ISS2000, TOKYO, OCT. 14-16, 2000, VPP-35 * * 1 Flux Pinning and Peak Effect in Y-123 Superconductor Daisuke Yoshimi, Minoru Migita, Edmund Soji Otabe, Teruo Matsushita Abstract_ investigated by measuring the chang* *e in the pinning prop- It is known that the peak effect in Y-123 superconduc-erty by addition of 211 * *phase particles in melt-processed tors is caused by oxygen deficient regions with lower Tc. In order to study the flux pinning mechanism ofYthese-defects,123 specimens, simil* *arly to the case of Nd-123 super- the effect of addition of 211 particles on thecpeakoeffectnwasductor. The resul* *ts on the critical current density and measured. It was found that the addition of 211tphasehde-e irreversibility fiel* *d were compared with the theoretical teriorated the pronounced peak effect at mediumpmagneticredictions of the flux * *creep-flow model. fields. This shows that the pinning mechanism of the oxygen deficient regions is different from the condensation energy interaction known for 211 particles. The results on the crit-II.Experimental ical current density and the irreversibility field were com- pared with the theoretical predictions of theSfluxpcreep-flowecimens were Y-123* * superconductor prepared by the model. melt process with different conditi* *ons of additions of 211 Keywords_ Peak effect, flux pinning mechanism,pmelt-articles and platinum to m* *ake the 211 particles fine. processed Y-123, flux creep-flow These conditions are listed in Tabl* *e 1. The size of four PACS 74.50.+r, 74.60.Jg, 74.62.Dh, 74.72.Bk, 74.80.Bj, * * 3 85.25.Dq specimens was about 3.14 x 2.09 x 0* *.82 mm . The c-axis was directed along the long axis of* * the specimen. The criti- I.Introduction calitemperaturenwasaaboutm90aK~91gK* *.nTheemagnetizationtic field along the c-axis was measured using a The peak effect of the critical current density,SJc,QisUob-ID magnetometer. Th* *e critical current density was es- served in oxide superconductors at certain regionstofitem-mated from the measur* *ed hysteresis of the magnetization perature and magnetic field. It is well knownathatnmelt-d the irreversibility f* *ield was determined by the field at processed Y-123 bulk superconductors have a broadwpeakhich Jcwas reduced to 1.0* * x105 A/m2. in Jc(B) at medium fields in the medium range of temper- ature. It has been revealed that oxygen deficient regions TABLE I with lower Tc are relevant to the peak effect, while 211Conditions of specimens. phase particles are not. As for the mechanism of these pin- ning centers, the attractive field-induced pinning_from_the____________________* *______specimenaddition2of11 particle condensation energy interaction and the repulsive pinning______________________* *_______211Ptsiz|e(~m)volume | from the kinetic energy interaction under the_proximity|ef-____________________* *______00nono~|1|010 Vo|l% fect have been proposed. ___________________________________* *______0125nw|t||%o~1|025 Vol% Mochida et al. [1] found that, while Jcincreased_in_the_______________________* *______10noye|s~12~3|V|ol%| low and high field regions by addition of Nd-422_phase_par-____________________* *______1125yw|t|%|es|~125 Vol% ticles to Nd-123 bulk superconductors, it decreased_and_the____________________* *______ | | | | peak effect reduced in the medium field region. In these su- perconductors the relevant pinning centers are considered as substituted regions where a Nd atom sits on a Ba siteIII.Results and Discuss* *ion and/or oxygen deficient regions. It is speculated that the pinning mechanism of these lower Tcregions isFnotithegat-. 1 shows the magnetic* * field dependence of Jc at tractive pinning centers but repulsive pinning7centers7[2]..3 K. The peak effec* *t was observed for specimen 00. The origin of the repulsive pinning is the kineticWenergyhen 211 phase particle* *s are added as in specimen 01, interaction under the proximity effect. In thisJmodelctheincreased in the low a* *nd high field regions, while it reduction of the peak effect is ascribed to thedinterferenceecreased and the pe* *ak effect disappeared at a medium between the repulsive pinning of the lower Tcfregionsiandeld. The disappearance* * of the peak effect is speculated the attractive pinning of Nd-422 phase particles.to be caused by an interferenc* *e between two different pin- To confirm this mechanism, in this study, thenelemen-ing mechanisms, because i* *f the pinning mechanism of the tary pinning mechanism relevant to the peak effectowasxygencdeficientrregionsei* *sasimilarlysattractive,eJcmust.in-Since 211 phase particles act as attractive* * @ D. Yoshimi, M. Migita and E. S. Otabe are withcKyushueInstitutenters due to th* *e condensation energy interaction, oxygen of Technology, 680-4, Kawazu, Iizuka 820-8502dJapane(Telephone:ficient regions * *are considered to act as repulsive pinning +81-948-29-7683, e-mail: yoshimi@aquarius10.cse.kyutech.ac.jp).centers. The kin* *etic energy interaction due to the longer T. Matsushita is with Kyushu Institute of Technology, 680-4, Kawazu, Iizuka 820-8502 Japan (Telephone: +81-948-29-7663,ce-oherence length un* *der the proximity effect is considered mail: matusita@cse.kyutech.ac.jp). as a candidate of the repulsive pin* *ning mechanism [2]. ISS2000, TOKYO, OCT. 14-16, 2000, VPP-35 * * 2 Fig. 1. Magnetic field dependence of criticalFcurrentidensitygat.77.32K. Criti* *cal current density at 77.3 K and 0.1 T vs effective in each specimen. surface area of 211 particles in * *a unit volume. It should be noted that the elementary pinningHforceeofretwehshalleanalyzefJcl* *foruspecimensx00candr01eusingep-flow model [5]. In the model, the pinning po- kinetic energy interactions decreases monotonicallytwithential, the most import* *ant quantity, can be theoretically the increasing magnetic field. Hence, the peakeeffectsistimated in terms of th* *e virtual critical current density considered to be caused by some transitionalichangenintthehe creep-free case, J* * , which is used as a parameter property of flux lines as assumed in the disorderrtransitionepresenting the pin* *c0ning strength. The temperature and [3]. This is consistent with the conclusion bymK"upferaetgal.netic field depend* *ences of J are given by [4] " #m * * c0 In the low field region, Jc increase monotonously with `T_'2 fl* *-1 increasing 211 phase particles. Fig. 2 shows the relation-Jc0= A 1 - Tc B * * ; (1) ship between Jcand the effective surface area of 211 phase particles in a unit volume at 77.3 K and at 0.1wT,hwhereefre A, m and fl are pi* *nning parameters. The distribution and d are the volume fraction and the size ofo211fparticles,Jc0is for simplicit* *y assumed to originate only from the respectively. This clearly shows that 211 particlesdareidom-stribution of A in * *(1) of the form: inant pinning centers at low fields. It seems to be strange ~ (logA - l* *ogA~)2 that the interference between the two kinds of pinningfin-(A) = Kexp -_________* *___m2;(2) teractions does not take place in this field regions. Because 2oe the spacing of flux lines is fairly long, elementarywfluxhpin-ere Am is the mos* *t probable value of A, oe2is a parameter ning interactions are performed almost independentlyrofepresenting the degree o* *f distribution width of the pinning each other, resulting in no significant interferencefbetweenorce and K is a con* *stant. The E-J characteristics were the two kinds of pinning interactions. calculated with the parameters list* *ed in Table 2. Am, m, fl, In the high field region, Jc shows again thegcorrelation2and oe2are adjusting * *parameters to get a good agreement with f/d. This suggests that the pinning interactionsbofetween the theory and e* *xperiment for Jc, where g2 is the 211 particles are again dominant in this fieldnregion.umInber of flux lines in * *the flux bundle. The values of m this region the flux line spacing is so shortathatnalmostdev-fl are approximate* *ly equal to 3/2 and 1/2, respectively, ery oxygen deficient regions are occupied bywfluxhlines.iInch are the theoretic* *al values for the pinning of 211 phase this case, if the superconductivity is inducedpinatheroxygenticles. In additio* *n, Am in specimen 01 is about two deficient regions, it brings about a high kinetictenergyiduemes as large as tha* *t in specimen 00. These results support to the spatial variation of the order parametertofhfluxelines.speculation that * *the pinning by 211 phase particles is Hence, it is considered that the superconductivitydinooxy-minant. Jc was determ* *ined from the E-J curve at the gen deficient regions and surrounding matrixeisldegradedectric field E = 1.0 x1* *0-9V/m. In Fig. 1, the solid lines and that the repulsive flux pinning interactionsadisappear.re the theoretical r* *esults of the flux creep-flow model for Thus, only the attractive condensation energysinteractionspecimens 00 and 01. * *It is found that the agreement is by 211 phase particles are considered to remain.gItoshouldod in the low and hig* *h field regions. be noted that the correlation between Jc and f/dTishnote irreversibility field * *are also determined theoretically as strong as in low field region. This is attributedwtoitheth the same definiti* *on as in experiment. The experimen- influence of flux creep. tal and theoretical results are com* *pared for specimens 00 ISS2000, TOKYO, OCT. 14-16, 2000, VPP-35 * * 3 TABLE II IV.Summary Pinning parameters used for calculation in allTrangeshofe elementary pinning m* *echanism for the peak effect field and temperature. was investigated for melt-processed* * Y-123 specimens. The _______________________________________________ specimen_|_Am______m_____fl____g2____oe2___________following results are obtai* *ned. 00____|1.30_x1091.69_0.68__4.17__0.04___________1.When 211 phase particles * *were added as in specimen 01____|2.31_x1091.68_0.52__4.39__0.04___________01, the critical current de* *nsity decreased and the peak effect disappeared in the medium* * field region. This interference shows that oxygen d* *eficient regions rele- vant to the peak effect act as r* *epulsive pinning centers by the kinetic energy interactio* *n under the proximity effect. 2.The peak effect in Y-123 superco* *nductors is consid- ered to originate from some tran* *sitional variation in the property of flux lines under* * a strong pinning by an oxygen deficient regions. 3.The flux pinning by 211 phase pa* *rticles is dominant at low and high magnetic fields.* * Especially, the irre- versibility field is determined * *by this pinning mecha- nism. Acknowledgments The authors would like to acknowle* *dge Dr. M. Morita of Nippon Steel Corporation for pre* *paring the Y-123 spec- imens for the measurement. References Fig. 3. Irreversibility lines of specimens[001and]01.T.pMochida,eN.rChikum* *oto,cT.oHiguchi,nM.dMurakami,.Adv.XSu-(Springer, Tokyo, 1998)p. 489. [2]T. Matsushita, E. S. Otabe, Phys* *ica C, 335 (2000) 153. [3]D. Etras, D. R. Nelson, Physica * *C, 272 (1996) 79. and 01 in Fig. 3. It is found that the agreement[is4good.]H. K"upfer, Th. Wolf,* * C. Lessing, A. A. Zhukov, X. Lancon, R. This also proves that the flux pinning by 211 phaseMparti-eier-Hilmer, W. Schau* *er, H. W"uhl, Phys. Rev. B 58 (1998) cles is dominant in the high field region. [52886.]M. Kiuchi, K.Noguchi, T. Ma* *tsushita T. Kato, T. Hikata, K. Sato, Physica C, 278 (1997) 62.