We measured the irreversibility temperatures, T{sub r}(H) for both oxides (pure and alloyed Y(123) and Bi(2212, 2223)) and metallic (Nb, NbTi, and Nb{sub 3}Sn) superconductors. These results are compared and discussed in terms of T{sub r}(H) being the depinning line and/or the melting line of the flux line (crystalline or disordered) lattice in the H-T plane

Ghosh, A.K.

Suenaga, M.

Welch, D.O.

Xu, Youwen

UNT Digital Library

, ,, Co,F--- °1°ii ._--/_iIrreversibility Temperatures in Oxide- and Metallic Superconductors', M. Suenaga, A. K. Ghosh, Youwen ,Xu, ahd D. O. WelchBrookhaven National Laboratory, Upton, NY 11973 USABNL--45324DE91 004021ABSTRACTWe measured the irreversibility temperatures, Tr(H) for both oxides [pure andalloyed Y(123) and Bi(2212, 2223)] and metallic [Nb, NbTi, and Nb3Sn]supercon-ductors. These results are compared and discussed in terms of Tr(H) being thedepinning line and/or the melting line OF the flux line (crystalline ordisordered) lattice in the H-T plane.KEY WORDS: Nb3Sn, NbTi, Y(123), Bi(2212, 2223), flux lattice meltingINTRODUCTIONNearly all of the applications for superconductors require high criticalcurrent densities at operating magnetic fields and temperatures. One of themajor disappointments associated with the high Tc superconductors was theirinability to have sufficiently high critical current densities Jc at hightemperatures. Although the values of Jc in thin films have improved to thepoint where they are useful, the intragranular critical current densities insingle crystal and ceramic oxide superconductors are still significantly lowerthan the desired values. It is believed that a part of this difficulty isassociated with very low values of the so-called "irreversibility temperature,Tr (H)" (the lowest temperature at which the flux lines can be movedreversibly). Also, these low vales of Tr(H) are related to very weak couplingbetween the conducting layers (CuO2) in these highly anisotropic layeredsuperconductors [I]. However, what this irreversibility line in the H-T planerepresents, whether a thermodynamic phase transition in the state of the fluxline lattice such as melting or a kinetic cross-over such as a depinning line,is not clear at this mcment and this question is still one of the highlydiscussed subjects in superconductor studies . In the conventionalsuperconductors, it is qenerally believed that since they are isotropic, thetemperature region of the reversibility is so small that it is very difficultto measure and is not cf practical importance. However, recently we havemeasured and reported [2] surprisingly large temperature regions where theflux lines are reversible in Nb3Sn and NbTi multifilamentary wires. Here, wewill compare these resul=s with similarly measured values of Tr(H) [3] for theoxide superconductors, Y(123) and Bi(2212, 2223), and discuss these in termsof various models for T.-(H) : crystalline [4] or disordered [5] flux latticemelting and thermally-assisted depinning [6].FXPERIMENTAL PROCEDUREFor the purpose of measuring the magnetic-field-dependen- irreversibilitytemperature T (H) and the critical temperature Tc(H) , bulk sintered specimensof the desired composit:ons were prepared [7,8]. The specimens were powdered(~I0 _m) and aligned alcng the c-axis. For the conventional wires, state-of-the-art multifilamentary Nb3Sn and NbTi wires were selected. In addition, inorder to study the effect of variations in the value of the Ginzburg-Landauconstant K on the irreversibility temperatures, a fine filamentary (~3Mm) _"cold-drawn Nb wire was also studied. These wires were cut in a length of -6 _mm segments and several lengths were placed horizontally (perpendicular to_l _ _IH_ _n a suDerconductinq quantum interference device (SQUID)magnetometer (Quantum Design, Inc.) while the aligned oxide powders we reIIIITIIIBUTIONOF THISLIIIOBIVENTISCIN/IMIrEII()' placed in the magnetometer such that the c-axis of the oxides are parallel to !the applied field. As previously described [3], a standard procedure formeasuring Tc(H) was used to determined, Tr(H).rThe results of such measurements of the magnetic moments for a Nb-Ti-alloy anda Nb wire are illustrated in Fig. i. Also shown in the figures are thecriteria which were used for determining the irreversibility temperature Tr(H)and the critical temperature Tc(H) [or the mean-field critical magnetic fieldHc2(H) ] . Here we define Tr(H) at the temperature which an observable deviationis noted in the moments after the temperature cycle. It was found that bythis criteria the difference in the moments for the warming and the coolingcycle was consistently-0.1% at Tr(H).5.0 1 , 1 ITc(H)I....._.|. .,-".,,,,--.-,o......... ,."/4.5'f --NbTi i I , l , , , ]C 4,0 Tr H)J./" H= 40kG< ,( :,/\/-1.0 _ 20_ . 0 N_S n "2.0I-I.5 10| • Y=_U307 (cllH) "'_,,,,.,_!, , , i 1 -2.0 oL_'' 0.7 0.8 o.g 1.0- 6,5 7.0 7.5 8.0 8.5 9.0 9,5Temperatur_( K) Tr ( H) / Tc ( 0 )Fig. i. The reversible temperature Fig. 2. A comparison of the tempera-regions for a Nb-Ti alloy and a pure ture region of the reversible mag-Nb wire are illustrated, netic flux line motion in Nb]Sn, NbTiand Y(123) .RESULTS AND DISCUSSIONThe results of the magnetic field dependence of the values of T_(H) for Nb3Sn,: Nb-Ti; and Y(123) and for the Bi(2212) and Bi(2223) specimens are summarizecin Figs. 2, 3 and 4. As shown in Fig. 3, the data for al/. of the pure andalloyed Y(123) specimens follow a relationship:H = a[l - T=(H)/Tc (0)]n (].)with n ~ I .5 and "a" is a constant of proportionality. We measured themagnetic hysteresis width AM [at T/Tc(0) --0.I and 0.5 and H = I0 kG] for eachspecimen. We find that "a" decreases nearly proportionally with decreasingAM. Thus, it is clear that the pinning strength of a specimen has a stronccorrelation with the irreversibility temperature in the case of alloyed= YBa2Cu30_. Also, it was found that the data above do not fit the predicteadependence of T_(H) on H predicted on the basis of flux-lattice melting byHoug._;ton et al. [4] . In the case of the Bi-oxides, the relationship in Eq. (I)does not fit at all as shown in Fig. 4. However, the relationship betweenthe strength of the flux pinning and the width of the reversible temperature_-- region is similar to that of Y(123), i.e. the stronger the pinning, the higher-_ the values of Tr (H) . Thus, these results indicate that the T r (H) lines in--these oxides are not likely to be a thermodynamic melting temperature of thecrystalline flux line lattice unless the melting temperature depends upon thestate of flux pinning.-Since both the flux creep model and Fisher's glass-to-liquid transition model[5] for Tr(H) depend on the strength of the pinning, we have to study otherDISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any spe-cific commercial product, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof.The views and opinions cf authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.4._ ,, i"14,0 • _l_cu I, _,__l'I .... 50i i I o Bi2223e22122,0t.5 yBaz(CUo.6eM.o_)_O ' 10otO , A I J i l-3.5 -3,0 -2,5 -2,0 -z.s -z.o -o.5 o,o 0 20 40 60 80 100In ( 1-Tr/Tc) Temperature( K )Fig. 3. The irreversibility tempera- Fig. 4. The irreversibility tempera-tures for a pure and alloyed Y(123) tures for a Bi(2212) and a Bi(2223)as a function of applied fields, as a function of applied field.10 -t -4 "J ' I I " I 1 ' ' ' ' 'I I •(a) • (b)• 0'__ • •10-z _ •e-- 8 A, , i ,, I * I , , , l •1 0 - _ ' .... _ .... 1.4 1.8 1,8 2.0 2,20 5000 10000 15000t(sec) In(Int)Fig. 5. The temporal dependence of the hysteresis width (the critical current)for a Bi(2223) .properties of these materials in order to distinguish these models. In orderto do this, we note that only in the limit of t -_ oo, the predictions of thetemporal dependence of the flux creep are distinguishable i.e. Fisher's modelpredicts,AM _ (In t)-I/_ where _ < 1 (2)while the creep model [6] predictsAM ~ e-st where _ is a constant. (3)If we assume that the regime AM -_ o is equivalent to t -_ oo, we can achievethis condition in the Bi-oxides at low fields (1-3 kG) and at temperaturesvery close to Tr. The result for the Bi(2223) specimen is shown in Fig. 5.This clearly shows that the creep in this specimen follows an exponentialtemporal dependence for t -_ _ [Fig. 5(a)] and does not follow the (int)-i/_dependence [Fig. 5(b) ] . Thus, we conclude that in these materials, Tr(H) ismore likely to be a depinning line in the H-T plane rather than a glass-to-liquid phase transition line.The present interest in studies of the magnetic state of the oxidesuperconductors als prompted reexamination of the conventional superconductorsto seek clarification of some of the observed phenomena in the oxides. Asshown in Fig i, state-of-the-art Nb-Ti wires exhibited rather largereversibility regions. Also, in Fig. 2 a comparison of the sizes of thereversibility _temperature regions is illustrated for Nb3Sn, Nb-Ti, and pureY (123) on a reduced temperature scale, T r (H) /Tc (0) . The regions, [Tc (H) -T.(H)], for Nb3Sn and Nb-Ti are nearly 1/3 as large as that for Y(123) . Thisis quite surprising, but what is mcre surprising is the fact that the fielddependence of Tr(H) for both the NbBSn and Nb-Ti wires fits the predicteddependence of the melting temperature calculated wit]_ the theory of Houghtonet al. [2] . In addition, the predicted _ dependence was also founded tofollow the theory as illustrated in Fig. I, i.e. smaller the value of K, thesmaller the reversibility region and _ for Nb is an order of magnitude smaller'than Nb-Ti. Furthermore, a large reduction in Jc (-5 times) or in the size offilaments in Nb-Ti wires did not _ result in observable difference in the sizeof [Tc(H) - Tr(H)]. Thus, all these results point toward flux lattice meltingin the high-_ and high-Jc conventional soperconductors. The only experimentalobservation which raises a question about this interpretation, is the factthat when the irreversibility temperature is measured by the cycling magneticfield at a constant temperature, this temperature Hr (T) is signi=icantlycloser to Hc2(T) than Tr(H). At present, the reason for this discrepancy isnot clear and further study is required to confirm the possible observation offlux-lattice melting in Nb3Sn and Nb-Ti.In summary, it appears that the irreversibility lines are flux-line depinninglines in the oxides and are possibly flux-line-lattice melting lines in theconventional superconductors. Particularly, the latter is very unexpected andfurther study is needed to understand the nature of flux line motion in thesematerials.ACKNOWLEDGMENTThis work was supported by the U.S. Department of Energy, Division ofMaterials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC02-76CH00016.REFERENCESI. For review, see for example, Tinkham M, (1991) IEEE Trans. Magn. Mag-27:(to be published) .2. Suenaga M, Ghosh AK, Xu Youwen, and Welch DO Phys. Rev. Lett. (submitted) .3. Xu Youwen, Suenaga M, Gao Y, Crow JE, and Spencer ND Phys. Rev. B (to bepublished) .4. Houghton A, Pelcovits RA , and Sudo S (1989) Phys. Rev. 40: 6763.5. Fisher MPA, (1989) Phys. Rev. Lett. 62: 1416.6. Griessen R, Flipse CFJ, Hagen CW, and Lensink J (1989) J. Less-CommonMetals 151' 39.7. Xu Youwen, Suenaga M, Tafto J, Sabatini RL, Moodenbaugh AR, and Zo!likerP (1989) Phys. Rev. B 39: 6667.8. Xu Youwen, Sabatini RL, Moodenbaugh AR, Zhu Yimei, Shyu Shln-guang,Suenaga M, Dennis KW, and McCallum RW (1990) Physica C 169: 205. !

Irreversibility temperatures in oxide- and metallic superconductors

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Irreversibility temperatures in oxide- and metallic superconductors

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