Strong anisotropy and weak flux pinning in (BIPB)2SR2CACU2O8 crystals

Ph}sica C 185-189 (1991) 2327-2328 North-Holland

STRONG ANISOTROPY AND WEAKFLUXPINNING IN (BiPB)2SR2CACU208 CRYSTALS Lu ZHANG, J.Z. LIU, M.D. tAN, Y. NAGATA, P. KLAVINS AND R.N. SHELTON University of California, Davis, CA 95616 Magnetic hysteresis and relaxation were m e a s u r e d on high quality single crystals of (BiPb)2Sr2CaCu208 to study the critical state behavior and thermally activated flux creep. The critical current densities show a large anisotropy and weak flux pinning in this material. The relaxation studies indicate that the flux creep is giant and flux motions are parallel to the ab plane in the both HIIc and H l c configurations. Growth of superconducting single crystals of (BiPb)2Sr2CaCu208 was first reported by a self flux method1. The preparation of single crystals (BiPb)2Sr2CaCu208 was optimized and reported elsewhere 2-3. The crystals have typical dimensions of 2xlx0.05 mms and superconducting t r a n s i t i o n s t e m p e r a t u r e s of 92-96K with transition temperature vddths of 2-4K, The h y s t e r e s i s loop and time relaxation measurements were conducted on a commercial SQUID magnetometer4. During magnetic hysteresis measurements, there was a 5-minute waiting time after setting a new field, so t h a t a stabilization of the magnetic field in the system could be obtained and the magnetization inside the sample could reach a relatively stable state. Figure 1 shows a measurement result of the full hysteresis loop of

flux pinning ~s observed in the ab plane. ~07 . . . . . . . . . . . . . . . . . . . . . . . . . . .

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a (BiPb)2Sr2CaCu208 crystal for HIIc at 6K where the data were taken after zero-field-cooling. The critical current densities can be estimated from hysteresis loops using an extended 2dimensional Bean model 5. Figure 2 shows the critical current densities at zero-field cooled (calculated from residual magnetization) as a function of temperature for both HIIc and Hlc. Jab The anisotropy ratio -~c is abou'~ 40 and weaker

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FIGURE 1 A magnetic hysteresis loop of a (BiPb)2Sr2CaCu208 crystal at T=6Kand HIIc

The magnetic relaxation e×perimen~s were performed at zero.field-cooledin a time period of 900-minutes. Fisure 3 shows the typica~ relaxation curves fo:" a (BiPb)2Sr2CaCu208 crysta~ at T=10K and H
(F)21-4534/gu$03.50 © 1991 - Elsevier Science Publishers B.V. AtI fights rcse~~ed.

L. Zhang et al. / Strong anisotropy and weak flux pinning in (BiPb);Sr2CaCu208 crystals

2328

magnetization is logarithmic in time, which can be explained by classical flux creep theory6. tt[llo

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FIGURE 3 Magnetization (ZFC) vs time at T=10K for fields parallel to the c-axis of a (BiPb)2Sr2CaCu208 crystal dM The maximum relaxation rates ~ at T=10K are 100 and 5 (emu/cm 3) for HIIc a n d H ± c , respectively. The large values of the relaxation rates reflect giant flux creep induced by thermal activated flux motion. The relaxation rates are strongly field dependent which indicates t h a t the t h e r m a l activated flux motions are strongly assisted by the driving force. The field dependent magnetization and magnetic relaxation will be discussed elsewhere 3. Relaxation rates show larger values for H[Ic as compared to those for H±c. The thermal activated energy barrier Uo's are estimated as Uo±=0.013ev and Uoll=0.022ev by kBT 1 dM using the formula - ~ o = Bo dlnt for H
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is the initial value of - Uoll

magnetic flux density3. The ratio of-u-~ is about 2. The small difference in Uo indicates the flux creep is the same for the two configurations. For the HIIc case, fluxons always move in the ab plane. For the H±c case, fluxons can move both perpendicular (Jc) and parallel (Jab) tO the ab plane. Due to the large anisotropy of the critical current density (Je<
to flux motion t h a t is m u c h stronger parallel to the plane (JcxB) t h a n in the perpendicular case. Our results explain why thermal activated flux motion perpendicular to the ab plane was not observed in prior studies 7. In conclusion, we m e a s u r e d the m a g n e t i c hysteresis and relaxations on (BiPb)2Sr2CaCu208 s u p e r c o n d u c t i n g crystals for two configurations: applied field parallel to the crystallographic c-axis a n d perpendicular to t h e caxis. The temperature dependence of the critical c u r r e n t density as well as the t e m p e r a t u r e independent of thermal activation energy were obtained at low temperatures and low fields. The suppression of flux motion perpendicular to the ab planes is the result of strong pinning by the insulating layers between the Cu-O planes. T h i s work is s u p p o r t e d by t h e U.S. D e p a r t m e n t of Energy u n d e r contract n u m b e r W-7405-ENG-48 to Lawrence Livermore National L a b o r a t o r y a n d by t h e National Science Foundation under grant n,~.~er DMR-90-21029. REFERENCES i. J.Z. Liu, G.W. Crabtree, L.E. Rehn, Urs Geiser, D.A. Young, W.K° Kwok, P.M. Baldo, J.M. Williams and D.J. Lama Phys. Lett. A 127, 444 (1988). 2. L. Zhang, J.Z. Liu, M.D. Lan, P. Klavins and R.N. Shelton, Bull. Am. Phys. Soc. 36, 667 (1991). 3. L. Zhang, J.Z. Liu, M.D. Lan, Y. Nagata, P. Klavins and R.N. Shelton, (submitted to Phys. Rev. B).

4~ Quantum Designc Inc. San Diego, CA. 5. E.M. Gyory~ R.B. vanDover, K.A. Jackson~ L.F. Schneemyer and J.W. Waszczak~ App±. ~nys° Lett. 55~ 283 (1972).

6. A.M. Campbell and J.E. Events, Adv. Phys. 2_~i~ 199 {1972). 7. B°D° Biggs: M°N~ Kunchur, J.J. Lin, and So!° Poon~ Phys° Rev° B 3_99, 7309 (1989~