Irreversibility Field in Bi-2212 Thin Films Syunsuke Yamaura, Edmund Soji Otabe, Teruo Matsushita, Tetsuji Uchiya* *ma Abstract_ The irreversibility field along thepc-axisawasred with the result on* * silver-sheathed Bi-2212 supercon- measured for two Bi-2212 thin films with differentdthick-ucting thick tapes. nesses. This result is compared with the measurements for thick tapes. The irreversibility field at the same reduced temperature decreased according as the thickness decreased.II.Experiments This result suggests that flux lines are fairlyTstronglyhcou-e specimens were t* *wo Bi-2212 thin films prepared by pled along their length over the distance longer than the film thickness investigated here even in two-dimensionalasuper-laser ablation method* *. The films were deposited on MgO conductor. This contradicts the prediction ofstheupancakebstrates in an oxygen * *atmosphere of 400 mTorr. The vortex model. thickness of each thin film was abo* *ut 200 nm(Sample1) Keywords_ Bi-2212 thin film, irreversibilityafield,nfilmd 100 nm(Sample2). The* * critical temperature of these thickness, flux creep-flow model films was 72 K and 60 K, respective* *ly. The c-axis was di- rected normal to the flat wide surf* *ace of the film. The I.Introduction DC magnetization in a magnetic fiel* *d parallel to the c-axis Twas known that irreversibility field dependswonathes measured using a SQUID* * magnetometer. The critical I flux pinning strength and the dimension ofcsupercon-urrent density was estim* *ated from the magnetization hys- ductor. In addition, it was shown that the irreversibilityteresis and the irrev* *ersibility field was determined2by the field of a thin film is lower than that of afthickitapeewireld at which Jcwas r* *educed to 1:0 x 106A=m . for Bi-2223 superconductor [1], although the flux pinning strength of a thin film is stronger than that of a tapeIwire.II.Results and Dis* *cussion This result suggests that flux lines are fairlyFstronglyicou-g. 1 shows the cri* *tical current density in Sample1 in pled along their length even in two-dimensionalwsupercon-ide range of temperatu* *re. The critical current density ductors. This is explained by the flux creepdtheoryefromcreases monotonously wi* *th increasing magnetic field and the viewpoint of difference of the flux bundledsize.oThateis,s not show the pea* *k effect. The critical current den- the flux bundle size is limited by the film thicknesssunderiaty in Sample2 was * *similar but lower than that in Sam- magnetic field along the c-axis. This leads topthelsmallepin-1. Fig. 2 shows th* *e temperature dependence of the irre- ning potential in thin films, because the pinningvpotentialersibility field for* * Samples 1 and 2. The temperature was is proportional to the volume of flux bundle.nTherefore,ormalized by the critic* *al temperature because the criti- the flux bundle size strongly influences thecfluxacreeplandtemperature is diffe* *rent between the two samples. The the irreversibility field. irreversibility field in Sample2 is* * fairly lower than that in It was found that the longitudinal elastic correlationSample1 at the same redu* *ced temperature. length of flux lines is much longer than the distanceTbe-his result is compared* * with the irreversibility field tween the CuO2 layers [2], [3]. This indicatesothatffluxsilver-sheathed multil* *ayered Bi-2212 tape wires with lines are fairly strongly coupled along theirtlengthheveneinaverage thickness o* *f superconducting layers of about two-dimensional superconductors and is consistent with the thickness dependence of the irreversibility field. This speculation is quite different from the prediction of the pancake vortex model [4]. It is necessary to investigate directly the thickness dependence of the irreversibility field for the most two-dimensional Bi-2212 in order to ascertain the above speculation. In this study, the irreversibility field of two Bi-2212 thin films with different thicknesses is mea- sured. This result is compared with the measurements for silver-sheathed thick tapes [5]. The observed result is com- S. Yamaura is with Kyushu Institute of Technology, 680-4, Kawazu, Iizuka 820-8502 Japan (Telephone: +81-948-29-7683, e- mail: yamaura@aquarius10.cse.kyutech.ac.jp) E. S. Otabe is with Kyushu Institute of Technology, 680-4, Kawazu, Iizuka 820-8502 Japan (Telephone: +81-948-29-7683, e- mail: otabe@cse.kyutech.ac.jp) T. Matsushita is with Kyushu Institute of Technology, 680-4, Kawazu, Iizuka 820-8502 Japan (Telephone: +81-948-29-7663, e- mail: matusita@cse.kyutech.ac.jp) T. Uchiyama is with Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro 152-8551 Japan (Telephone: +81-3-5734-2454,Fig. 1. Critical cu* *rrent density vs magnetic field in Sample1. e-mail: te2@ap.titech.ac.jp) Fig. 2. Temperature dependence of irreversibilityFfield.ig. 3. Thickness de* *pendence of irreversibility field at T=Tc= 0:42. Each line shows the theoretical p* *rediction. 5 ~m(Tape1) and 10 ~m(Tape2) [5]. The critical tempera- ture of Tapes 1 and 2 was 86.0 K and 90.0 K,irespectively.s numerically analyze* *d for each sample. The results of These results are also plotted in Fig. 2. Itnisufoundmthatetherical analysis ar* *e shown by lines in Fig. 3. The differ- irreversibility field in Tape1 is highest andethenirreversibil-ce of each line * *comes from different pinning parameters. ity field decreases monotonically with the decreasingTthick-he irreversibility * *field generally increases with increasing ness of superconductor. d and is saturated when d is longer* * than L. It was found According to the collective flux creep theorytthehpinningat d is shorter than * *L in Samples 1 and 2 and longer than potential, U0, is described in terms of the virtualLcriticalin Tapes 1 and 2. current density, Jc0, in the creep-free case.TIthiseknownathatbove results are * *contradictory to the prediction U0depends on the flux pinning strength and theovolumefofthe pancake vortex mode* *l. That is, the flux lines are the flux bundle. The longitudinal flux bundlecsize,oL,nwassidered to be fairly * *strongly coupled along their length even in the most two-dimensional Bi* *-2212 superconductors. ` '1=2 This is supported by the measuremen* *t of longitudinal cor- L = __Baf_2ss~; (1) relation length of the order of 10 * *~m [5]. 0Jc0 with af denoting a flux line spacing. If the thickness of IV.Summary superconductor, d, is thinner than L, we haveT[6]he irreversibility field was m* *easured for two Bi-2212 2kBJc0d thin films with different thickness* *es. The observed result U0= 4:23g_____; (2) was compared with the result on sil* *ver-sheathed Bi-2212 2ssB1=2 superconducting thick tapes. The fo* *llowing results are ob- where g2 is a number of flux lines in the fluxtbundle.aOnined.1.The irreversibi* *lity field of thicker specimen was gener- the other hand, if d is thicker than L, we haveally higher. This agrees with th* *e theoretical prediction 2kBJ1=2 of the flux creep-flow model. U0= 0:85g____c0_: (3) 2.This shows that the flux lines m* *agnetically strongly (2ss)3=2B1=4 coupled along their length even * *in two-dimensional su- It is assumed that the dependences of Jc0on temperatureperconductors and do not* * behave as predicted by the and magnetic field are expressed as pancake vortex model. " ` '2#m ~ ~ffi References Jc0= A 1 - T_T bfl-11 - B__; (4) [1]N. Ihara, T. Matsushita Physica * *C 257 (1996) 223. c Bc2 [2]M. Kiuchi, H. Yamato, T. Matsush* *ita Physica C 269 (1996) 242. [3]T. Matsushita, M. Kiuchi, H. Yam* *ato, T. Hirano, N. Koshizuka where A, m, fl and ffi are the pinning parameters.Physica[C4277](1997)J185.. R.* * Clem Phys Rev B 43 (1991) 7837. Fig. 3 shows the thickness dependence of the[irreversibil-5]M. Tagomori, M. Ki* *uchi, T. Matsushita, T. Hasegawa Advances ity field at T=Tc= 0:42, where the points are[experimentalin6Superconductivity]* *XT(Springer-Verlag,.Tokyo)M(1998)a529.tsushita, T. Tohdoh, N. Ihara Physica C* * @ values. The thickness dependence of the irreversibility field