1016 IEEE WANSACTIONY ON APPLIED SUPERCC)NDULTIVITY, VOI,. 10, NO. 1, MARCH 2000

Flux Pinning Strength of Different Superconducting Artificial Pins in Nb-Ti Multifilamentary Composites

Yun Zhn, Oake Miura, Taknshi Okubo, Dsisuke Ito Department of Elcctrical Engineering, Tokyo Mctropolitan University, 1-1 Miriami-osawa, Hschioji, Tokyo 192-0397 Japan

Sakaru Endo Siipcrconducting Products &pnrtmcnt, The Furukown Electric Co.,I,td., 500 Kiyotnki, Nikko, Tochigi 321-1493 Japan

ALs~Fuc~-To imprwe tlic critical current density (Jc) of multifilementary Nb-Ti supercnnductors, the pinning charactcristics nf wires hnviiig diffcrcnb superconducting artificial pilining msterids wnd pili sizcs wcrc studitd. Nb, Nb- 7,51vt.%Ti1 and Te pins were introduccd into each Nh-Ti filumcnt KS an artificinl pinning center (APC). Magnetizntinn w a s mcasured tn cihtain tlic mapctic ficld dependcncc of the pinning forcc dcnsity (FP) at vwioiis tcmperaturcs. A significant cr~ltanccment of F I ~ IIK", bcen obseircd for APC wires in comparism to conivcntionel wires. Br incrcascs in rst.c~~ding order uf Tv, NLTR and Nh pins. This rcsult rctlects the clemcntary pinning fwcc (CP) cnlculated by the Girrzburg-

intri each Nb-'Ti filament. To avoid interfilamentuiy-proximity coupling, Cu-Ni ulloy was used as tlic m&X material. The wires wexc drawn IO a diameter whcrc the pin size wus companihlc to the flux line l a t h spacingat low liclds. Nb, Nb-7..5wtvt.%'l'a and Ta wcrc chosen as pin materials and the volumc fraction of pins was fixed at 17%. Conventional multifilamentary wires with no srtiticid pins were preparcd for conparismi. 'I'hie wire spcimcns are shown in Table 1.

TAl3LI; 1 Specificattioil ol's&mem

Nb(l70/), Nh-T~(17%), Ta(l7%),. Landnu (G-L) theory. The slope of the FI, curve for Tw pins E I I R I I ~ C S st 4.4 K, whcrc II supcr-~~uIT" transitiori occurs. We nlso invcstigrtcd thc FI, prowrties in n magnetic ficld prrnllcl

Speciineiis [Volumo 1:raction 74 Con. (0%)

tr, thc wirc-&s. In this cas;, thc perk of ib shifts to Rhighcr magictic ficld comp;md with that of P pcrpendkular inagictic 1:ilament Dinmcler dl him) I).270 0.518

Wire Dinniotcr (mm)

Filamunt Spncing d (pin) 0.196 0.370 Pin Spcinp & (nm) * 77.9 149.9 PLi Diamctcr dp (nm) * 42.1 50.9

anrlnoI,p

0.414 0.794

ficld. This rcsult suggcsts Ihnt the numhcr of cffcctivc pinning sites in a parallcl magnctic field is larger than in n pcrpciidicular magnctic field duc to thc nnisotnipic microstructurc of' thc pins. Furthermore, we report on their flux piriiii~ig scaling bhnviars.

+ Convcnrivnal sFcilnens

I3y niwiis of introducing Arlilicial Pinning Centcrs (APC) into Nb-Ti sopcrwndiicting wires, it is kcusible to produce o. high critical current density (Jc) aiid control its value. Thc A l T method is regarded as n key technology for increasing J{: because i t has 1hc ability to chmgc pin materials and piii size fieely, lhus making it possiblc to optimize ihe flux pinning forcc and research pinning mechanisms. Wc have been studying AFC technology for scvcral years to beltcr undcrstaiid the flux pinning phenomena [1]-[3]. In this work, wc havc investigated the teinpcrature dependence of thc flux pirinirig strcngth (FP) for Nb-Ti multifilummtary wires with different supcrconiltictitig piii niutcrids and the results me discussed on thc basis of the G-L Ihaory. Furthermore, we have studid the .I>inning behavior in magnctic fields parallel and pcrpmdiailar to tlic wirc axis and have related it to the anisotropic microstructure ofthe pins.

II. EXPERIMENI'AI.

APC wires wcrc fabricated by a doublc slacking method. Seven round pins in a triangular arrniigment were introduccd

Scanning eloctron niicroscopy (SEM) wus donc to investigate thc miCrostructiire of tlic crms-sectiori of ~iii M'C filfimcnt. The SEM photographs of the Nb aiitl Ta pins arc shown in Figs. I a-b. 'I'he pins art? defmnicd inlo n riL)h,bnn-hkc shape it1 hoth spccimens by Ihc hrge drawing strain, though the dcsigned pins had U round shape. Morcovcr, some Ilmluntioris of the cross-sectional area of thc 'Ja pins arc observed tmd suggcst the occurrcnw of pin sausaging. A larger dcloi-maiiori of the pins is cibst.rvd in A l T wires with t l i i m i ~ ~ lilnrnent diameters. Thc cfkct of this change in microstructure on thc nux pinning force dcnsity is discussd.

(a) Ab APC for dfLO.S1 8 ptn, (h) Tn APC for 4.71.08 I'm.

1051-8223/013$10,00 Q 2000 IEEE

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l'hc tanpmturc dependeiica of Fp cxlracted surl'ticz pinniiiy effect 101- each N'C wirc at 1 T, where APC wires iiidicnte k g c r Pp cimnlw-otively, is 1dotted in Fig. 3. As tzinpcmtiire riscs, FP vnlue for each APC wirc dccreascs linearly.

tliz inrignctic field dcpendcnce of pinning h c c dciisity (1:~). lilux pinning properties wcre invcsiigated in rnngnctic fields both ~ " d l c l niid pcrpciidiculnr to thc wirc rixis. 'Uic critical tcmperatturc (I(:) WIS definud as thc tetnpcrntiee :it which (lit

mngrietization tit 1 mT changes from U diamngiietic to :I pminngnctic sttiic. Sqicrcotidiicting pamnicters, such a s tlic Iliwtndynnmic critical fcld niid cthcrenw lcngth ( E ) , far thc pin maicrials wcre calculntetl kom thc magneht ior i nicnsuremcnts. 'Thc practical uppcr critical field (Ti,!,*) rvns defiincd as thc ricld nt which Fp cxtrnpolutd to XTO.

111. I&SUI..TY AND L ~ S C I J S S I O N

Pigs. 2c13 show thc magnelic I'ield dcpcndencc of YP liir the differen1 types t r l hl'C w i m colnpnrcd to f i convcn~ionnl wirc at the s m e tilumrrit dioiiieter (0.518 pm) a1 2.5 K and 5.0 K , rcspectivcly. At holh tanpcralures, a hrge enbnnccnicnt of thc pinning forcc density (FIX) is observcd for thc N ' C wires conipnmd to tlic convcnlional wirc. FP increases in the ordcr of Til, Nb-'ln and Nb. The peak ficld of Fp shifts liom 1.5 T to 1 'I' when Ihc temperature rises fi-cm 2.5 K to 5.0 K. Thc d i ikcnce in Fr betwccn APC wircx and cwivcniionaI wires hoccmes smaller ill highcr fields and tcmperaturcs, In patticiiiar, at 5.0 K, PI, Cor the 'In APC wire drops tu ncnrly thc stimc vnluc B S the conventionnl wirc above 5 T.

1 :,

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0.35

0.2

11.2 1

I S E $9 0 1

4 .05 ti -.., .-.L -... I ..-_-. ._-..! . I .-.-. ~ ..-..

2,s 3 3 5 1 4 5 5

Fig. 4. Tmipemtorc dcpendcncc oftlimrdicd elernmtnry pirniiig forcc (E,) TlKl

l'hc elemcntaiy pinning forw (&) for very 1hin superconducting pins, in which the spntirrl varintion trf thc order lxrrametcr is quite sniall due to thc strong proximity clTect, can be cxpresscd by tlic following equations [4,5J.

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Fig. 4 shows the temperature dcpcndclicc of 111~‘ theoretical t;., where the avuagc pin tlilckncss is measured to hc 37 I l t n Compared with the cxpcrimciilllul FIB viilues shown in Fig. 3, tlic ~cniperatirre dependcnce of fp for cnch APC wire is in gaod ngreeiiicnt with lhul u l Pp. Consequently it appwrs that the valuc oi’Fp dircc(1y reflects the vdne of fp.

Pig. 5 shows the tmipernturc dcpcndence of F , for cadi hl’c wirc with df = 0.51 8 pm at 1 1’. It is natLwnrtliy tlint thc slope of Fp varies only for thc Ta APC at 4.4 K, which is lhc TC of Ta. This rcsiilt suggests that a super-nomnl (S-N) iransition has occurrcd in thc Ta pin. Even though a S-N trnnsition occurs, there is no mnrkcd junip in Pp. It is helpfill to study the h x pinning mechanism, laking into account the proximity cll‘cct to expl:iin this phenomenon 161. When nonnal mntcrial has U strong proximity effect and the pin sizc is d f i c i d y annll, fp is calculated as

J-- G 1.56(1 +OX1 - c ) -1.23(1 - c y , f P 0

where {s, & i s thc cohcrcnce length for supcrcmduding a ~ d iionnnl rnntcrint, rcspeclively. fm is thc clcmcntary pinning force when there is no cfkcct. Iu our cxperiment, the cohcrcncc lctigth tit 4.4 K fir NI)-Ti and ?‘a is cstiinatcd to bc = 5.5 nn rrnd tN = 80 ntn, respectively. Thc nitio of pin layer. thickness to Nb-Ti layer thickncss w i i s measured to be ds : d p 1 : 3 , :mi calcrilations using thcsc vnlues give the ratio fI) / $6

= 0.34, ‘Therefore, the strong proximity effect catises thr: degratlation of Pp b r Tn pins. Moreover, as shown in thc SEM Iinage of the Ta pin, the non-uniform cross-section of nrcii is rmother candidate for thc cause otTp reduction.

” ‘ ~ ”

Fig. 6 shorvs the tmpcmturc and magnetic ficld dcpcndcncc of FP for the Nh APC wire at df = 0.518 pm. ‘l’hough the mnxiin FP vduz of each oricmution is almost the snmc, h e peak in parallel fidd shifts to il higher magnetic IicId in comparison to that in pcrpendiculnr field. ‘This bchnvior rcsults from the anisotropic microstructure of the ribbon-shaped pins, To nnalyze the scaling propertics ut‘ I:p, the tempcrnturc dcpmdcnce of the practical uppcr critical ttiagnztic field (ljC2*) in purallul and perpzndiculnr magncric ficld is shown in Fig. 7.

The ohserved RC2* of kith Nb and Nb-1’~ specimens at rlinbrcnl wire dinmeteis in parallel magnctic ricld is murkcdly higher thau ilia[ in pcrpendiculnr field. 21) superconducting chrractcristics similar to multi-layer systems reportcd in referaice 1171 may bc thc causc.

Vis. 6. l’empzrilture and mngnctic ficld dupendcnci! ofFF fur thu Nb wirc n l di -0.518 puli iii prnl le l ai~dpctpendio~llornlRgnetic fields.

5.5 6 6.5

TIKI

Fig. 7. & a * in pnrnllcl and pcrpendiculor mngnatiu iid& HI various tcmpcmnlra.

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[GI E. S. Olabr: and T. MAtsushiln, Critionl Currelit Uviirity in Srqxmmrluctiirg Nb-l'i under pinxitniiy effect, Cryogcnios, 33, p, 5 3 1 , 1993,

171 Y . Kiiwnsawn. T. Tounkn, A. IJchiyamo, S. Matsudi and S. Nnkruio Phyxica, C175,p. 187, 1991.

[S] A. M. Canipkll and J. E. Evccfts, Ah. l'hys., 21, 1j.372, 1972.

Figs. Xn-h sliow the scaling hchuvioru d the NI> spmirncn 111 df = 0.5 I X pm in pnrr~llcl and perpendicular inugnetic fields I;ig. I) also shows h e temperature scaIhg characteristics of thesc sl~ccimcns. As s h o w iii these figures, good scaling behaviors for both specimens WO contirind The scaling parameter p riscv from 0.3 1 in pcrpciidiculat magnetic field lo 0.64 in pnrnllcl field wliilc thc suuling parameter m r i m from I .7 to 2 ,2 . According to scaling thcory [RI, the increase of both 13 and n i nicans that the numhcr oi' ci'fcctive pinning sites in pnrnllcl iiiagnetic iiceld is largcr than that in pc-rpcndicular mrigIictic iicld due tu thc' anisnt1o)pic microstructure of thc pi ii s.

IV. CONCLUSION

We hnvc studicd tbc dux pinning strength d different snpcrconductiiig arliticinl pins in Nb-Ti iiiidtililfimcnWy cr)inposiks and have foimd the following. 1. 'I'hc I r of Al'C wires is largcr 111an that of conventional

wircs and increascs in thc order of'I'a, Nb-l'a, and Nh. 2. Thc I;r Vi\lUc crf tliiYerent A P C wires directly ridccts thr: f:J

v;ilue CUI culatcd by the G-L theory. 3 . . A chnngc in the flux pinning mcchmism for the 'i'n APC

doe to the S-N tlansitioii was observed. 4. When 11 mugnclic ficld is applied parrillel to the Wirc axis,

the 1:p peak shi€ts 10 t i h i g h magnetic field and Bc~' increriscs in coinparison to n perpendicdar mignetic field. By scaling nnalgsis, it was revenled that tha effective pinning sitcs increase in p"le1 field. This hcliavior was rclutd tr) the anisotropic Inicrostructurc of thc pins.

Wc would likc to thnnk 1)r. 1'. 1. T,ce ol' I ~ C University of Wisconsin Ibr prcwiding thc SEM images.

1

n.8

0.6

0.4

0.2

0

0.2 0 4 0.6 0.8 I h =T3/Bc2*

Fig. 8. Megncticfieldxcnling choractaisticofh'bPSC [or dr=I).518 pm in (a) IMnmllcI ningnctic field, (11) Iqwdic11lnr ma@c ticld.

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