Correlation of magnetic and magneto-optical properties with microstructure analysis in melt-textured ž Nd Sm Gd / Ba Cu O

Transcription

1 Ž. Physia C Correlation of magneti and magneto-optial properties with mirostruture analysis in melt-textured ž Nd Sm Gd / Ba Cu O y A. Das 1, M.R. Koblishka ), M. Jirsa 2, M. Muralidhar, S. Koishikawa, N. Sakai, M. Murakami SuperondutiÕity Researh Laboratory, DiÕision 3, International SuperondutiÕity Tehnology Center, Shibaura, Minato-ku, Tokyo , Japan Reeived 29 Marh 1999; reeived in revised form 13 April 1999; aepted 27 April 1999 Abstrat Magneti properties of the melt-proessed ternary superondutor Ž Nd Sm Gd. Ba Cu O Ž NSG y with non-homogeneously distributed NSG-211 partiles were investigated by means of magneti and magneto-optial measurements. Flux distributions obtained by means of magneto-opti imaging after zero field-ooling and field-ooling were analyzed in order to understand the flux penetration behavior of this ompound. The urrent-arrying length-sale was determined from the reverse leg of the magnetization hysteresis loops and ompared to the atual dimensions of the sample. This was done both on the whole sample and on three individual domains to whih the sample was subsequently divided. Magnetization hysteresis loops of all samples exhibited a strongly developed seondary peak in the temperature range 30 KFTF77 K. The ritial urrent and pinning fore densities derived from these data were normalized to the values at the peak position, Jmax and F max, respetively, and analyzed as a funtion of applied field redued both to the respetive peak field and irreversibility field. q 1999 Published by Elsevier Siene B.V. All rights reserved. PACS: E; Ge; Jg Ž. Keywords: Magneti and magneto-optial properties; Nd Sm Gd Ba Cu O ; Melt-textured superondutor y 1. Introdution Light rare earth Ž LRE. Ba Cu O Ž 2 3 y where LRE denotes Nd, Eu, Sm, Gd. superondutors melt- ) Corresponding author. Fax: q ; koblishka@iste.or.jp 1 Present address: Solid State Physis Laboratory, Delhi , India. 2 On leave from Institute of Physis, ASCR, Na Slovane 2, CZ Praha 8, Czeh Republi. proessed in low oxygen partial pressure exhibit a high irreversibility field, B irr, aompanied by a large ritial urrent density, J, in intermediate fields, at the maximum of the seondary Ž fishtail. peak. At Ts77 K, B irr )7 T and J usually ap- proahes the order of magnitude of 10 9 Am y2 at the peak position w1,2 x. A harateristi feature of the LRE-123 superondutors is the existene of a solid solution between the LRE atoms and Ba, whih leads to the formation of a LRE-rih phase with weaker superonduting properties. Using the oxygen-on r99r$ - see front matter q 1999 Published by Elsevier Siene B.V. All rights reserved. Ž. PII: S

2 170 A. Das et al.rphysia C trolled melt growth Ž OCMG. proess to prepare the samples, the amount of this LRE-rih phase an be ontrolled so that a high superonduting transition temperature, T, results wx 3. Very reently, it has been reported that OCMG proessed Ž LRE, LRE, LRE Ba 2Cu 3Oy superondutors Žthat is, three different LRR elements mixed together at the rare earth site. show the superonduting transition temperature T in the range of 93 K and J even higher than in the samples pre- pared under normal oxygen pressure wx 4. The large values of J in these materials are again due to a pronouned fishtail peak. The analysis of the fishtail effet in 123 Ž i.e., Y and Tm. ompounds w5 7x led to the onlusion that the random pinning disorder due to oxygen defiient zones is responsible for the formation of the peak. The even better magneti properties of the ternary ompounds are related to the inreased disorder on the rare earth site whih auses a more uniform distribution of the LRE-rih phases wx 8. Furthermore, it was found that this ombination of three LRE elements enables the formation of submiron-sized Gd BaCuO Ž Gd partiles, whih an effetively inrease the ritial urw9,10 x. These so-alled 211 partiles Žor 422 in the ase of Nd. are randomly distributed rent density throughout the Ž LRE, LRE, LRE.-123 matrix w11 x A pronouned fishtail effet is ommonly found in the ternary ompounds; the peak position at 77 K is typially loated at around T, and the peak height usually exeeds that of the entral peak in the magnetization loop. The typial fishtail shape is usually modified, espeially on the low-field side of the peak. This implies that aside of the random pinning disorder another type of pinning struture is ative in the intermediate field range. The orresponding pinning mehanism should have different field and temperature harateristis. The role of the non-superonduting Y-211 andror Nd-422 partiles in the pinning proess requires therefore a detailed study. One has to take into aount also the inhomogeneity of the distribution of the non-superonduting partiles in a given sample. The inhomogeneity is losely related to the variation of growth ondiw12x for the ase of tions as doumented in Ref. Y-211 partiles trapped in Y-123 rystals. The phenomenon of entrapment of Y-211 partiles in melttextured Y-123 rystals was qualitatively explained by the pushingrtrapping model wx 5. This model explains the behavior of a foreign partile existing in the front of an advaning solid liquid interfae during solidifiation and shows that the amount and size of the Y-211 partiles varies with distane from the seed. Generally, for haraterization studies, small retangular speimens are ut from the original pellet, rather far from the seed in order to ensure a homogenous partile distribution. To reah a better insight how the non-superonduting partiles influene the pinning properties Ž J B harateristis., we studied in detail a zone where the distribution of 211 partiles strongly varies. Mirostruture analysis oordinated with both magneti haraterization and magneto-optial Ž MO. investigation of flux distributions should help to understand the role of the non-superonduting partiles in the flux pinning. In the present paper we deided to ut a large sample from the pellet near the seed. After magneti and MO measurements the sample was further divided into smaller piees and suessive magneti and MO measurements were performed to investigate how the magneti properties vary within the sample. The mirostruture analysis was arried out using a polarization mirosope and a sanning eletron mirosope Ž SEM.; the loal flux distributions were studied at 30 K using MO imaging. 2. Experimental Powders with the atomi ratio of ŽNd 0.33Sm Gd. Ba Cu O Ž NSG y were sintered and pressed into pellets of about 20 mm in diameter, whih were then subjeted to the OCMG proess in 0.1% O2 in Ar atmosphere. The details of the sample preparation are given elsewhere w1,4 x. From the melt-textured NSG pellet a retangular speimen with Ž. 3 dimensions of a=b= s2.0=1.40=0.38 mm was ut in a distane of about 6 mm away from the seed. The sample was mehanially polished to obtain a surfae roughness of 0.5 mm. We used the MO imaging with a ferrimagneti Bi-doped Y Fe garnet thin film with in-plane anisotropy to observe the flux distributions w6,7 x. Our MO detetion system is equipped with an eletromagnet to apply a field up to 0.5 T, a helium gas flow ryostat, and an optial mirosope attahed to a olor CCD video amera.

3 MO investigations were performed at Ts30 K; the magneti field was applied parallel to the -axis of the sample. Integral magneti properties were studied using a ommerial SQUID magnetometer ŽQuantum Design models MPMS 5 and 7. in the temperature range between 30 K and 77 K and in fields up to 7T. A. Das et al.rphysia C Results and disussion 3.1. Mirostruture analysis A polarized light image of the NSG sample is presented in Fig. 1a, revealing the twin plane pattern in the Ž a,b. -plane. The dark spots are due to several holes and 211 partiles. Furthermore, a grain boundary pattern is visible dividing the sample into three domains, labeled D1, D2 and D3, respetively. Along these boundaries 211 partiles lustered so that these areas appear blak in the polarization image. One domain was wedge-shaped and strethed aross the entire length of the sample. We denoted this domain as D1. Its surfae was 1.06 mm 2. In the lower part of the image, two other domains are seen. The smaller square domain, with surfae area of 0.75 mm 2, was denoted as D2. The third domain Ž D3. was the largest one, with a surfae measuring 1.13 mm 2.Itis important to note that the ourrene of suh domains are not typial for the NSG ompound. Fig. 1b presents domains D2 and D3 in an SEM mirograph. Here, we an learly distinguish between the dark holes and the 211 partiles Ž white.. The size of the 211 partiles varies between 10 and 100 mm; also some very small Ž f1 mm size. white spots an be deteted. Entrapment of 211 partiles is ommonly observed in melt-textured Ž LRE. Ba 2- Cu 3Oy samples even when the starting omposition is stoihiometri as in the present ase. Note also that no additions like Pt or CeO2 were used to refine the 211 partile size. Eletron miroprobe analysis revealed that the 211 partiles are of the NSG-211 type, i.e., Nd, Sm, and Gd is ontained in the 211 partiles in the same ratio as in the superonduting matrix. In domain D2, the 211 partiles are relatively uniformly distributed, whereas D3 shows only a small overall onentration of 211 partiles, and a large portion of the domain is free of 211 partiles. Fig. 1. Ž. a Polarization image of the NSG sample revealing the twin plane pattern in the Ž a,b. -plane. The dark spots are due to several holes and 211 partiles. Furthermore, a grain boundary pattern is visible dividing the sample into three domains, labeled D1, D2 and D3, respetively. The hathed area was removed from part D1 before the magneti measurements Žsee also Figs. 2 and 3.. Ž b. Represents a SEM mirograph of domains D2 and D3. Here, we an learly distinguish between the dark holes and the 211 partiles Ž white.. The size of the 211 partiles varies between 10 and 100 mm; also some very small Ž f1 mm size. white spots an be deteted Magneto-optial studies All flux patterns presented in this paper are taken at a temperature Ts30 K in order to ahieve large enough ontrasts; the exposure time of the amera is always kept onstant during an experimental run in order to allow for a diret omparison of the images to eah other. In the olor representation, flux of positive sign is imaged as green; well-shielded regions are represented as brown. Vorties of opposite polarity Ž after a hange of field diretion. are imaged yellow. The MO investigations at 30 K reveal a quite inhomogeneous flux penetration into the sample. In

4 172 A. Das et al.rphysia C Fig. 2a h, we present the initial flux penetration Ž i.e., along the virgin urve.. The sample is ompletely shielded up to an applied field of 30 mt Ž. a. In Ž. b, 45 mt, flux starts to enter the sample through a grain boundary between domains D2 and D3, whereas the remainder of the sample is still ompletely shielded. In Ž., 75 mt, Ž. d, 120 mt, and Ž. e, 150 mt, flux ontinues to penetrate along the grain boundary network until the sample is ompletely deoupled into three magnetially independent domains. From Ž. f, 180 mt, on, vorties begin to enter into these domains as well. The flux penetration starting from the grain boundaries is evidently easier than starting from the sample edges, whih Fig. 2. Flux penetration and trapping of the entire OCMG-proessed NSG sample Ž W. observed by MO imaging at Ts30 K. The series of MO images: Ž. a 30 mt, Ž. b 45 mt, Ž. 75 mt, Ž. d 120 mt, Ž. e 150 mt, Ž. f 180 mt, Ž. g 330 mt, and Ž. h 510 mt, represents the initial flux penetration Ž i.e., the virgin urve. into the zero field-ooled sample. The marker is 1 mm long. The Meissner state is imaged brown; the loal magneti field, B is represented in green. The shades of green reflet the field strength. z

5 A. Das et al.rphysia C is a lear indiation of a surfae Ž entry. barrier. Finally, in Ž. g, 330 mt, and Ž. h 510 mt, a more or less regular flux penetration into the domains an be observed. Here, it is important to mention that the grain boundaries are not weak links. In suh a ase, even a very small applied field would be suffiient to destroy the oupling between the domains almost instantaneously. The present grain boundaries are apable of arrying a transport urrent, whih auses a gradual deoupling of the domains. This gradually deoupling is typial for high-t samples as dis- ussed in Refs. w13,14 x. Fig. 3 presents the redution of the applied field towards the remnant state Ž m H s 0 T. 0 a after applying the maximum field of 510 mt. In Ž. a, the field is redued to 150 mt. Vorties are leaving the sample also through the grain boundaries. The series Ž.Ž. b e illustrates the further redution of the field. In Ž. d, 45 mt, vorties of opposite polarity Ž negative vorties ; yellow. beome stable inside the grain boundaries. Around these negative vorties, an annihilation zone with Bs0 T is formed. On further dereasing the external field, more and more suh negative vorties are reated until the remnant state is established in Ž. f. The generation of these negative vorties takes plae first where the pinning properties are weakest. Therefore, the observation of negative vorties allows to identify weak-pinning areas. In order to hek if the grain boundaries are not only at the sample surfae, but indeed separate the sample into three domains through the entire thikness, the sample was further thinned down by another 100 mm. ZFC studies were repeated on the thinned NSG-whole sample and the stepwise flux Fig. 3. Redution of field toward the remnant state at Ts30 K Ž sample W., after applying a field of 510 mt. Ž a. 150 mt, Ž b. 105 mt, Ž. 75 mt, Ž. d 45 mt, Ž. e 15 mt, and Ž. f 0 mt Ž remnant state.. The reversal of the stray field is indiated by the olor hange from green Ž positive field. to yellow Ž negative field.. The marker is 1 mm long.

6 174 A. Das et al.rphysia C Fig. 4. Ž. a The flux penetration through the grain boundaries Ž sample W. in an applied field of 270 mt, Ts30 K. Ž. b Trapping of flux in the remnant state after field-ooling in a field of 270 mt. The field was removed after Ts30 K was reahed. In both ases, vorties of opposite polarity an be seen along the grain boundaries, but the field-ooled state in Ž. b is fully penetrated. The marker is 1 mm long. entry was again observed, thus onfirming that the sample is definitely multi-domain in the whole thikness. Fig. 4 shows flux patterns on this thinned down sample; in Ž. a a field of 270 mt is applied. We notie that the three domains are deoupled magnetially. The ZFC remnant state an show if the sample is fully penetrated or not. It was impossible to fully penetrate the sample even after thinning down as our maximum applied field available was only 0.51 T, while the full penetration field, H U, of the sample Ž. was around 2.0 T. In ontrast to this, b presents a state obtained after field-ooling the sample in a field of 270 mt. After the temperature of 30 K is reahed, the external field is removed ŽFC remnant state.. In this ase, a fully penetrated sample results. Also in this remnant state, negative vorties are generated and enter the sample through the grain boundaries and other weak-pinning areas. However, as the sample is fully penetrated, the annihilation of vorties of opposite polarity effetively sans these weak-pinning areas; even in a bulk superonduting sample. Therefore, we proposed suh field- Fig. 5. MO studies on domain D2 at 30 K. The marker is 100 mm long. Ž. a Domain D2 in zero-field ooled remnant state after a maximum field of 510 mt and then subjeted to a negative field of y15 mt. Negative fields failitate the entry of negative vorties and thus an give information about the weak hannels. Ž. b 90 mt field-ooled remnant state of D2 subjeted to a field of y510 mt and brought bak to the remnant state. The field is removed after reahing 30 K. This learly shows that a field of 510 mt is not suffiient to penetrate to the enter of the sample and annihilate the field-ooled trapped flux.

7 A. Das et al.rphysia C ooled remnant states as effetive means to analyze bulk superonduting samples by means of magnew15 x. to-opti imaging After these experimental runs, the domains were separated mehanially and the domain D2, whih is rih in 211 phases, was studied individually. The Fig. 6. Ž. a FC remnant pattern of domain D2 at Ts30 K, after FC in a field of 90 mt. The marker is 100 mm long. The resulting flux pattern is nearly uniform to the eye, but a loser inspetion reveals important differenes; see Ž. b. Ž. b 3D ontour plot of the field distribution of the state shown in Ž. a. The ontour plot of the loal field, Bz reflets the morphology of the sample surfae. Furthermore, several islands of trapped flux an be seen whih are due to the presene of 211 partiles.

8 176 A. Das et al.rphysia C magneto-opti studies on the domain D2 are represented in Fig. 5. In Ž. a, the sample was zero field ooled, then a maximum field of 510 mt was applied and subsequently swithed off. Afterwards a small negative field of 15 mt was applied. The remnant state shows that the flux penetration is not isotropi, even though the sample geometry is not perfetly symmetri. The small negative field has pushed the trapped flux from the border of the sample towards the interior. There are three distint regions: The enter part is flux free Ž Meissner phase.. This region is surrounded by an area of positive flux, whih in turn is bordered by negative vorties. In Fig. 5b, the remnant state is presented after the sample was field-ooled in a field of 90 mt and then a maximum negative field of 510 mt was applied at 30 K and subsequently swithed off. Applying a negative field to the remnant state auses vorties with opposite polarity to enter the sample through possible weak hannels. The penetration of negative vorties reates annihilation zones when they meet the positive flux. The study of negative vorties enables us to map the regions of weak hannels. In Fig. 5b, there is a flux trapped in the entral region in ontrast to the ZFC remnant state where the entral region is in Meissner regime. The annihilation zone is now in the interior of the sample and not at the border as before. This learly shows that a field of 510 mt is not suffiient to penetrate up to the enter of the sample and annihilate the field ooled trapped flux. The non-uniform distribution of the seond phase partiles is learly refleted in the asymmetri flux pattern in the remnant state. Nevertheless the shape of the flux pattern in the enter of the sample is in both ases similar, indiating that the weak hannels are used for flux entry as well as for the flux exit. To orrelate the flux lattie state with the mirostruture we plotted the 90 mt field ooled remnant state Ž Fig. 6a. as a three-dimensional Ž 3D. ontour plot Ž Fig. 6b.. The ontour plot shows the morphology of the surfae. We realize that in only about 20% of the area in the entral part high magneti flux an be found, indiating that flux has esaped from the sides. 211 partiles surrounded by a dense struture of ontour lines an be learly observed w15 x. The strain around the 211 partiles evidently affets and traps vorties, whih indiates that the interfae of the non-superonduting partiles and the normal matrix an at as an effiient pinning medium. 4. Magneti measurements 4.1. Critial temperature determination To determine T of the samples, zero-field-ooled Ž ZFC. urves were measured by means of a SQUID magnetometer, in an applied magneti field of 1 mt. The entire sample had T s93.1 K with a superon- duting transition width less than 1 K. The sample D1 had the same T value but the width of the transition inreased to 2.5 K. The domain D2 showed the highest T of 94 K with a sharp transition within 1 K. ZFC and field ooled Ž FC. urves of all the samples are plotted in Fig. 7. The whole sample Ž denoted by W. had the smallest Meissner signal indiating better pinning properties than have the individual domains. This indiates that the interfaes substantially ontributed to the flux pinning. The domain D2, ontaining a larger amount of non-superonduting partiles, expelled magneti flux less than D Hysteresis measurements Ž. Magneti hysteresis loops MHLs were measured by a SQUID magnetometer in the no overshoot Fig. 7. T measurements on the entire NSG sample W, domain D1, and domain D2, performed using an applied field of 1 mt.

9 A. Das et al.rphysia C mode in the temperature range of K. The magneti field was always applied parallel to the -axis. Critial urrent densities, J, were alulated from the MHL height Dm using the extended Bean model for the retangular speimen w16,17 x, J s Dmrr with rs1r2 adbyar3 2 Ž., where d is the thikness of the sample and a and b are its lateral dimensions, bga. Keeping in mind that these samples ontain an inhomogeneous distribution of 211 partiles, we heked the effetive urrent length-sales within the sample. As suggested by Angadi et al. w18 x, the J value obtained from Bean model depends on the length sale on L whih the urrent flows. A nondestrutive method for L estimation is based on the analysis of the magnetization loop w18 x. In the disk of the radius r and thikness d, magnetized along the normal, the initial slope of the reverse leg of the MHL is related to the effetive urrent lengthsale as 2 3 dmrd HsyŽ pl. r lnž 8rrd. y0.5. Ž 1. Ž y2 Here m is the magneti moment in A m. and Ž y1 H is the applied field in A m.. We approximated our nearly retangular samples by a disk with an effetive radius giving a surfae area equal to the surfae of the atual sample. The length-sale of the sample W was about 64% of the sample radius. pl 2 was about 45% of the atual surfae of the sample. None of the individual domains had suh a large surfae, the biggest domain extended to only 36% of the total sample surfae area. This indiates that the domains were not ompletely magnetially independent. Similar alulations for the individual domains after their separation revealed that the domains D1 and D2 had length sales of 77% of their radii. This means that a high J flew at only 60% of the domain area. The above analysis an be taken as only a qualitative measure as the approximation of the retangular samples by a disk is rather rude. Nonetheless, we an onlude that before separation the domains were magnetially oupled and only a small portion of the whole sample arried a high ritial urrent. From this analysis also follows that J averaged over the whole sample volume was lower than the values in the individual domains after their separation. Fig. 8 demonstrates that this was the ase at all measured temperatures. The domain D1 had always the highest Fig. 8. J as a funtion of the applied field for samples W, domain D1, and domain D2, at Ž. a T s30 K, Ž. b T s50 K, and Ž. T s77 K. J, whereas J of W was always found to be the lowest of all. All three J Ž B. urves, for W, D1, and D2, showed a lear peak effet Ž PE. without any evidene of the twin struture ativity w19 x. The fishtail peak Bpk of all three samples varied with temperature; the fishtail maximum position and irreversibility field saled with temperature approximately equally: Bpk and Birr of domain D1 were always low, in D2, always high; the whole sample always exhibited intermediate values of both quantities. Note a rather strong distortion of the J Ž B. urve of W at 77 K. At higher temperatures the orresponding field range ame out of the experimental range of fields Fit of the experimental urões It is a ommon feature of PE that the maximum shifts to lower fields and the magnitude dereases

10 178 A. Das et al.rphysia C with inreasing temperature. It also appears that a shift of the fishtail maximum to higher fields usually brings about also an inrease in irreversibility field. To ompare the PE of different samples at different temperatures it is useful to normalize the urves with respet to some harateristi value. If the fishtail maximum oordinates, Ž B J. pk max, are used as suh a harateristi value, the normalized PE shape of the hysteresis loop in RE-123 superondutors an be well approximated by the expression w20x n JnŽ b. sb exp Ž 1yb. rn Ž 2. where Jn sjrj max, and bsbrb pk. n is parameter desribing field dependene of the harateristi atiw x yn vation energy 21,22, U0 AB, and the harateristi ritial urrent density is assumed to be a linear funtion of field w21 23 x, j0 AB. In Fig. 9a we present the J Ž B. urves of the samples W, D1, and D2, normalized to their fishtail maxima. The data measured at 77 K were hosen as at this temperature nearly all the urves fit into our experimental field window of 5 T. These urves were analyzed by means of Eq. Ž. 2. The fits are in the figure represented by lines. Sample W fitted with n s 1.85 but the urve was somewhat distorted, probably due to inomplete eletrial inter-onnetion of the individual domains. Although the J Ž B. urves of domains D1 and D2 look not muh alike Ž Fig. 8., after normalization they nearly ollapsed to one urve and from their fit by Eq. Ž. 2 we obtained ns1.57 and ns1.51, respetively. Fig. 9b shows the same data presented in terms of the pinning fore density, F s BJ. For the FŽ B. dependene normalized to the fishtail peak oordinates Ž B, F., the relation equivalent to Eq. Ž 2. fpk max reads w23 x, Ž. Ž. Ž. 2 n Fn bf sbfexp 1ybf 2rn 3 with Fn s FrF max, bfs BrB fpk. The fits of the F Ž b. data by Eq. Ž 3. n f resulted in similar values of the fitting parameter as in the previous ase, ns1.77, ns1.55, and ns1.56 for samples W, D1, and D2, respetively. The slight variation in the n values is due to different effetive weights exerted on individual experimental points in both representations, espeially in the low-field range. Finally, we normalized the field variable to irreversibility field. The resulting urves are presented in Fig. 9. Ž. a The normalized urves from Fig. 8 where Jns Jr Jp is the redued ritial urrent, and bs BrBp is the redued magneti field. Ž B, J. p p are the fishtail maximum oordinates. The data were fitted by Eq. Ž. 3. Ž. b The same data as in Ž. a, in the pinning-fore-density representation. The urves were normalized to the respetive fishtail maximum oordinates and fitted by Eq. Ž. 3. Ž. The same as in Ž. b, with the applied field normalized to the irreversibility field. Fit was done by means of Eq. Ž. 4. Fig. 9 together with fitting urves aording to a Ž. modified Eq. 3, 2 n n irr irr Ž irr. F Ž b. s Ž b. exp 1y Ž b. 2rn Ž 4. with birr s BrBfirr and b s BfirrrB fpk. Note that bf s b irr.1rb desribes the mutual position of the fishtail peak with respet to irreversibility field. In the fits of the data in Fig. 9 both n and b were taken as free parameters. The introdution of an additional fitting parameter aused a slight hange in the n values. We note that the n values orresponding to different data representations are lose to eah other. They are, however, signifiantly lower than those met sometimes in NdBa Cu O single rys

11 A. Das et al.rphysia C tals. The same applies for the parameter 1rb that was found in all three ases signifiantly lower than f 0.4, the value observed in some NdBa 2Cu 3O7 single rystals w23 x. Fig. 9a reflet some ommon features of different data representations. For example, the distortion of the urve measured on the whole sample is apparent in all three plots and the n value is in all ases signifiantly different from those of individual domains. Another ommon feature is the ollapse of the normalized data of domains D1 and D2 onto one urve. From this we onlude that the rather massive 211 partiles, if ative at all, at as a random pinning disorder, pinning on whih results in the observed typial fishtail shape of the magnetization urve. The higher onentration of the 211 partiles in domain D2 might be onsistent with the enhanement of both Bpk and Birr in this domain. Although the ritial urrent densities in domains D1 and D2 signifiantly differ, shapes of the normalized urves of D1 and D2 are pratially idential. For a detailed insight into the role of 211 partiles in flux pinning more experimental and theoretial work is needed. 5. Conlusions A sample of the ternary ompound NSG possessing a three-domain struture was studied, first on the sample as a whole, then on separated domains, by means of the mirostruture analysis, magneti and magneto-optial measurements. SEM showed that the sample had a non-uniform distribution of NSG- 211 partiles, in size from 2 to 100 mm. The MO images proved that magneti flux enters the whole sample step-wise, first penetrating along grain boundaries, then entering grains. The ritial urrent sale-length analysis showed that the domains were partially oupled before separation. After the separation the domain D1 had a high J, while D2, rih in 211 partiles, exhibited intermediate J but signifi antly higher Bpk and Birr than the other two sam- ples. The fishtail shape of the MHLs of individual domains an be well fitted by the exponentially deaying funtions following from TAFC model. The values of the free parameters n and b indiate that magneti properties of the studied material an be further improved. An effetive ontrol of the pinning site distribution and size is neessary to optimize the pinning mehanisms in the melt textured samples. Aknowledgements This work was partially supported by NEDO for the Researh and Development of Industrial Siene and Tehnology Frontier Program. AD, MJ and MRK thank the Japanese Siene and Tehnology Ageny Ž STA. for the support of this work by the provided fellowships. The work was ompleted under the partial support of GA ASCR No. A The authors thank T. Mohida and S.J. Seo for their tehnial assistane and valuable disussions. Referenes wx 1 M. Murakami, S.I. Yoo, T. Higuhi, N. Sakai, J. Weltz, N. Koshizuka, S. Tanaka, Jpn. J. Appl. Phys. 33 Ž L715. wx 2 S.I. Yoo, N. Sakai, H. Takaihi, T. Higuhi, M. Murakami, Appl. Phys. Lett. 65 Ž wx 3 M. Murakami, N. Sakai, T. Higuhi, S.I. Yoo, Superond. Si. Tehnol. 9 Ž wx 4 M. Muralidhar, H.S. Chauhan, T. Saitoh, K. Kamada, K. Segawa, M. Murakami, Superond. Si. Tehnol. 10 Ž wx 5 A. Endo, H.S. Chauhan, Y. Shiohara, Physia C 273 Ž wx 6 M.R. Koblishka, R.J. Wijngaarden, Superond. Si. Tehnol. 8 Ž wx 7 M.R. Koblishka, Superond. Si. Tehnol. 9 Ž wx 8 M.R. Koblishka, M. Muralidhar, M. Murakami, Appl. Phys. Lett. 73 Ž wx 9 M. Muralidhar, M. Murakami, Appl. Superond. 5 Ž w10x M. Muralidhar, M.R. Koblishka, T. Saitoh, M. Murakami, Superond. Si. Tehnol. 11 Ž w11x M. Murakami, in: Melt Proessed High Temperature Superondutors, World Sientifi, Singapore, 1993, p w12x A. Das, S. Koishikawa, T. Fukuzaki, M. Muralidhar, M. Murakami, Appl. Superond. 6 Ž w13x M.R. Koblishka, Th. Shuster, H. Kronmuller, Physia C 211 Ž w14x M.R. Koblishka, Th. Shuster, H. Kronmuller, Physia C 219 Ž w15x M.R. Koblishka, A. Das, M. Muralidhar, S. Koishikawa, N. Sakai, M. Murakami, Jpn. J. Appl. Phys. 37 Ž L1227.

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