Flux Pinning Property in Superconducting Sm-123 Powder Edmund Soji Otabe, Teruo Matsushita, Yoshitaka Takahama, Hisayuki Suematsu, His* *ao Yamauchi Abstract_ The size dependence of flux pinning property TABLE I was investigated for superconducting Sm-123 powderAspec-verage and standard dev* *iation of particle size in each imens. It was found that the peak effect disappeared when the particle size became smaller than the longitudinalscorre-pecimen. The ratio* * of the average size to the longitudinal lation length. This suggests that the peak effect originatescorrelation length * *is also listed. from a certain cooperative phenomenon associated with the transitional change in the state of flux lines but not directly from the elementary pinning mechanism such as the field- induced pinning mechanism. specimen 1 2 3 * *4 5 Keywords_ Sm-123, peak effect, disorder transition,_size______________________* *________average4size9.(~m)88.55.04.42.2 dependence ___________________________________* *________standard1deviation5.44.52.93.01.3 _______(~m)________________________* *________ I.Introduction =l44 5.83 0.82 0.55 0* *.12 0.06 ___at_77.3_K,_1.5_T________________* *________ HE critical current density vs. magnetic fieldpcurveeinak effectyes(yes)(yes* *)no no T RE-123 (RE = Sm, Nd, Y) frequently shows a peak in a medium magnetic field. Although it is known that oxygen vacancies or regions where Ba site is substituted by a RE element are relevant pining centers to the peak ef- II.Experimental fect, the pinning mechanism of these regions isSstillpunderecimens were prepare* *d by a sintering method and a discussion. On the other hand, the disorder transitionpofost annealing was perf* *ormed for pellets at 350OC for 40 flux line lattice was proposed to explain thehpeakieffectn[1].an oxygen gas flo* *w. The pellet was first powdered According to this model, the peak effect is aacooperativend then separated in f* *ive specimens with different average phenomenon of flux lines accompanied by a plasticpdefor-article sizes by using * *sieves of different mesh sizes and mation of the flux line lattice, which occursaindependentlyn elutriation techni* *que in which the size dependence of of the kind of elementary pinning mechanism.tThathis,ethefalling velocity of pa* *rticles in ethanol is used [2]. The peak effect is caused by an enhancement of theapinningvef-erage value and the s* *tandard deviation of particle size ficiency due to the plastic deformation of fluxalines.rThere-e listed in Table * *1. Specimen 1 has the largest particle fore, for the occurrence of the peak effect,stheispecimenzsizee of about 50 ~m * *and other is less than 10 ~m. The should be larger than the longitudinal correlationplength,owder was mixed with * *insulating Y2O3 powders for the l44, over which length each flux line moves coherently.pThisurpose of the restr* *iction of the percolating current among suggests that, when the specimen size is smallertthanhl44,e superconducting pow* *ders, and then pressed into a form the disorder transition of flux line latticeoisfconsideredptoellet. The c-axis * *of the specimen was roughly oriented be diminished, resulting in no peak effect. to the press direction. In this work, superconducting Sm-123 superconductingThe magnetization of speci* *men in the magnetic field par- powder specimens with different particle sizesawerelpre-lel to c-axis was measu* *red with a SQUID magnetometer. pared and the peak effect of the critical currentTdensityhwase critical current* * density was estimated from the hys- investigated from the viewpoint of the relationshiptbetweeneresis width of the * *magnetization. the particle size and the longitudinal correlation length. III.Result and Discussion E. S. Otabe is with Kyushu Institute of Technology, 680-4, Kawazu, Iizuka 820-8502 Japan (Telephone: +81-948-29-7683,Fe-ig. 1 shows the ma* *gnetic field dependence of the criti- mail: otabe@cse.kyutech.ac.jp) cal current density, Jc, at 77.3 K * *and 80 K for each spec- T. Matsushita is with Kyushu Institute of Technology,i680-4,men. The peak effe* *ct of J is appreciable in specimen 1 Kawazu, Iizuka 820-8502 Japan (Telephone: +81-948-29-7663, e- * * c mail: matusita@cse.kyutech.ac.jp) and slightly observed in specimens * *2 and 3, while it is not Y. Takahama is with Tokyo Institute of Technology,s4259,eNagat-en in specimens* * 4 and 5 with much smaller particle sizes. suta, Midori-ku, Yokohama 226-8503, Japan (Telephone:S+81-45-pecimen 1 with the* * largest size shows the lowest J at low 924-5315) * * c H. Suematsu is with Nagaoka University of Technology,f1603-1ields. The density* * of pinning sites is supposed not to be Kamitomioka, Nagaoka, Niigata 940-2188, Japang(Telephone:+81-reatly different a* *mong the specimens. It is considered that 258-47-9894, e-mail: suematsu@nagaokaut.ac.jp)the degree of flexibility of flux* * lines to meet pinning sites H. Yamauchi is with Tokyo Institute of Technology, 4259, Nagat- suta, Midori-ku, Yokohama 226-8503, Japan (Telephone:i+81-45-s higher for a spe* *cimen with the particle size smaller than 924-5315), e-mail: yamauchi@metaria.titech.ac.jpthe longitudinal correlation le* *ngth, l44, because the area in aThenratiodof1the.average5particleT* *size,wtohl44ate77.3rKe the peak effect is observed, is listed in Table 1. The obvious peak effect i* *s observed only when =l44is enough larger than unity,* * while even an indica- tion of the peak effect is not seen* * for specimens 4 and 5 in the opposite case. This result agre* *es with the prediction as abovementioned. That is, the enh* *ancement of the crit- ical current density in medium magn* *etic fields originates from the enhanced flexibility of th* *e flux line lattice as pre- dicted in [1]. This is consistent * *with the tendency that the specimen with smaller particle * *size has lower Jcat low fields. That is, the peak effect or* *iginates from the cooper- ative phenomenon associated with th* *e transitional change in the state of flux lines, but not* * directly from the elemen- tary pinning mechanism such as the * *field-induced pinning mechanism. The irreversibility field, Bi, is * *determined by the mag- netic field at which Jcdecreases to* * 107 A/m2. It is found that the specimen with smaller part* *icle size shows lower Bi. This is attributed to the lower* * pinning potential. That is, the pinning potential, U0, is g* *iven by U0= bU0V; * * (2) where bU0is the pinning energy dens* *ity averaged over unit volume and V is the volume of the f* *lux bundle [3]. In the above the volume is V = l44l266when* * d > l44, while it is restricted as V = dl266when d < l44* *, where l66is the trans- verse correlation length. Therefore* *, when d is small, the effect of the flux creep is pronoun* *ced, resulting in smaller Bi. IV.Conclusions o The peak effect is only observed* * when the average par- ticle size, , is larger than * *the correlation length, l44. This shows that the enhancement * *of Jcis attributed to the enhanced flexibility of the * *flux line lattice, and sup- ports the mechanism of the disor* *der transition of flux lines. At the same time it is co* *ncluded that the peak effect is not caused by the elem* *entary field-induced Fig. 1. Magnetic field dependence of the criticalpcurrentidensitynatning mecha* *nism. 77.3 K and 80 K for each specimen. o The irreversibility field is sma* *ller in the specimen with smaller particle size, since the* * pinning potential is re- stricted by the size of supercon* *ductor. which the interference among pinning forces takes place is limited by the particle size. Here, l44is theoretically givenReferences by ` ' [1]T. Matsushita, "Flux pinning in * *superconducting 123 materials", 1=2 ` Ba ' 1=2 Supercond. Sci. Techno. vol. 13, * *pp. 730-737, 2000. 153. l44= C44_ff ' ___f__ ; (1) [2]Y. Takahama, H. Suematsu, T. Mat* *sushita, H. Yamauchi, "De- L 2ss~0Jc pendenceSofmpeakBeffectaon2theCpa* *rticleusize3inOsuperconducting7-ffi: Physica C, vol. 338, pp. 115-120, 2000. where C44= B2=~0is the elastic tilt modulus of[flux3lines,]T. Matsushita, T. To* *hdoh and N. Ihara, "Effects of inhomoge- ffL is the Labusch parameter and afis the fluxnlineespacing.ousvfluxopinninglst* *rengthtandafluxgfloweoncscalinghofacurrent-racteristics in high-temperature s* *u@ This value can be estimated using observed resultsPofhJc.ysica C, vol. 259, pp.* * 321-325, 1996.