-Goodman relation in the dirty limit, we get K=I30 and ~*(0)=70 A. Aside from the physical origin of a peak effect, the existance of the extremely low critical current in our specimen, can be caused by the fact that atomic

configurations in an amorphous system is strongIx distorted on the scale much smaller than ~x(O) so that the pinning center (<70 ~) can not be effective to pin the flux. It suggests that the quality in the amorphous state on the microscopic scale ~*(0) is sensitively reflected to the flux pinning effect. It is highly desirable to clarify what kinds of the structural defects can cause the finite values of Jc measured in our amorphous system. It is concluded from our results that the amorphous state brings about a high K and extremely "soft" type II superconductivity. REFERENCES i) W. L. Johnson, S. J. Poon and P. Duwez, Phys. Rev. BII 150 (1975). 2) T. Masumoto, A. Inoue, S. Sakai, H. Kimura and A. Hoshi, Trans. JIM 21 115 (1980). 3) J. W. Ekin, Phys. Rev. BI2 2676 (1975) and W. L. Johnson, S. J. Poon, J. Durand and P. Duwez, Phys. Rev. BI8 206 (1978). 4) A. Inoue et al., to be submitted to Proc. Rapidly Quenched Metals 4th, Sendai, 1981. 5) K. Noto, H. G. Wener, R. P. Huebener, Cryogenics Nov. 626 (1978). 6) A. M. Campbell and J. E. Evetts, Critical Currents in Superconductivity, (Taylor and Francis Ltd, London 1972). 7) For example, W. L. Johnson, Rapidly Quenched Metals 3rd 2 1 (1978). 8) Y. B. Kim and R. D. Stephen, Superconductivity, ed. R. D. Parks (Mercel Dekker Inc, 1969) ch. 19