Fishtail effect, magnetic properties and critical current density of Gd-added PMP YBCO

Ž .Physica C 297 1998 75–84

Fishtail effect, magnetic properties and critical current density ofGd-added PMP YBCO

Yong Feng a,b,c,), Lian Zhou a, J.G. Wen b, N. Koshizuka b, A. Sulpice c,J.L. Tholence c, J.C. Vallier c, P. Monceau c

a Northwest Institute for Nonferrous Metal Research, P.O. Box 51, Xi’an, Shaanxi 710016, Chinab SuperconductiÕity Research Laboratory, ISTEC, 1-10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan

c CRTBT and LCMI, CNRS, BP166, 38042 Grenoble Cedex 9, France

Received 1 September 1997; revised 24 September 1997; accepted 13 November 1997

Abstract

The magnetization curves of YBa Cu O and Y Gd Ba Cu O samples prepared by a powder melting process2 3 y 0.4 0.6 2 3 y

technique were measured with a superconducting quantum interference device magnetometer at different temperatures.Fishtail effects are observed below 70 K with the HHc configuration in the Gd-added sample, while no peak effect is foundin YBCO. The origin of the fishtail is discussed. It is found that J and flux pinning can be increased by the Gd addition.c

The magnitude of the improvement of J increases with the magnetic field. The reduction of the size of Y BaCuOc 2 5

particles, stress-field pinning and magnetic pinning induced by the substitution of Gd for Y may explain the enhancement ofJ and flux pinning. q 1998 Elsevier Science B.V.c

PACS: 74.72B; 74.60G; 74.60J

Keywords: YBCO; Fishtail effect; Flux pinning; Gd addition

1. Introduction

For the practical application of high-temperaturesuperconductors, it is necessary to obtain high criti-

Ž .cal current densities J . Unfortunately, J is disap-c cŽ .pointingly low in sintered YBa Cu O YBCO2 3 y

samples because of the serious weak links and thegranularity in these materials. In zero field and at 77K, only J values up to several 1000 Arcm2 arec

obtained and J dramatically drops even in a veryc

) Corresponding author. Tel.: q86 29 6224487; Fax: q86 29623 1103; E-mail: [email protected].

small applied field. In recent years, much effort hasbeen spent in enhancing the critical current densityof YBCO and great progress has been made. Todate, several methods such as melt-textured growthw x w x w x1 , liquid phase process 2 , quench melt growth 3 ,

w xand powder melting process 4 have been developedto fabricate high-J YBCO superconductors. Re-c

w xcently, Egi et al. 5 have prepared high JcŽ .NdBa Cu O Nd123 single crystals by a travelling2 3 y

Ž .solvent floating zone TSFZ approach. Moreover,w xYao et al. 6,7 have systematically investigated the

growth dynamic of Nd123 and have prepared largeNd123 single crystals. Despite the remarkable en-

0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0921-4534 97 01847-9

( )Y. Feng et al.rPhysica C 297 1998 75–8476

hancement of the low-field J , its performance at ac

high magnetic field is disappointing. Therefore, it isnecessary to further improve J in a high magneticc

field in order to bring about their application toelectrical engineering. One of the most effectiveways is to introduce strong flux pinning centers inthe YBCO system.

The flux pinning process in YBCO is a verycomplex phenomenon, which is not yet clearly un-derstood and should be further investigated. Recentstudies show that oxygen defects, stacking faults,dislocations, twin boundaries, and columnar defectsintroduced by ion irradiations are found to be effec-

w x w xtive pinning centers in YBCO 8–12 . Jin et al. 13suggested that fine-scale defects created during thedecomposition of YBa Cu O may act as pinning2 4 8

centers in YBCO. Furthermore, although the exactŽ .flux pinning mechanism of Y BaCuO Y211 is still2 5

unknown, it is widely accepted that both mechanicalproperties and flux pinning of YBCO are improved

w xby the fine Y211 embedding 14 . Some scientistsalso found that the thickness of the interfaces be-tween Y123 and Y211 is comparable to the coher-ence length in the ab-plane of YBCO superconduc-tors. To date, the effectiveness of possible pinningcenters has not been established.

However, there is much evidence that the fluxpinning of YBCO can be further enhanced through

w xchemical doping of Pt, Rh, CeO , etc. 15,16 . Some2

authors found that J values and flux pinning can bec

improved by adding elements to YBCO samples. Hfand Hf–Ca doping at the Y site could increase theintragrain J in the YBCO system. It is thought thatc

the preferential substitution of Hf or Hf–Ca for Yw xcan act as a pinning center 17,18 . We found that

the substitutions of Ho for Y and Sn for Cu lead tow xan improvement of J and microstructure 19,20 .c

Because chemical doping can be easily controlledand is non-destructive and very effective in improv-ing J , the further investigation of chemical dopingc

is of great importance both for physical understand-ing and practical applications. In the present paper,the YBa Cu O and Y Gd Ba Cu O supercon-2 3 y 0.4 0.6 2 3 y

ductors were prepared by the powder melting pro-cess method and were investigated through differen-tial thermal analysis, AC susceptibility, and super-

Ž .conducting quantum interference device SQUIDmagnetometer. The effects of the Gd substitution for

Y on superconducting properties and flux pinningare described.

2. Experimental

Samples with nominal compositions ofŽ .YBa Cu O YBCO and Y Gd Ba Cu O2 3 y 0.4 0.6 2 3 y

Ž .Gd06 were fabricated by the powder melting pro-cess method. The detailed description of the prepara-

w xtion process has been reported previously 21 . Inshort, Y211 and BaCuO powders were synthesized2

through a solid state reaction technique using Y O ,2 3

Gd O , BaCO and CuO. Then, these powders were2 3 3

well ground in an appropriate ratio and were coldpressed into a rectangular shape. The bars were putinto a tube furnace with the highest temperaturesbetween 1030 and 10808C. The moving rate wasaround 2 mmrh. Finally, the samples were annealedat 5508C for 40 h in flowing oxygen to insurecomplete oxygenation of the samples. Also, anneal-ing was continued to 4008C at a slow cooling rateŽ .38Crh for possible additional oxygen loading andthen to room temperature at 58Crmin. The sampleshape is rectangular and the dimensions are 0.8=0.3=0.06 cm3 and 0.9=0.35=0.07 cm3, respectivelyfor the YBCO and Gd06 samples.

In the experimental process, the magnetic field isparallel to the longer dimension of the samples. Itmeans that the field is perpendicular to the c-axis ofthe sample. In order to confirm this, we first mea-sured the magnetic hysteresis loop at 60 K in fieldperpendicular to the longer dimension of the speci-men. Then, we slowly rotated the sample and testedagain. It is found that the direction of the appliedfield in which the magnetic hysteresis loop is largestis just perpendicular to the longer dimension of thesample. This process insured that the longer dimen-sion of the sample is perpendicular to the crystallinec-axis.

Critical temperature was measured by a supercon-Ž .ducting quantum interference device SQUID mag-

netometer. Magnetization measurements were carriedout using a SQUID magnetometer with the magneticfield perpendicular to the c-axis of the sample atdifferent temperatures. All the magnetization mea-surements were performed by first cooling the sam-ple in zero field and then applying a field to begin

( )Y. Feng et al.rPhysica C 297 1998 75–84 77

Table 1Critical temperatures for the two samples

Ž . Ž .T K DT K

YBCO 92 1.0Gd06 92.8 1.1

the measurement. After each measurement finishedat a given temperature, the sample was warmedabove T to drive out any trapped flux and thenc

cooled to the measuring temperature.We measured several YBCO and Gd06 samples.

The results reported below are representative of thesesamples. Other results are very similar to thosereported here.

3. Results and discussion

Table 1 shows critical temperatures of YBCO andGd06 samples. It is found that T values for the twoc

samples are around 92 K. This result is in agreementwith the previous work, in which the magnetic mo-ment of rare-earth elements has no detrimental effecton T . The transition width is about 1 K, indicatingc

that there is a good homogeneity in these samples.The typical temperature dependence of DC magneti-zation for the Gd06 specimen is shown in Fig. 1. Theapplied field was perpendicular to the c-axis. The

Ž .DTA result reveals that the melting temperature Tm

is changed by the Gd addition. T is increased frommŽ . Ž .9758C YBCO to 10328C Gd06 in air.

Fig. 1. Temperature dependence of DC magnetization for theGd06 sample with H Hc.

Fig. 2. X-ray diffraction pattern of the Gd06 specimen.

X-ray diffraction pattern was performed on aPhilips 1700 diffractometer with Cu Ka radiation.The results show a strong enhancement in the strength

Ž .of the 001 peaks in the YBCO and Gd06 samples,which indicates a perfect c-axis orientation in thesespecimens. The typical X-ray diffraction spectrum ofthe Gd06 sample is given in Fig. 2. Also, it can beobserved in Fig. 2 that there is a small peak of the211 phase in this spectrum. The weak peak revealsthe presence of the 211 particles in the sample.

Fig. 3 shows the SEM photographs of the Gd06sample. These observations indicate that the plate-shaped 123 crystals are well oriented with theirab-planes parallel to the longer dimension of thesample. The sample is very dense, showing hardlyany voids or microcracks. In addition, grain bound-aries are discontinuous and disappear in many re-gions. Thus, 123 crystals can grow with each other.The intergrowth between 123 grains can result in theelimination of weak links. Furthermore, it can befound that many dispersively distributed 211 parti-cles exist in the 123 matrix.

Figs. 4 and 5 give the field dependence of magne-tization curves for the YBCO and Gd06 samplesmeasured at different temperatures with the magneticfield perpendicular to the c-axis. It can be observedthat the hysteresis of the magnetization increaseswhen the temperature drops, which means that theflux pinning force of the specimens is graduallyimproved. This is because the hysteresis is attributedto the presence of flux pinning sites in materials.

w xAccording to the results of Campbell et al. 22 ,these magnetization curves belong to the typically


Ž .Fig. 3. Fracture photographs of the Gd06 sample. a Low magni-Ž .fication. b High magnification.

Fig. 4. The magnetization curves for the YBCO specimen atdifferent temperatures with field perpendicular to the c-axis.

Fig. 5. Magnetic hysteresis loops for the Gd06 sample at varioustemperatures with H Hc.

magnetic hysteresis loops with strong flux pinning. Itcan be seen from Figs. 4 and 5 that the maximumdiamagnetic field, H changes with temperature.m

w xPreviously, Malozemoff 23 found that H could bem

described by

H sAJ 1yD , 1Ž . Ž .m c

where A is a constant related to the dimension of thesamples and D is the diamagnetization factor. Thisequation suggests that H should be relevant to them

dimension of the specimens, which has been con-firmed by some authors. They observed that Hm

w xdrops with decreasing the dimension 23 . Figs. 6and 7 illustrate H as a function of temperature form

all the samples. H decreases as the temperaturem

increases, a behavior which is similar to that of the

Ž .Fig. 6. The variation of maximum diamagnetic field H withm

temperature in the YBCO sample.


Ž .Fig. 7. The maximum diamagnetic field H as a function ofm

temperature for the Gd06 sample.

critical current density. For the Gd06 sample, H ism

about 0.3 T at 50 K, while it drops to 0.05 T whenTs80 K.

On the other hand, it can be observed that theshape of the curves for the Gd06 sample stronglysuggests the superposition of a reversible componentand a hysteresis component at high temperatures.The reversible component can be related to paramag-netic Gd ions with negligible interaction between thelocal magnetic moments and the superconductingelectrons. The temperature dependence of magnetiza-tion at Hs3 T for the Gd06 specimen is given inFig. 8. It is found that the magnetization above Tc

can be well described by the Curie–Weiss law1rmA Tyu , 2Ž . Ž .where u is the paramagnetic temperature. By com-puter fitting, u is around y4.15 for the Gd-added

Fig. 8. Temperature dependence of magnetization for the Gd06Ž .sample at 3 T H Hc .

Fig. 9. Critical current density J vs. magnetic field at differentcŽ .temperatures for YBCO H Hc .

sample. These observations clearly show that there isparamagnetism in the Gd06 specimen although Y isonly partially substituted by Gd.

The critical current density was calculated byw xusing the Bean model 24 . In the case of field

perpendicular to the c-axis of a sample, J is givenc

by the following equation

J s20 MqyMy rd. 3Ž . Ž .c

Here, Mq and My are magnetization moments atincreasing and decreasing magnetic fields, respec-tively, and d is the sample thickness along thedirection of the field penetration. Figs. 9 and 10illustrate the J –H properties of the YBCO andc

Gd06 samples at different temperatures with theHHc configuration. As for the YBCO sample, Jc

decreases quickly with the magnetic field below 1 T,whereas J falls off much more slowly above 1 T.c

Fig. 10. Magnetic field dependence of J at various temperaturescŽ .for the Gd06 sample H Hc .


The result indicates that the weak link is signifi-cantly overcome and the flux pinning is relativelystrong.

In addition, no fishtail effect can be observed inYBCO, while this anomalous phenomenon is foundin the Gd06 sample. Furthermore, the similar anoma-lies can be also seen in other high-T superconduc-c

tors including melt-processed YBCO, NdBCO, Bi–w xSr–Ca–Cu–O, etc. 25–27 . To date, two kinds of

fishtail effects have been found in these materials. InYBCO, broad and temperature-dependent fishtailsare observed. As for Bi–Sr–Ca–Cu–O superconduc-tors, a sharp and temperature-independent peak isfound. Although the origin of this phenomenon is notfully understood, several mechanisms have been de-

w xveloped to interpret it. Daeumling et al. 28 at-tributed the fishtail effect to the flux pinning createdby the oxygen-deficient regions. However, studies onmelt-textured YBCO subjected to prolonged oxy-genation show that the peak effects continue to existw x29 . The location and shape of the fishtail effect inthe specimen subjected to 40 h oxygenation are thesame as those in the sample subjected to 90 hoxygenation, so it appears that the peak effect is notdue to the flux pinning induced by the oxygen-defi-cient region. Moreover, the synchronization effectsof the increased disorder in the vortex lattice and thematching effect between the vortex lattice and thetwin structure are proposed to explain the fishtail

w xeffect 30 . Recently, some authors suggested that thepeak effect in melt-processed YBCO can be welldescribed by the collective pinning theory at high

w xtemperatures 31 . Also, a crossover from single tocollective flux creep is believed to be the origin of

w xthe fishtail effect 32 . Here, we do not think that theflux pinning induced by the oxygen-deficient regionis responsible for the peak effect in the Gd06 sample.It is interesting to note that the fishtail is observedfor the HHc configuration in the Gd06 sample,which has not been seen in the oxygen-deficientYBCO. In addition, the peak effect is absent in Fig.10 when Ts80 K. Therefore, we believe that theorigin of fishtails in the Gd06 specimen are notoxygen defects. A similar result is found in NdBCOand SmBCO, in which the peak effects are created

w xby Nd or Sm substitutions for the Ba sites 33 . It isŽ .important to note that the peak field H at whichp

J reaches its maximum value is strongly tempera-c

ture dependent and H shifts to a higher field withp

decreasing temperature as shown in Fig. 11. So, thefishtail effect is not attributed to the matching effect.On the other hand, it can be observed from Fig. 11that H decreases linearly with temperature andp

ssd H rdTs0.1 TrK. This is much smaller thanpw xthe previous reports of ss0.7–10 TrK 34 . Klein

w xet al. 35 found a power law behavior H s4.2=p5Ž .3r210 1y t in an untwinned YBCO single crystal.

They proposed that the peak effect may be due to apercolating network of reversible zones since thetemperature dependence of H is similar to that ofp

the irreversibility field. However, the linear tempera-ture dependence of H is found in the Gd06 sample.p

This kind of relation was also obtained in REBCOw xsingle crystals with REsY, Yb and Dy 36 . Thus,

the percolating network of reversible zones is alsonot the reason of the fishtail in our sample. It isconsidered that the fishtail effect in the Gd06 speci-men may be due to the cation defects orrand theparamagnetism in the sample induced by the localsubstitutions of Gd for Y. The exact mechanism ofthe peak effect in our sample is not clear and shouldbe further investigated.

Fig. 12 gives a plot of the reduced critical currentŽdensity J rJ J corresponding to the highest Jc cp cp c

.value vs. the reduced field HrH at different tem-p

peratures. The curves at 50 K and 60 K can be scaledin a single master curve, which means that themagnetization behavior is dominated by a single typeof pinning center at 50 K and 60 K. Unfortunately,

Fig. 11. The peak field H as a function of temperature for thep

Gd06 specimen.


Fig. 12. The reduced critical current density J r J vs. thec cp

reduced field HrH in the Gd06 sample.p

the plot at 70 K cannot be scaled in the same curveas those at 50 K and 60 K. This indicates that theflux pinning behavior at 70 K is different from thoseat 50 K and 60 K.

In order to investigate the flux pinning character-istic, the flux pinning force density is calculated.Figs. 13 and 14 give the field dependence of F forp

the samples at various temperatures. F increasesp

with increasing the field within the observed field at50–70 K. When temperature is raised to 80 K, Fp

initially increases with the field and reaches a maxi-mum value. Then, F drops as further increasing thep

field. There is a single peak in each sample at 80 K.The dependence of J on temperature at differentc

fields was studied and Figs. 15 and 16 show Jc

values as a function of temperature for all the sam-ples. It can be observed that the decrease of J onc

Fig. 13. The flux pinning force density F as a function of fieldp

and temperature for YBCO.

Fig. 14. Flux pinning force density F vs. magnetic field in thep

Gd06 specimen.

temperature at low fields is much lower than that athigh fields, which implies that the flux pinningmechanism is different in different fields. Martinez

w xet al. 37 systematically investigated the J behaviorc

of melt-textured YBCO in wide temperatures andmagnetic fields. They found that the interfaces be-tween Y123 and Y211 particles are the dominantpinning centers in the low field region, while othercrystal defects become more active in high fields.Our recent work shows that the stacking faults canact as very effective pinning centers in the low fieldsand high fields, but the role of stacking faults aspinning centers is different under different fieldsw x38 . In addition, the behavior of other defects suchas point defects and dislocations changes with themagnetic field.

From Figs. 9 and 10, it can be seen that J isc

increased by the Gd addition ranging from 1.05 to

Fig. 15. The dependence of J on temperature at different fieldsc

for the YBCO sample.


Fig. 16. J as a function of temperature for the Gd06 sample.c

Ž .3.5 times in different fields. J is about 8400 YBCOc2 Ž .and 24 500 Arcm Gd06 at 60 K in 3 T. Fig. 17

illustrates the ratio of J in YBCO over J in thec c

Gd06 sample. The magnitude of the enhancement ofJ in high fields is higher than that in low fields. Itc

increases with the field from 1.1 in 0.1 T to 3.5 in 4T at 50 K. This result clearly indicates that the Gdaddition has different contributions to the increase inJ at high fields and low fields. The improvement ofc

J over pure YBCO is very interesting and it isc

expected that J will be enhanced high enough forc

electrical engineering applications through furtherrefinement of the composition and optimization forprocessing conditions.

Based on the above discussion, we can concludethat the substitution of Gd for Y can help improve Jc

and flux pinning in YBCO superconductors. Also,some researchers have found that J can be in-c

Fig. 17. Field dependence of the ratio of J in YBCO over J inc cŽ .Gd06 J r J .c,Gd c,Y

creased by other rare-earth element additions. The20% substitutions of Sm and Eu for Y in sinteredYBCO lead to an enhancement of the intragrain Jc

from 11 000 Arcm2 to 25 000 and 27 000 Arcm2 atw x77 K in 0.9 T 39 . Recent work demonstrates that

the flux pinning is improved by the partial substitu-tions of Pr for Y. It is thought that the increase influx pinning by Pr ions is mainly induced by thesuppression of superconducting order parameters inthe vicinity of Pr ions via a magnetic pair breakingw x40 . In addition, Nd O rLa O additions have been2 3 2 3

found to result in enhancement of the flux pinning inmelt-textured YBCO. It is considered that the in-creased J may be due to pinning effects created byc

NdrLa ions being present on Y andror Ba sites inw xthe YBCO lattice 41 .

However, although the effects of the rare-earthelement additions in YBCO samples on flux pinninghave been investigated, the mechanism of the in-crease in J and flux pinning is not clear. On thec

basis of our results, we think that the enhancement ofJ may be related to the following reasons. First, thec

size of Y211 particles is significantly reduced fromŽ . Ž .3.2 mm YBCO to 0.96 mm Gd06 . The size of

211 particles was determined by a chemical method.The size distributions of 211 for YBCO and Gd06are shown in Figs. 18 and 19. The reduction of the211 size will be helpful to diminish microcracks andwill lead to more interfaces between Y123 and Y211phases. The interface of Y123 and Y211 is found tobe effective pinning centers in YBCO. As a result, Jc

and flux pinning are improved. This is confirmed by

Fig. 18. The distribution of the size of 211 particles in YBCO.


Fig. 19. The size distribution of 211 particles in the Gd06specimen.

other reports, in which the refinement of the Y211size caused by the additions of PtO or CeO can2 2

w xenhance J and obtain better microstructure 42 .c

Secondly, it is well known that the flux pinning willbe introduced by the elastic interaction between thevortex lattice and the stress field in conventional

w xhard superconductors 43 . It is expected that thelocal lattice mismatch can be created by the partialsubstitutions of rare-earth elements with differentionic radii for Y in YBCO, which will lead to theformation of the stress field. Consequently, the fluxpinning can be introduced. There is a stress fieldcaused by the Gd addition because the radius ofGd3q is larger than that of Y3q. Thus, an additionalstrong flux pinning will be formed in Gd-dopedYBCO sample. So, J is increased. A recent calcula-c

w xtion has given an evidence for it 44 . Furthermore, itcan be observed in Fig. 8 that there is paramagnetismin the Gd06 specimen, which will induce the mag-netic pinning. This may also be responsible for theimprovement of J .c

4. Conclusion

We have carried out magnetic measurements ofYBa Cu O and Y Gd Ba Cu O prepared by2 3 y 0.4 0.6 2 3 y

the powder melting process method through a SQUIDmagnetometer. Anomalous peaks are found in theGd-added sample below 70 K with the magneticfield perpendicular to the c-axis, whereas no fishtail

can be observed in YBCO. The peak field H de-p

creases linearly with the field. The fishtail may beattributed to the cation defects orrand the paramag-netism in the Gd-doped sample created by the Gdaddition. It is found that J can be improved by thec

substitution of Gd for Y, which may be due to thereduction of the size of Y211 particles, magneticpinning and stress-field pinning. This should be fur-ther investigated to separate the relative contribu-tions of these aspects.

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Fishtail effect, magnetic properties and critical current density of Gd-added PMP YBCO

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