Critical current densities in singlecrystal Bi2.2Sr2Ca0.8Cu2O8+δR. B. van Dover, L. F. Schneemeyer, E. M. Gyorgy, and J. V. Waszczak Citation: Applied Physics Letters 52, 1910 (1988); doi: 10.1063/1.99741 View online: http://dx.doi.org/10.1063/1.99741 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/52/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Properties of critical current density in heterostructures of Y1Ba2Cu3O7− y /Y1Ba2(Cu1− x Ni x )3O7− ysuperconducting films AIP Conf. Proc. 251, 108 (1992); 10.1063/1.42070 Highfield critical current densities in Bi2Sr2Ca1Cu2O8+x /Ag wires Appl. Phys. Lett. 55, 2441 (1989); 10.1063/1.102295 Improvement of the critical current density in the silver sheathed BiPbSrCaCuO superconducting tape Appl. Phys. Lett. 54, 1582 (1989); 10.1063/1.101319 Irradiationinduced enhancement of the critical current density of epitaxial YBa2Cu3O7−x thin films Appl. Phys. Lett. 54, 1051 (1989); 10.1063/1.101423 Anisotropic critical current density in superconducting Bi2Sr2CaCu2O8 crystals Appl. Phys. Lett. 54, 72 (1989); 10.1063/1.101437

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Critical current densities in single .. crystal 812.2 Sr2CSQ.a CU20S+8 R. B. van Dover, L. F. Schneemeyer, E. M. Gyorgy, and J. V. Waszczak AT& T Bell Laboratories, Murray Hill, New Jersey 07974

(Received 4 April 1988; accepted for publication 18 April 1988)

We report transport and magnetic measurements of critical current densities in single crystals of high Tc superconductors in the Bi-Sr-Ca-Cu-O system. Transport measurements of Jc were made using a pulse method, while a vibrating sample magnetometer was used to measure M-H loops of oriented crystals in fields up to 15 kOe. Values for transport Jc in zero applied field are higher than those of Ba2 YCU30 7 single crystals at comparable temperatures. Also we observe a strong magnetic field dependence for J c at temperatures above ~ 20 K.

The discovery of materials which are superconducting at temperatures above 77 K has created the tantalizing possi­bility of widespread use of these materials for practical appli­cations, such as electrical power transmission, high-speed signal propagation on microstriplines, and a wide variety of magnet-based devices. In general these applications depend on the ability of the superconductor to carry reasonable su­percurrent densities. Measurements of critical current densi­ties in ceramic samples indicate fairly low values for J" even at zero applied magnetic field. I The behavior of ceramic samples appears in general to be dominated by weak cou­pling at grain boundaries or interfaces between the particles comprising the ceramic.

Recently, Biz.2Sr2CaO.8CUZOg.H;' with a supercon­ducting transition temperature between 79 and 84 K, was discovered and identified.2

-6 This new material is similar to

its predecessor, Ba2 YCn 307' in structural features such as two-dimensional square-coordinated Cu-O bonds (planes), and in physical properties such as a very high upper critical field He2 (0), a short coherence length 5(0), and a moderate thermodynamic critical field, He (0).7 However, there are important differences between the two materials such as the absence of twinning and invarient oxygen content of Bi 22 Sr 2 Cao.R CUz Os + {j' It is therefore interesting from a materials physics perspective to compare the critical current densities of crystals of these two high Tc oxides. Additional­ly, since individual Biz.zSr2CaO.gCU20S f fj crystals show evidence for transitions at both 84 and 110 K, we can com­pare the current sensitivity of the two transitions and gain insight into the amount and distribution of the 110 K phase.

In this letter we describe transport an.d magnetic measurements of the critical current density of Biu Sr 2 CaO.8 CU2 0 8 "" Ii' Both measurements yield values for Je (T) which are much higher than those measured in ce­ramic samples. In zero field these values are higher than the critical current densities in Ba2 YCU3 0 7 single crystals and are indeed comparable to those of the best Ha2 yeu3 0 7 thin films.s However, we observe a strong dependence of Jc on external magnetic field, H, for fields on the order of 1 kOe at temperatures above - 20 K, in marked contrast to the be­havior of Ha2 YCu) 0 7 •

Single-crystal samples of Bin Sr 2 Cao.s CU2 Os + b were grown from alkali chloride fluxes as described prevlously.9 The crystals grow as rectangular prisms with the c axis nor­mal to the basal plane. Since the crystal thickness is small ( 1-4 pm), transport measurements of the critical current

density could be carried out over a reasonable range of Tc - T-= AT. High quality contacts were reproducibly made

by depositi.ng a thin film (250 urn) of Ag onto a crystal face through a metal mask, using low-pressure (2.5 mTorr) Ar sputtering. Fine (25 f.lm diameter) Ag wires were attached to the sputtered pads using a Ag cement which was fired at 400 "c. Contacts made by this procedure were nonrectifying with specific contact resistances less than 10 - 5 n cm2

, and the contacts had considerable mechanical strength. The temperature dependence of the sample resistance was moni­tored by applying a low-frequency (21 Hz) ac current across two contacts to the sample and monitoring the voltage on the other contacts with a phase sensitive detector. Transport measurements were made using a pulse method to minimize sample heating. A 2 Ji-V criterion (which translates to a 10 f..lV /mm field criterion) was used to define the critical cur­rent at a given temperature.

Crystals measuring approximateiy 1 X2XO.OOl mm3

with a single transition at 81 K were selected for magnetic characterization. The superconducting transition tempera­tures of these crystals were measured using a superconduct­ing quantum interference device magnetometer. A vibrating sample magnetometer with a field range of ± 15 kOe was used to measure the magnetization as a function of field. Temperature control to ::::: ± 0.5 K was achieved using a helium flow system. To obtain the critical current from the magnetization data, we used the Bean model. 1o For such thin crystals, only measurements with the applied field (H) par­allel to c could reliably be interpreted. In this configuration, the thin crystal platelet approximates a disk.

Typical results are shown in Fig. 1, where we plot M as a function of H at 5.4 and 28 K for a Bi2.2 Sr 2 Cao.s CU2 0 8 ~ {j

sample with dimensions 2.3X 1.2 X 0.004 mm3• The sample

was cycled through ± 15 kOe before the magnetization data were recorded. The low-temperature magnetization loop, as seen in Fig. l( a), very closely approximates the shape pre­dicted by the Bean model.'! At higher temperatures (> 20 K), the M-H loops deviated from this shape and showed increased sensitivity ofthe critical current density to the ap­plied field. A typical loop, taken at 28 K, is shown in Fig. 1 (b).

The Bean model givesJc = 30AM / D (lc in A/cm2, llM in G, and D in em), where aM is the magnetization differ­ence for increasing and decreasing fields and D is the diame­ter of the disk. We evaluated 11M at fields of 1.5 and 9 kOe and use this to calculate the values of J c plotted as a function

1910 Appl. Phys. Lett. 52 (22), 30 May 1988 0003-6951/88/221910-03$01.00 © 19B8 American Institute of Physics 1910

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FIG. 1. Magnetization M as a fUJlction of the applied field H at (a) 5.4 aJld (b) 28 K for a 1.2X2.1XO.OO4 mm' platelet crystal of Hi2 .2 Sr2Cae .• CuPs-; , with H !Ie.

of temperature in Fig. 2. Measurements on a second crystal gave comparable values. The use of Bean's model for infer­ring critical current densities is subject to certain caveats, as this model was derived for superconductors for which Je (D)

is independent of B, Le., for which the pinning force density is exactly proportional to B. This assumption has no a priori justification, even for the low Tc superconductors, but the Bean model has empirically been found to accurately de­scribe the behavior of many of those materials. Recently, Kumar and Chaddah II have extended the Bean model by assuming an exponential decay of Jc (B), a choice which may be relevant for the high Tc superconductors. They find that the original Bean model would underestimate Je at low fields in this case.

:------------------------------------1 107~ .

'l'E

"

r ,. I g 103~ \ J: I § I:~; ~o. ) TRANSPORT I I

r .1.5.08) MAGNETIZATION * I lOL_~ 9~~~ ______ L __ __L ____ L_

o 20 40 60 80 100

TEMPERATURE [K]

FIG. 2. Temperature dependence of the critical current density as deter­mined from magnetization data at 1.5 and 9 kOe, and from transport data at o and 1.2 kOe. The calculated depairing current, Jd , is indicated by the dotted line.

1911 ApI'\. Phys. Lett., Vol. 52, No. 22, 30 May i 988

Figure 3 shows the temperature dependence of the resis­tivity for a different single-crystal sample of Bi2.2 Sr2 Cao.R CU2 0 8 -> 0 grown from the same alkali chloride flux as the sample of Fig. 1. The sample dimensions were O.13X L3XO.OOl mm3

, with the voltage contacts 0.17 mm apart. Zero resistance in such samples is obtained at tem­peratures varying between 79 and 84 K, and is achieved at 82 K for the sample shown. The other resistance drops between 88 and 110 K are also associated with superconductivity. We suggest that the sample comprises an inter growth of several phases with superconducting transitions at 81, 104, 108, and 110 K. By examining the sensitivity of the various transi­tions to current (inset to Fig. 3), we conclude that relatively little of the 104 K phase is present, that the 108 and 110 K phases are present in substantial amounts, and that the bulk of the sample is the 81 K materiaL This interpretation of course assumes that the critical current densities of the var-

ious phases are at least comparable. Critical current densities determined from transport

measurements are shown in Fig. 2. Data shown are for the sample of Fig. 3; however, the data are typical of the three crystals we measured indicating that the presence of the 110 K phase neither enhances (for example, by increased pin­ning) nor diminishes the observed critical current. The dot­ted !ine indicates the depairing current, Jd ( T), expected for this material: Jd(T) =Jd(OHl- ;2)(1- t4 )--I/2, where t = T ITc and Jd (0) = He (O)/3v61TfloA(O) = 1.5 X 107

A cm-- 2• Here we have used He (0) = 0,62 T and

".1.(0) = 140 nrn, based on preliminary measurements.7 For zero applied field, the observed J c (T) is within a factor of20 ofJd (T). [By comparison, ale ofl X 106 A cm- 2 at 77 K in Sa2 yeu] 0 7 is roughly a factor of 15 less than J d C 77 K) obtained by a corresponding estimate. J At these current densities H> He! at the crystal edges, so flux penetration is expected and the high Je would seem to imply strong pin­ning. However, with a 1 kOe applied field, J c is strongiy depressed, as seen in Fig. 2. This observation is consistent with the trend seen in the magnetic measurements: Je is strongly field dependent at temperatures above - 20 K.

r------------------------------------l

E 200r .. ",.-4','SAc/m_z_ I u ~,':;;:~m 2

'i 1- I "SOOOA cm-2

t I I f: wo~- I I

i 9h~-*1O'- I r I I I

° L _____ L ____ L ____ "-__ .-L __ ._L ___ J o 100 200 300

TEM"ERATURE (l<J

FIG. 3. Temperature dependence of the resistivity for a single crystal of Hi,! Sr 2 Cae.s CU2 O. 4- 6 showing mUltiple transitions. (Inset: The resistive transition near 100 K as measured with various applied current densities.)

van Dover et at. 1911

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Such a field dependence might be attributed to weak-link connections in the 120 X 170 f.lm region of the crystal which is sampled by the transport technique, but this interpretation is difficult to reconcile with the extremely high zero-field critical currents observed. Alternative explanations for the unusual temperature and fieid dependence of the critical current density, such as the presence of a surface barrier to flux entry, are presently being investigated.

A discrepancy between the values for Jc ( n obtained by transport and magnetization measurements is readily appar­ent in Fig. 2. Rough extrapolation of the data from the two measurements indicates that values of Jc inferred from mag­netization are almost a hundredfold smaner than the trans­port measurement at comparable fields. Some of this dis­crepancy might be attributed to weakly coupled regions within the crystal: the induced shielding currents would then circulate only within each region so that the effective sample size would be smaller than the gross measured size. We have found, for example, that rough handling resulting in plastic deformation of the micaceous crystals can induce weak-link behavior in the region of the deformation, as observed by transport measurements. It is also possible for weak links to form during growth of the crystal platelets, although we do not have direct evidence for this. We note that significant flux creep (a decrease in the magnetization) is observed over short times at zero field and low temperatures, which may signify flux motion through weakly coupled regions, al­though it may also be due to a more complex behavior of the flux lattice. In any case, the effective sample size would have to be ~ 20 f.lm to fully account for the discrepancy-this size is unreasonably small, as the samples used for transport are ten times larger. It is possible that the Bean model does not correctly describe the behavior of this material, although its validity was demonstrated 12 for thin-film Ba2 YCu3 0 7 ,

These results and their implications will be the subject of further investigation.

Even at an applied field of 1 kOe, the critical currents measured here are comparable to those of Ba2 YCu} 0 7 sin­gle crystals. Since Ba2 YCU j 0 7 single crystals are typically a-b microtwinned, the high critical current densities mea­sured in that material have been attributed to pinning at the twin boundaries. Perfect single crystals in general should be expected to have low critical current densities because of poor flux pinning. Our measurements on (untwinned) Bi2.2 Sr 2 CUC.R CU2 Os + /j crystals indicate that twinning is not required to obtain high J, values in zero field, while the strong field dependence may point to weak or fieid-depen­dent pinning forces. In any case, the critical current densities

19i2 Appl. Phys. Lett., Val. 52, No. 22, 30 May 1988

of single-crystal samples are much higher than those of ce­ramic samples, 13. ,4 presumably because the grain boundaries of the ceramic act as weak links, so in that regard the behav­ior of Biz.2 Sr2 Cao.s CUz 0 8 + {j is quite analogous to that of Ba2 YCU\07-

The high values of Jc we find in Bi2.2 Sr 2 C~1.8 CUz 0 8 + 15

single crystals at low fields imply a good potential for some applications, e.g., microcircuit interconnects, but the strong decrease at moderate fields is unacceptable for many other uses. The origin of this field dependence is unclear and may reflect the difficulty of working with Bin Sf 2 Cao.s CUz OR + 15

or may be related to the intrinsic nature of these complex oxide superconductors.

We gratefully acknowledge helpful discussions with R. M. Fleming, S. Martin, G. S. Grader, R. J. Cava, and D. W. Murphy. We would especially like to thank T. Palstra and B. Batlogg for communicating results prior to publication.

IR. J. Cava, B. Batlogg, R. B. van Dover, D. W. Murphy, S. Sunshine, T. Siegrist, J. P. Remdka, E. A. Rietman, S. Zahurak, and G. P. Espinosa, Phys. Rev. Lett. 58, 1676 (1987).

2H. Maeda, Y. Tanaka, M. Fukutoml, and T, Asana (unpublished). JR. M. Hazen, C. T. Prewitt, R. J. Angel, N. L. Ross, L. W. Finger, e. G. Hadidiacos, D. R. Veblen, P. J. Heaney, P. H. Hor, R. L. Meng, Y. Y. SUII, Y. Q. Wang, Y. Y. Xue, Z. J. Huang, L. Gao, 1. Bechtold, and C. W. Chu, Phys. Rev. Lett. 611, 1174 (1988).

4S. A. Sunshine, T. Siegrist, L. F. Schneemeyer, D. W. Murphy, R. J. Cava, E. Bat/ogg, R. B. van Dover, R. M. Fleming, S. H. Glarum, S. Nakah8xa, R. Farrow, J. J. Krajewski, S. M. Zalmrak, J. V. Waszczak, J. H. Marshall, P. Marsh, L. W. Rupp, and W. F. Peck (unpublished).

'J. M. Tarascoll, Y. LePage. P. Barboux, B. G. Bagley, L. H. Greene, W. R. McKinnon, G. W. Hull, M. Giraud, and D. M. Hwang (unpublished).

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tional Meeting, New Orleans, LA, March 1988. 14S. Jin, T. H. Tiefel, R. e. Sherwood, R. B. van Dover, and R. A. Fastnacht

(unpublished) .

\ian Dover et a/. 1912

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