Microstructural and magnetisation study in melt-grown YBaCuO samples

PHY$1CA ELSEVIER

Physica C 244 (1995) 106-114

Microstructural and magnetisation study in melt-grown Y-Ba-Cu-O samples R. Gopalan a, T. Roy a, T. Rajasekharan a,., G. Rangarajan b, N. Haft Babu c a Defence Metallurgical Research Laboratory, Hyderabad-500 258, India b Department of Physics, Indian Institute of Technology, Madras-600 036, India e School of Physics, University ofHyderabad, Hyderabad-500 134, India

Received 29 November1994; revisedmanuscriptreceived24 January 1995

Abstract The effect of Y2BaCuO5 (211 ) and silver on the microstructure of melt-grown YBa2CU3OT_x(123) has been systematically investigated. An optimal amount (28 mol%) of211 in the starting composition of 123 with 5 wt.% of silver gave critical current densities > 104 A/cm 2. Magnetisation measurements using a SQUID magnetometer have been utilised to characterise the superconducting properties of the material at various stages of optimisation. Bean's and Kim's critical-state models are used to analyse the magnetisation of the optimised composition ( 123 + 28 mol% + 5 wt.% Ag). An anticlockwise tilt observed in the magnetisation curve above 70 K in the optimised sample has been explained as being due to the presence of an additional paramagnetic contribution arising from 211 particles present in the 123 matrix.

1. Introduction High critical current density (Jc) values have been reported in YBa2Cu307-x (123) processed by a meltgrowth technique by several authors [ 1-7]. Although there are some variations in the reported processes with respect to the starting material and temperatures used, all the variants of the melt-growth process involve the formation of 123 from Y2BaCuO5 (211) and liquid phases (BaCuO2+CuO) while very slowly cooling the sample through the peritectic formation temperature ( 1010°C, in air) of 123. The final microstructure consists of many rather large domains of 123 with several parallel platelets of 123 within. The platelets have been reported to be parts of a single crystal [8]. The specimens with the largest reported J~s have, in addi* Correspondingauthor. 0921-4534/95/$09.50 © 1995 ElsevierScience B.V. All rights reserved SSD10921-4534 ( 95 ) 00056-9

tion, a large number of 211 precipitates within the domains and contain some amount of silver [9]. The low J¢ in polycrystalline 123 can be attributed to a lack of textured growth, the presence of second phase at grain boundaries, microcracking and the absence of pinning of the flux within the grains. The work reported in this paper has been carried out with two objectives: (1) To understand the modifications in the microstructure as a function of 211 and silver and to study the superconducting properties through magnetisation measurements at various stages, and (2) to arrive at an optimum composition that yields a high Jc in the bulk.

2. Experimental Sintered samples of 123 with various concentrations of 211 (0 mol%, 20 mol%, 28 mol% and 50 mol%

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R. Gopalan et al. / Physica C 244 (1995) 106--114

211) introduced into the matrix were melt processed as reported elsewhere [ 10]. All the samples were initiaUy partially melted at around 1130°C. They were then cooled from 1030°C through the peritectic formarion temperature of 123 ( 1010°C in air) at a rate of l°C/h in flowing oxygen. The samples were annealed at 930°C for 24 h and then cooled to room temperature with holds between 600°C and 400°C in 02 atmosphere. Microstructural features were analysed using optical microscopy with polarised light and a Philips EM430T transmission electron microscope (TEM). Magnetisarion measurements were performed using a Quantum design SQUID magnetometer on rectangular bar samples cut from the melt-grown samples. Samples were cut carefully such that they contain a single domain of oriented platelets. The magnetisation results reported here have been obtained with the applied magnetic field parallel to the slab thickness.

3. Results and discussion 3.1. Melt-grown stoichiometric 123

Melt-grown 123 samples are known to exhibit a microstructure showing domains in which the 123 platelets tend to crystallise as large parallel grains. All the 123 platelets within a single domain will have a common c-axis. An X-ray pole figure study carried out on a melt-grown stoichiometric 123 sample revealed that the c-axes of the individual domains are misoriented at angles of 38 ° and 15-20 ° with respect to the sample surface normal [ 11 ]. It should be mentioned here that the microstructural and magnetisation results reported in this paper pertain only to the observations made within single domain. Fig. l ( a ) shows a polarised optical micrograph of melt-grown 123 of stoichiometric composition. The sample exhibits large oriented 123 platelets with twin boundaries cutting across them. An important feature in this sample is the presence of "cracks" of about 2.8 p,m width between the aligned platelets. The J~ estimated from the magnetization data using Bean's critical-state model [12] was only 10a A/cm 2 in the absence of field and it dropped to 2 × 102 A/cm 2 at 0.1 T (Fig. 2(a) ). The J~ values are not very large and are comparable to that of a well sintered polycrystalline 123 sample. This shows that the excellent grain align-

Fig. I. Opticalmicrostructuresof melt-grown(a) stoichJome~c123 sampleand (b) 123 containing20 mol% 211 sample. t'8I~ ~, L,

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• 123 0 123 + 5wt% A9 & 123 • 20mo~ % 211 123 ÷ 28mol%211*~w~%Ag

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Fig. 2. Criticalcurrentdensity (Jc) as a functionof appliedmagnetic field for melt-grownsamples at 77 K: (a) stoichiometric123, (b) 123+ 5 wt.% Ag, (c) 123+ 20 tool%211 and (d) 123 + 28 mol% 211 +5 wt.% Ag.

R. Gopalan et al. / Physica C 244 (1995) 106-114

108

ment within the domains alone is not sufficient to produce a significant improvement in the critical current density values in melt-grown stoichiometric 123. The other defects observed in the microstructure such as "cracks" occurring between the 123 platelets and the absence of flux-pinning centres have to be remedied in melt-grown stoichiometric 123 before the samples can support large JCs.

/

3.2. Effect of 211 Fig. l(b) shows an optical micrograph of a meltgrown sample containing 20 mol% 21 I. It is found that the average width of a 123 platelet and the average crack width between the 123 platelets decrease with addition of 211. A similar observation of a decrease of 123 grain width with 211 addition has been discussed in the literature [ 5,7 ]. Magnetisation measurements on these samples at 77 K yielded a Jc value of 4.5 × 103 A/cm 2 at zero field and 2.4 × 103 A/cm 2 at 0.1 T (Fig. 2(c) ), which is one order of magnitude higher than that obtained in stoichiometric 123 sample. From these measurements it can be noticed that melt-grown material with 211 addition exhibited a larger magnetic Jc at high fields indicating an increased intragranular J¢. However, since the average size of 211 particles is much greater than the coherence length (7-30/~,) and also greater than the flux-line lattice spacing (af= 1.075 (~o/B) 1/2), 211 particles cannot directly contribute to flux pinning in large magnetic fields ( > 1 T). It is possible that structural defects around a 123/ 211 interface may contribute to flux pinning. Mironova et al. [ 13] have reported that 211 particles with a large surface curvature are associated with numerous dislocations and they are believed to be responsible for the high J¢ obtained in 211 added 123 samples. Fig. 3 shows a TEM bright field image of a 211 inclusion in 123 matrix. The particle has an elliptical shape.

Fig. 3. A TEM micrograph showing 211 inclusion of elliptical morphology with extended defects around 123/211 interface in meltgrown 123 containing 20 tool% 211 sample. These defects are observed where the curvature of the interface is large. The radius of curvature at which the defects start appearing is < 0.25 p.m.

Extended defects are observed at the 123/211 interface only when the interface curvature is large. It can be measured from the micrograph (Fig. 3) that only when the radius of curvature of the interface is < 0.25 p.m the extended defects start to appear. Our TEM study in melt-grown 123 containing 20 mol% 211 sample confirmed the observation of Mironova et al. [ 13]. This suggests the need for making 211 particles of small size for better pinning of the flux. Table 1 gives the microstructural parameters obtained in melt-grown samples in the present study. Figs. 4(a) and (b) show the variation of the average width of the 123 platelets and average crack width between the 123 platelets as a function of the 211 concentration. We have also observed in the present work that the particle size of 211 inclusions becomes finer as the 211 content is increased (Fig. 4(c) ). The plateletwidth and crack-width values are averaged over several grains and the results reported could be reproduced in several experiments. It can be seen that the average crack width between the aligned platelets decreases

Table 1 Microstructural parameters obtained in melt-grown samples processed in the present study Sample

Average width of 123 platelet (l~m)

Average crack width (p~m)

211 particle size (p.m)

123 123 + 20 mol% 211 123 + 28 mol% 211 123 + 5 0 mol% 211

15 8 6.5 4.5

2.8 1.5 0.45 0.125

8-10 4-6 I-3

R. Gopalan et al. / Physica C 244 (1995) 106-114

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Fig. 4. (a) Dependence of 123 platelet width on 211 concentration. (b) Variation of the average crack width at 123 platelet boundaries as a function of 211 concentration, and (c) a plot of average 211 particle size vs. 211 concentration.

with increasing concentration of 211. Although these results in the present study indicate that as the 211 content increases, the crack width decreases, "cracks" cannot be completely closed. The minimum crack width observed at 50 mol% 211 was 0.125 I~m with 123 platelet width of 4.5 p.m. Our experimental results showed that the decrease in crack width was rather rapid up to 28 mol% 211 and thereafter it seems to

109

decrease gradually to a constant value, A resistivity measurement showed that in a sample containing 50 mol% 211, zero resistance was not obtained and this could be due to the presence of a large number of 211 particles breaking the superconducting path. These results led to selecting 28 mol% 211 as an optimal amount of 211. It is observed in the present work that though Jc values in melt-processed 123 with 211 additions, showed an increase with 211 content, they were only in the range 103 A / c m 2 at 77 K in zero field. Since further modifications of the microstructure were not possible using 211 alone for an improvement of Jc, other methods were explored to increase J¢ further. It is interesting to note in this context that the very small levitation effects shown by the melt-grown 123 samples containing 211 could be attributed to the relatively lower J¢ and the presence of cracks in the sample limiting the radii of the screening current loops [9]. The complete closure of the cracks at the boundaries between 123 platelets was achieved by the addition of silver to 123 with optimised 211 content (28 mol%).

3.3. Effect of silver Silver can be added to eliminate the cracks as there is evidence that the presence of silver improves the mechanical strength [9,14--16]. Our melt-growth experiment with 5 wt.% silver in 123 showed retention of silver particles isolated as well as filling some of the grain boundary cracks (Fig. 5 (a)). Jc obtained from a magnetisation study on these samples was 8 × 102 A/ cm 2 at 77 K and at H = 0 . 1 T (Fig. 2 ( b ) ) which is higher than that obtained in stoichiometric 123. The improvement of Jc in silver-containing samples can be attributed to the elimination of some of the grainboundary cracks by the addition of silver. However, Jc is not very high in the presence of a magnetic field suggesting that silver acts only to reduce the weak links, improving the intergrain J¢. Addition of 5 wt.% silver in melt-grown 123 with 28 mol% 211 was found to close most of the grain-boundary cracks. However, we could not detect any silver at the grain boundaries. Fig. 5 (b) shows the optical microstructure of such a sample. The mechanism by which the closure of grain-boundary cracks occurs with silver addition is not clear. One can invoke a mechanism wherein the reported partial solubility of Ag in 123 [ 17] leads to the closure of

110

R. Gopalan et al. / Physica C 244 (1995) 106-114

Table 2 Comparison ofJ c values at H=0.1 T obtained at 77 K for meltgrown samplesprocessedin the presentstudy Sample

Jc (A/cm2)

123 123+20 mol%211 123+5 wt.% Ag 123+28 mol%211 + 5 wt.% Ag

0.2x 103 2.4× 103 0.8x 103 1.8× 104

A/cm 2 at 1 T. Such a slow fall of J~ with field can be due to the strong coupling between the grains in meltgrown samples. The J~ values obtained at 77 K in nearly zero field for the melt-grown samples processed in the present study are compared in Table 2. It can be seen that the sample containing 28 mol% 211 and 5 wt.% Ag has a large Jc value. This large J~ value is obtained essentially through a microstructure control such as introduction of pinning centres associated with 211 particles, reduction of weak links by silver additions, etc. This sample showed a large levitation effect and suspension effect. The sample containing 28 mol% 211 and 5 wt.% Ag was further characterised for its magnetic properties using critical-state models.

Fig. 5. Opticalmicrostructureof melt-grown(a) 123+ 5 wt,%silver and (b) 123+28 mol%211 +5 wt.% silver. cracks by the modification of the phase diagram of Y Ba--Cu--O system. The J¢ of the sample containing 28 mol% 211 and 5 wt.% Ag at H = 0.1 T estimated from magnetisation measurements is 1.8 X 104 A/cm 2 (Fig. 2(d) ). Transport J¢ exceeding 16 000 A/cm 2 at 0 T could be obtained in this sample [ 18]. A feature of the J¢ versus magnetic field plot (Fig. 2) is that J~ after an initial steep fall remains essentially constant over the entire field range. This steep fall in J¢ with the magnetic field is a common phenomenon in oxide superconductors. Kupfer et al. [ 19] gave a comprehensive set of data on Jc versus B on powdered 123 samples. The sharp fall in J¢ is usually ascribed to the effect of magnetic-field penetration through weak links. However, the Jc drop observed in a low field range in the case of melt-grown samples is not very steep as compared to the sintered samples. For example in the optimised composition ( 1 2 3 + 2 8 mol% 2 1 1 + 5 wt.% Ag) J¢ drops from 3.8 X 104 A/cm 2 from zero field to 0.9 × 104

3.4. Analysis of the magnetisation curves in meltgrown 123 + 28 mol% 211 + 5 wt. % Ag sample

In this section we present an analysis of the magnetisation curves carried out using critical-state models. For hard superconductors, the critical current density (Jc) can be obtained from the magnetisation curves based on the critical-state model which was suggested by Bean [ 12] and Kim et al. [20]. In Bean's model J~ was considered to be a constant independent of H. In Kim's model J~ is expressed as a function of the local internal field (Hi) and is given by Jc(Hi) = k l ( H o + H i ) , where k and Ho are the constants. Since then, several different Jc(HO functions have been reported in the literature [21-23] to explain the Jc dependence on H. Chen and Goldfarb [24] have given the magnetisation curves for type-II superconductors of an orthorhombic geometry following Kim's critical-state model and explained how to apply the derived results to high-T~ superconductors.

R. Gopalanet al. / PhysicaC 244 (1995)106-114 In the present work, we have obtained analytically the magnetisation curves for the melt-grown 123 sample containing 28 mol% 211 and 5 wt.% Ag of an orthorhombic geometry based on Bean's model as well as Kim's model. One of the motivations for using Kim's model for the derivation of the magnetisation curve is that out of all the models discussed in the literature, this one is the most general. The expression for Jc (Hi) given by the Kim's model is somewhat complicated because there are two constants (k and Ho) involved. These constants are further related by a parameterp to the full penetration field Hp by

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Fig. 6. High-field magnetisationas a function of field for melt-grown 123 + 28 tool% 21 ] + 5 wt.% Ag sampleat 5 K. (F1) representsthe experimentally obtainedM(H) curve. (C)) representsthe computed M(H) curve for Bean's model.

where a is the sample dimension perpendicular to the field direction. Hence, in order to deduce J¢ (Hi) using Kim' s model, values of the constants k and Ho have to be obtained. These values can be obtained by generating the magnetisation curves using the expressions obtained for the hysteresis loops at different field ranges [24]. We first attempted fitting the experimentally obtained high-field magnetisation data at 5 K to the M(H) expressions for orthorhombic samples in the Bean limit. This was attempted first because the experimentally obtained AM at 5 K from the magnetisation data was found to be almost independent of field. Hence, Bean's model can be applied to derive the magnetisation curve. M(H) equations have been given by Chen and Goldfarb [24] for the high-field hysteresis loop for Hp < Hm, where Hm is the maximum applied field. Using these equations the hysteresis loop was generated and a good fit was obtained for the M(H) data experimentally with Hp = 5150 G. The experimentally obtained and analytically derived M(H) curves are shown in Fig. 6. In Bean's limit, J¢ is given by the relation J¢ = Hp/a and a value of 10s A/cm 2 is obtained at 5 K using the Hp value. Watanabe et al. [25] have reported in QMG 123 samples a J¢ of 107 A/cm 2 at 4.2 K. However, many flux jumps were observed in their sample. In the present study though flux jumps were observed in melt-grown stoichiometric 123, no such effect was observed at 5 K in melt-grown 123+28 mol% 211 + 5 wt.% Ag samples and this may be attributed to strong pinning of the vortex lattice.

M(H) curves in Bean's limit, we next attempted to fit the low-field magnetisation data using Kim's model. The equations governing the shape of the hysteresis loops for the low-field case, where the maximum applied field (Hm) is less than Hp have been given by Chen and Goldfarb [ 24]. In these equations Ho and k are related to Hp and p. With appropriate values of Hp and p one can generate the magnetisation curves. A computer program was used to calculate the M(H) values for the expressions given by Chen and Goldfarb [24]. We have used the experimentally obtained M(H) curve with a maximum applied field of rim = 2000 G at 5 K for analytical fitting. A good fit was obtained for the experimentally obtained M(H) data with Hp = 7190 G andp = 0.64 (Fig.7(a)). Using these values o f p and Hp, the constants Ho and k were calculated and were found to be 37 110 A / c m and 4X 109 A2/cm 3, respectively. Jc(Hi) can be now obtained from Kim's equation. Variation of Jc as a function of the local internal field measured from Kim's model is shown in the inset to Fig. 7(a). Next we attempted to fit analytically the experimentally obtained M(H) curve at 77 K for the high-Hm case (Hm=5.5 T). We observed that the experimentally obtained M(H) curve does not fit the analytically derived M(H) curve well at high fields ( > 2.3 T). This is due to the fact that the experimentally obtained M(H) curve has an anticlockwise tilt about the origin. This indicates that there could be a positive susceptibility term contributing to the M-Hcurve. In order to account

with

K n o w i n g the Hp value f r o m analytically derived

R. Gopalanet al. I Physica C 244 (1995) 106-114

112

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high-Hm case so as to have M ( H ) t o t ~ l = M ( H ) + x H where X corresponds to a positive susceptibility. This imposes an additional fitting parameter X in addition to Hp and p for the analytical derivation o f the M(H) curve. With H p = 6 9 9 G, p = 1.52 and X = 0 . 8 X 10 - 4 e m u g - 1 G - 1 , we could get a good fit to the experimentally obtained M(H) curve at 77 K and this is s h o w n in Fig. 7 ( b ) . The appearance o f a positive susceptibility in the M(H) curve at 77 K signals a paramagnetic contribution in addition to the diamagnetic contribution from the superconducting phase. In order to ascertain this, M versus H curves were obtained at different temperatures and these were analytically fitted with the additional positive susceptibility term xH in the M ( H ) expressions (Fig. 8). The reciprocal of the X values obtained at various temperatures, when plotted against T (inset to Fig. 8) was found to s h o w a C u r i e - W e i s s

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Fig. 7. ( a ) Magnetisation curve for maximum applied field (Hm) of 2 0 0 0 G at 5 K for melt-grown 1 2 3 + 2 8 mol% 2 1 1 + 5 wt.% Ag sample. Symbols ([3) and ((3) represent experimentally and analytically obtained M ( H ) curves. The calculation conditions for analytically derived M(H) curves are Hp = 7 1 9 0 G and p = 0 . 6 4 . Variation of Jc as a function of the local internal field measured from

Kim's model is shown in the inset. (b) Magnetisation curve for maximum applied field (Hm) of 5.5 T at 77 K for melt-grown 123 + 28 mol% 211 + 5 wt.% Ag sample. Symbol ((3) and the solid line represent experimentally and analytically obtained M(H) curves, respectively. Analyticallyderived M(H) curves are obtained by adding an additional term xH to the magnetisation equations given by Chen and Goldfarb [24] with the fitting parameters of Hp= 699 G, p = 1.52 and X = 0.8 × 10- + emu g- 1 G- 1. The variation of Jc as a function of the magnetic field measured from Kim's model is shown in the inset.

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Fig. 8. Magnetisation curves obtained at different temperatures for melt-grown 123 + 28 mol% 211 + 5 wt.% Ag sample. Symbols represent experimentally obtained M(H) curves. Solid lines represent analytically derived M(H) curves with an additional term XH added to the equations given by Chen and Goldfarb [24]. The plot of X- 1 vs. temperature is shown in the inset. X values are obtained from analytical fitting of the M(H) curves at different temperatures.

R. Gopalan et al. / Physica C244 (1995) 106-114

constant was estimated from the slope of the 1/X versus Tplot as ~ 0 . 6 × 10 -3 emug -1 G -1 K. Thompson et al. [26] have reported an additional paramagnetic contribution to the magnetisation in GdBa2Cu307_x high-temperature superconductors. The magnetic susceptibility of this material, which has nearly the same superconducting transition temperature as YBa2Cu3OT_x was shown to follow the Curie-Weiss law above and below T¢. The paramagnetic susceptibility was interpreted as arising from the gadolinium sublattice where Gd 3÷ ions carry a magnetic moment. The authors concluded that superconductivity and field-induced paramagnetism exist largely independently of one another in the compound. Similar observations have been made by Liu et al. [ 27 ] for the TIo.sPbo.sCao.sEro.2Sr2Cu2Oysuperconductor. The observation of paramagnetism cannot be due to the above reasons in the present case because there is no magnetic moment on the yttrium ion in YBa2Cu307_ x. It is known that the susceptibility of the "green" phase Y2BaCuO5 (211) shows a CurieWeiss behaviour above ~ 30 K [28,29]. The Curie constant obtained in our work is close to the values for the 211 phase reported in the literature [28]. From the above it can be concluded that the paramagnetic contribution observed in the present study could arise from some of the 211 inclusions present in the sample. We did not observe any paramagnetic contribution below ~ 70 K in the present study. It is known that YBa2Cu307 -x when made deficient in oxygen can give rise to different T¢s with Tc varying from 90 K at 06.96 to about 60 K in the vicinity of 06.67 [30]. Owing to the difficulties associated with oxygenating the highly dense melt-grown samples, there is a possibility of some amount of superconducting material remaining in the lower-T¢ form. Hence it can be speculated that the 211 inclusions in this phase, which are shielded from the applied field at lower temperatures, get exposed to the magnetic field when the lower-T¢ phase turns normal and contribute to the paramagnetic susceptibility.

113

ties at various stages of processing were monitored by measuring the M-H curves using a SQUID magnetometer. We have observed that the microstructure of meltgrown 123 consists of several large domains within which the 123 platelets are oriented with cracks at the platelet boundaries. Our study showed that the 123 platelet width and crack width at platelet boundaries decrease with similar dependences on increasing the 211 content. Our TEM study revealed secondary defect at the 123/211 interface in elliptical 211 particles, suggesting that the defects associated with the 123/211 interface can be the possible pinning sites as observed by Mironova et al. [ 13 ]. Our study enables us to find a critical interface radius below which the defects occur. The magnetic Jc obtained by SQUID measurements showed a steady increase with increasing 211 content. In a fully optimised sample ( 123 with 28 mol% 211 and 5 wt.% Ag), we could obtain a high Jc and large levitation effects. The large magnetic J~ obtained in the optimised composition can be attributed to the presence of pinning centres associated with the 211 particle and almost the complete elimination of the weak links by closing of the cracks between the 123 platelet boundaries. An anticlockwise rotation has been observed in the M-H curves above 70 K in 123 + 28 mol% 211 + 5 wt.% Ag sample which is the optimised composition that gives the maximum critical current density and also the best dependence of J¢ on H. Such a tilt is, however, not observed below ~ 70 K. This rotation is shown to correspond to an additional paramagnetic contribution to the susceptibility from the samples and the rotated M-H curves could be fitted with an additional term (xH) added to the expressions for the magnetisation given by Chen and Goldfarb [24]. The reciprocal of the X values obtained at various temperatures, when plotted against T is found to show a Curie-Weiss behaviour. The additional paramagnetic contribution is suggested to originate from 211 inclusions in the poorly oxygenated portions of the sample.

Acknowledgements 4. Conclusions We have studied the effect of systematically varying the content of 211 inclusions and that of silver additions in the melt-grown 123 material. The magnetic proper-

The authors are thankful to the Director, Defence Metallurgical Research Laboratory, Hyderabad for the permission to publish this work and to the National Superconductivity Science & Technology Board

! 14

R. Gopalan et al. I Physica C 244 (1995) 106-114

(NSTB), Department of Science & Technology (DST), New Delhi, for financial support.

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