Flux pinning in ternary (Nd 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O y melt-processedsuperconductorsM. R. Koblischka, M. Muralidhar, and M. Murakami Citation: Applied Physics Letters 73, 2351 (1998); doi: 10.1063/1.122458 View online: http://dx.doi.org/10.1063/1.122458 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Flux pinning in ternary (Nd–Eu–Gd)Ba 2 Cu 3 O 7−δ : Optimization for the highest pinning Appl. Phys. Lett. 77, 2033 (2000); 10.1063/1.1312863 Enhancement of J c by 211 particles in ternary ( Nd 0.33 Eu 0.33 Gd 0.33 ) Ba 2 Cu 3 O y melt-processedsuperconductors Appl. Phys. Lett. 76, 91 (2000); 10.1063/1.125666 Flux pinning in melt-processed ternary ( Nd–Eu–Gd)Ba 2 Cu 3 O y superconductors with Gd 2 BaCuO 5 addition J. Appl. Phys. 86, 5705 (1999); 10.1063/1.371582 Enhanced flux pinning in (Nd 0.33 Sm 0.67 )Ba 2 Cu 3 O 7−δ melt-processed superconductors by Arpostannealing Appl. Phys. Lett. 75, 259 (1999); 10.1063/1.124341 Evidence of strong flux pinning in melt-processed ternary (Nd–Eu–Gd)Ba 2 Cu 3 O y superconductors Appl. Phys. Lett. 75, 253 (1999); 10.1063/1.124339

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.240.225.44 On: Sat, 20 Dec 2014 09:01:33

Flux pinning in ternary „Nd0.33Eu0.33Gd0.33…Ba2Cu3Oy melt-processedsuperconductors

M. R. Koblischka,a) M. Muralidhar, and M. MurakamiSuperconductivity Research Laboratory, International Superconductivity Technology Center, 1-16-25,Shibaura, Minato-ku Tokyo 105, Japan

~Received 27 July 1998; accepted for publication 15 August 1998!

The flux pinning characteristics of ternary melt-processed~Nd0.33Eu0.33Gd0.33!Ba2Cu3Oy ~NEG!superconductors are studied in the temperature range 60<T<90 K. NEG samples exhibit a stronglydeveloped peak effect in the dependence of the critical current densities on the external field,Ha .The scaling of the pinning forces versus the reduced fieldh5Ha /H irr ~where H irr denotes theirreversibility field! yields a peak ath050.5 which is an indication of pinning provided by a spatialvariation of the transition temperature. The presence of a weaker superconducting second phase isdemonstrated by means of field cooling and warming experiments in fields up to 7 T. Furthermore,we discuss the possible effect of the magnetic moments of Gd and Nd on the flux pinning. ©1998American Institute of Physics.@S0003-6951~98!01742-2#

The preparation of melt-processed~LRE!Ba2Cu3Oy su-perconductors (LRE5light rare-earth elements, i.e., Nd, Sm,Eu, Gd! using the oxygen-controlled melt-growth~OCMG!process has proven to yield samples with an onset of super-conductivity atTc,onset'96 K and a high critical surrent den-sity, j c , exceeding 20 000 A/cm2 at 77 K, 2 T.1 Characteris-tic for all LRE compounds is a solid solution between theLRE atoms and Ba, which provides composition fluctuationsthroughout the sample. It was demonstrated in Ref. 2 thatthese composition fluctuations lead to spatial variations ofthe transition temperature,Tc , and hence to increased fluxpinning by means of theDk-3,4 or dTc-pinning5 mechanism,which is mainly active at elevated temperatures ('77 K). Acharacteristic behavior of this pinning mechanism is the tem-perature dependence of the pinning potential, which shows amaximum before decaying towards 0. Correspondingly, ef-fects of flux creep in this temperature range are considerablyreduced.6

The quest to increase the current densities even furtherled to the preparation of so-called ternary compounds of theLRE elements Nd, Eu, and Gd with a nominal compositionof ~Nd0.33Eu0.33Gd0.33!Ba2Cu3Oy ~NEG!. Preliminary resultsshowed indeed, an improvement ofj c , and also of the irre-versibility field, H irr .7 As the composition fluctuations areincreased by means of mixing several rare earth elements, itis expected that thedTc pinning is even stronger.

A fruitful tool to study the pinning properties of a super-conductor is a scaling of the volume pinning forces,Fp5 j c

3Ba , whereBa denotes the applied field, versus a reducedfield, h, at various temperatures. For high-Tc superconduct-ors, several authors8 have shown that the appropriate scalingfield is H irr , where by definition Fp5 j c50. ForNdBa2Cu3O72d ~NdBCO! superconductors, the position ofthe maximum,h0 , was found to be at 0.42 for an OCMGprocessed sample, and at 0.48 for a single crystal.2 Such ahigh value ofh0 is not reached in any other high-Tc systemhaving both a highj c and H irr , as YBa2Cu3Oy2d ~YBCO!

shows the peak at around 0.33,9 and Bi2Sr2CaCu2O81d ,Bi2Sr2Ca2Cu3O101d have 0.22–0.25.9,10 As a consequence,flux creep effects are dominating at fields aboveh0 .

In this letter, we present the results of such a scalinganalysis of the pinning forces of a pure NEG sample in thetemperature range 65 K<T<90 K, where all required pa-rameters likeH irr and the maximum of the volume pinningforce, Fp,max, are directly accessible to our experimentalfield range. From the scaling of the pinning forces, we de-duce the character of the microscopic pinning mechanism.

Bulk samples of NEG were prepared using the OCMGprocess in 0.1% partial pressure of oxygen. The crystalliza-tion process was initialized by a MgO100& seed which wasplaced at the center on top of the basal pellet surface. Thedetails of the heat treatment schedules are described in detailelsewhere.7 Finally, a sample is cut to dimensionsa3b3c52.631.731 mm3.

Magnetization loops~MHLs! are measured using a com-mercial superconducting quantum interference device~SQUID! magnetometer11 with a maximum field of67 T;Haic axis. To minimize field inhomogeneities, the scanlength is set to 1 cm. Furthermore, the homogeneity of thesamples was ensured by investigating them by means ofmagneto-optic imaging.12 Critical current densities,j c(Ha ,T), are obtained from the MHLs by means of theextended Bean formalism. Due to the large paramagneticmoment of Nd and mainly Gd, corrections to the MHLs haveto be carried out carefully.

Figure 1 presentsj c(Ha ,T) data in the temperaturerange 65 K<T<90 K. The secondary peak effect~PE! isstrongly developed in the NEG system, as is clearly visiblefrom this graph. At T577 K, the peak positionBp is'2.8 T. The PE is visible up to 88 K before it vanishes.Note that the width of the peak is also considerably largerthan in other superconductors.13 At temperatures below 70K, this large peak width leads to aj c being quasi indepen-dent of field in a broad field range, which is a very importantachievement for future applications.

In Fig. 2, we present the result of the scaling analysis ofa!Electronic mail: koblischka@istec.or.jp

APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 16 19 OCTOBER 1998

23510003-6951/98/73(16)/2351/3/$15.00 © 1998 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.240.225.44 On: Sat, 20 Dec 2014 09:01:33

Fp , normalized by its maximum value,Fp,max, and plottedversus the reduced field,h5Ha /H irr . In the temperaturerange 65–90 K, all parameters are directly accessible to ourexperimental field range.H irr is determined from the MHLsusing a criterion of 105 A/m2. As H irr depends on the sweeprate of Ha , it is important to compare only data with thesame electric field across the sample~i.e., identical sweeprate ofHa).14 The resulting scaling is found to be excellent;necessary corrections toH irr by means of the scaling werewithin the experimental error. The peak in theFp diagram islocated ath0'0.5. In Refs. 2 and 9 it was shown that such ahigh peak position can only be explained assuming a domi-

nant pinning provided bysuperconductingdefects with apinning interaction volumeVpin;j2d ~j denotes the coher-ence length andd the intervortex spacing!, instead of theusual normal–conducting inclusions like Y2BaCuO5

particles.9 These latter particles lead to a peak ath050.33,commonly found in Y-based 123-superconductors.8,9 Thehigh position of the peak in the NEG superconductors causeseffects of flux creep to be considerably reduced at elevatedtemperatures, which is also an important aspect for applica-tions.

In order to prove the existence of a weaker supercon-ducting phase providing the additional flux pinning, tempera-ture scans in both field cooling~FCC! and field warming~FCW! modes were carried out in various fields between 10mT and 7 T. These scans are performed in a continuoussweep mode with a sweep ratedT/dt535 mK/min in thetransition region. This ensures a large number of data pointseven in a sharp superconducting transition. In Fig. 3, the dcsusceptibilityx(T) is plotted for various applied fields. Thedata shown were recorded during FCC runs; the FCW dataare omitted for clarity. The inset presents the superconduct-ing transition for an ordinary measurement, i.e., warming upthe sample in a field of 1 mT after zero-field cooling. Thisclearly confirms that the sample appears to be single phase,and has aTc,onsetof 93.1 K and a transition width of'1.5 K.When performing field-cooling measurements in large exter-nal fields, the magnetic moments of Nd and Gd cause a con-siderable increase ofx on decreasingT. Therefore, the mag-netic moment is positive even in the superconducting state.Note the behavior ofx(T) aboveTc , which is following aCurie–Weiss law. In fields between 1 mT and 2 T, the su-perconducting transitions are quite sharp;Tc is found to re-

FIG. 1. Critical current densities,j c(Ha ,T), inferred from the magnetiza-tion loops in the temperature range 65 K<T<90 K. Note, the strongly de-veloped peak effect and the large width of the peak below 77 K.

FIG. 2. Scaling of the normalized pinning forces,Fp /Fp,max, vs the reducedfield, h5Ha /H irr . The scaling is found to be excellent; yielding a peak ath0'0.5.

FIG. 3. The dc susceptibility of NEG as a function of temperature forvarious applied fields. The data shown are taken during continuous coolingin field ~FCC!; the FCW data are omitted for clarity. The arrow points to thekink in the transition, which indicates the presence of a weaker supercon-ducting phase. Note also the paramagnetic moment mainly caused by Gd.The inset shows the transition measured after zero-field cooling and subse-quently applying a field of 1 mT.

2352 Appl. Phys. Lett., Vol. 73, No. 16, 19 October 1998 Koblischka, Muralidhar, and Murakami

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.240.225.44 On: Sat, 20 Dec 2014 09:01:33

duce slightly as a function of field to 91.5 K at 5 T.x(T)below Tc still increases on decreasingT. However, at fieldsof 5 T and above, a step in the superconducting transitionregion is clearly visible which we ascribe to the presence ofa slightly weaker superconducting second phase. Such a stepis also seen in NdBCO single crystals and OCMG samples asreported elsewhere.15 This phase is due to a Nd-rich phase,as observed by scanning tunneling microscopy and transmis-sion electron microscopy.16 Up to now, this phase could notbe detected in any magnetic measurements.17 In a small ap-plied field, there is only one detectableTc due to proximityeffects leading to a quasihomogeneousTc throughout thesample. In FCC measurements, the difference inTc ~or k!may become visible above a certain field, which is largeenough to suppress the weaker superconducting phase.

Furthermore, in NEG samples the magnetic moments ofthe Gd and Nd atoms are another possible source which mayincrease the flux pinning. Earlier studies of polycrystallineGdBa2Cu3O72d ~GdBCO! showed that the paramagnetismand superconductivity can coexist; however, pure GdBCOproved to be inferior as compared to YBCO.18 Thesesamples were not prepared in reduced oxygen atmosphere;so the smallj c values obtained can be explained. Here it isimportant to note that the diamagnetic moments in oursamples are considerably larger than the paramagnetic con-tribution; at lowT there is no detectable background distort-ing the shape of the MHLs. The moments of Gd or Nd maycause pair breaking when located on the Ba sites; and thismay contribute to increase the spatial variation of the super-conducting properties; especially if the Gd atoms are homo-geneously distributed in the sample providing a uniform dis-tribution of the pinning sites ~‘‘constant pinningwavelength’’4!. Pinning provided by the magnetic interac-tion alone will not cause a large contribution toFp as thecore interactions dominate in high-Tc superconductors (k>50) by a factor ofk/4 ln k.19 Therefore, the main source ofpinning in NEG samples is the spatial variation ofTc , whichis enhanced here as compared to both melt-processed andsingle-crystalline NdBCO. This may be due to a uniformdistribution of the superconducting defects throughout thesample.

In conclusion, we have performed a scaling analysis ofFp vs h, and found evidence for a strong pinning provided byspatial variations ofTc . By means of field cooling andwarming measurements, we could prove the existence of a

weaker superconducting second phase. The large magneticmoment of Gd does not create magnetic pinning, but cancontribute to the pinning by pair breaking when located atthe Ba sites. The pinning provided by the variation ofTc isincreased for the NEG samples as compared to pure NdBCO,which we ascribe to an improved uniform distribution of thesuperconducting defects.

1M. Murakami, N. Sakai, T. Higuchi, and S. I. Yoo, Supercond. Sci. Tech-nol. 9, 1015~1996!.

2M. R. Koblischka, A. J. J. van Dalen, T. Higuchi, S. I. Yoo, and M.Murakami, Phys. Rev. B58, 2863~1998!.

3H. Ullmaier, Irreversible Properties of Type II Superconductors~Springer,Berlin, 1975!, Chap. 3; A. M. Campbell and J. E. Evetts, Adv. Phys.21,199 ~1972!.

4D. Dew-Hughes, Philos. Mag.30, 293 ~1974!.5G. Blatter, M. V. Feigel’man, V. B. Geshkenbein, A. I. Larkin, and V. M.Vinokur, Rev. Mod. Phys.66, 1125~1994!.

6A. J. J. van Dalen, M. R. Koblischka, K. Sawada, H. Kojo, T. Higuchi,and M. Murakami, Supercond. Sci. Technol.9, 659 ~1996!.

7M. Muralidhar, H. S. Chauhan, T. Saitoh, K. Kamada, K. Segawa, and M.Murakami, Supercond. Sci. Technol.10, 663 ~1997!.

8L. Civale, M. W. McElfresh, A. D. Marwick, F. Holtzberg, C. Feild, J. R.Thompson, and D. K. Christen, Phys. Rev. B43, 13 732~1991!; L. Klein,E. R. Yacoby, Y. Yeshurun, A. Erb, G. Mu¨ller-Vogt, V. Breit, and H.Wuhl, ibid. 49, 4403 ~1994!; J. S. Satchell, R. G. Humphreys, N. G.Chow, J. A. Edwards, and M. J. Kane, Nature~London! 334, 331 ~1988!;H. Yamasaki, K. Endo, S. Kosaka, M. Umeda, S. Yoshida, and K. Ka-jimura, Phys. Rev. Lett.70, 3331~1993!.

9M. R. Koblischka, Physica C282-287, 2197 ~1997!, and referencestherein.

10P. Fabbricatore, C. Priano, A. Sciutti, G. Gemme, R. Musenich, R. Parodi,F. Gomory, and J. R. Thompson, Phys. Rev. B54, 12 543~1996!.

11Quantum Design, San Diego, CA 92121, Model MPMS7 and Model XL.12M. R. Koblischka and R. J. Wijngaarden, Supercond. Sci. Technol.8, 199

~1995!.13Scalings ofFp and j c were reviewed recently by M. Jirsa and L. Pu˚st,

Physica C291, 17 ~1997!. The peak width is further, an important param-eter in a newly developed critical state model incorporating the PE; see T.H. Johansen, M. R. Koblischka, H. Bratsberg, and P. O. Hetland, Phys.Rev. B56, 11 273~1997!.

14H. H. Wen and Z. X. Zhao, Phys. Rev. B50, 13 853~1994!.15M. R. Koblischka, M. Muralidhar, T. Higuchi, K. Waki, N. Chikumoto,

and M. Murakami~unpublished!.16Using STM and TEM on NdBCO samples, several regions of different

color were found on the sample surface with a dimension of about 10 nmand a spacing of about 30 nm. These defects are ascribed to the compo-sition fluctuations. For details, see, T. Egi, J. G. Wen, K. Kuroda, H.Unoki, and N. Koshizuka, Appl. Phys. Lett.67, 2406 ~1995!; N. Chiku-moto, J. Yoshioka, and M. Murakami, Physica C291, 79 ~1997!.

17B. Chen, K. Kuroda, and N. Koshizuka, inAdvances in SuperconductivityIX ~Springer, Tokyo, 1996!, p. 211

18H. Theuss and H. Kronmu¨ller, Physica C242, 155 ~1995!.19R. P. Hubener,Magnetic Flux Structures in Superconductors~Springer,

Berlin, 1979!.

2353Appl. Phys. Lett., Vol. 73, No. 16, 19 October 1998 Koblischka, Muralidhar, and Murakami

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.240.225.44 On: Sat, 20 Dec 2014 09:01:33