The study of inter and intragranular pinning behavior of oxygen irradiated textured polycrystalline Bi2Sr2CaCu2O8+δ and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ superconductors

Physica C 407 (2004) 55–61 www.elsevier.com/locate/physc

The study of inter and intragranular pinning behavior of oxygen irradiated textured polycrystalline Bi2Sr2CaCu2O8þd and Bi1:84Pb0:34Sr1:91Ca2:03Cu3:06O10þd superconductors Pintu Sen

a,*

, S.K. Bandyopadhyay a, P.M.G. Nambissan b, R. Ganguly c, P. Barat a, P. Mukherjee a a b

Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Calcutta 700064, India Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Calcutta 700064, India c Bhabha Atomic Research Centre, Mumbai 400085, India

Received 3 December 2003; received in revised form 13 March 2004; accepted 22 March 2004 Available online 7 June 2004

Abstract Inter and intragranular pinning potentials of 116 MeV oxygen ion irradiated textured polycrystalline Bi2 Sr2 CaCu2 O8þd (Bi-2212) and Bi1:84 Pb0:34 Sr1:91 Ca2:03 Cu3:06 O10þd (Bi-2223) have been studied from dc magnetization at low (50 Oe) and high (10,000 Oe) fields respectively. There is an increase in intergranular pinning potential ðU0 Þ in Bi2223 in contrast to anomalous decreases in Bi-2212 with increasing dislocation density. Intragranular pinning potential is governed by the concentration of vacancy type defects rather than its size as observed in proton irradiation. Pinning has been explained in the light of statistical dilute pinning model.
2004 Elsevier B.V. All rights reserved. PACS: 61.80.J; 74.60 Keywords: Oxygen irradiation; Inter and intragranular pinning; Magnetization; BSCCO superconductors

1. Introduction Cuprate based high temperature superconductors (HTSC) are nonstoichiometric, particularly with respect to oxygen content. Charged particle irradiation causes knock-out of oxygen thereby generating oxygen vacancies, which contribute significantly towards pinning of fluxes leading to

*

Corresponding author. Fax: +91-33-23346871. E-mail address: [email protected] (P. Sen).

the enhancement of critical current density ðJc Þ. We had earlier carried out 15 MeV proton irradiation studies on Bismuth based superconductors Bi-2212 and Bi-2223 with respect to Jc and pinning potential [1,2]. There was a remarkable difference between these two systems with respect to irradiation induced enhancement of Jc and pinning potential which could be explained in light of irradiation induced knock-out of oxygen and their differences in these two systems. We have also carried out irradiation with heavy ions like oxygen of moderate energy (116 MeV) with large energy

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2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.250

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P. Sen et al. / Physica C 407 (2004) 55–61

deposition as compared to 15 MeV protons, revealed from the values of displacement per atom (d.p.a.) of these ions (d.p.a. of 15 MeV protons and 116 MeV oxygen ions at dose of 1 · 1015 ions/ cm2 are 1.2 · 105 and 6 · 103 , respectively). Oxygen ions have been seen to be intercalated in Bi–O layer as revealed from decrease in bulk life time ðsb Þ in positron annihilation lifetime spectroscopy (PALS) [3]. Also, there have been generation of significant amount of dislocations presumably in the high energy grain boundary region along with considerable amount of vacancy type defects in the intragranular region [3]. HTSC systems are granular in nature and transport critical current density depends on both intergranular and intragranular network. Intergranular critical current density and the pinning due to weak Josephson vortices can be assessed by magnetization at low field, i.e. far below Hc1 (100 Oe), where magnetization current flows essentially in the grain boundary region [4–6]. On the other hand, magnetization at high field reveals the nature of intragrain Abrikosov vortices as the flux enters inside the grain. In this paper, dc magnetization studies of 116 MeV oxygen ion irradiated samples at low field (i.e. 50 Oe) and high field (i.e. 10,000 Oe) have been done. We have studied the intergranular and intragranular pinning behavior in the light of defect morphology revealed in our earlier analysis of PALS for vacancy type defects and X-ray diffraction line profile (XRDPL) analysis for dislocations [3].

2. Experimental Textured polycrystalline Bi-2212 and Bi-2223 samples, identical with respect to Tc (variation within ±0.25 K), remanent magnetization (0.0245 emu/gm for Bi-2212 and 0.0285 emu/ gm for Bi-2223 at temperature of 5 K and field of 50 Oe) were chosen for irradiation with a beam of 116 MeV oxygen ion (Oþ5 ) obtained from Variable Energy Cyclotron Centre at Kolkata. The penetration depth of 116 MeV Oþ5 ion is 80 l. Samples have been irradiated from both sides to obtain the maximum volume of irradiation. Doses employed were 1 · 1013 , 5 · 1013 , 1 · 1014 and

5 · 1014 Oþ5 /cm2 . The beam current was 100 nA to avoid local heating. DC magnetizations were performed at a temperature of 5 K by EG&G PARC––4500 vibrating sample magnetometer. The magnetic field was applied along c-axis (i.e parallel to the direction of irradiation). The fields used for minor and major hysteresis loops were 50 and 10,000 Oe, respectively. Samples for magnetization measurements were in the shape of rectangular bar (2 · 2 · 0.5 mm3 ). The demagnetization factor incorporated in the magnetization measurements has been calculated on the basis of oblate spheroid model [7,8] and was assigned as 0.716.

3. Results and discussion 3.1. Intergranular pinning behavior At low magnetic field ð Hc1 Þ, the magnetic flux penetrates only in the grain boundary region and thus pinning potential is controlled by the defects present at that region. The intergranular Jcint for low field can be expressed as Jcint ðT Þ ¼ ½H =2d f ðDÞ

ð1Þ

The term d is the average range of intergranular pinning potential which is equal to the grain radius Rg for an isolated Josephson vortex in two-dimensional array of grains [9]. D is the demagnetization factor and H is the magnitude of field. For practical purpose, H has been taken as the remanent magnetization i.e. amount of trapped flux at zero field determined in our measurements of minor hysteresis loops (Fig. 1). Neglecting the mutual vortex repulsion and their creep phenomenon, the average pinning force density Fp can be written as [10] Fp ¼ U0 =½Rg Vb

ð2Þ

where Vb is the flux bundle volume and is given by the relation Vb ¼ 2Rg ½/0 =jBj . /0 is the flux quantum. The intergranular pinning potential U0 at zero field is assumed in its simple form as a

U0 ðT Þ ¼ U0 ð0Þ½1  ðT =Tc Þ

ð3Þ

P. Sen et al. / Physica C 407 (2004) 55–61

5x1013 1x1014 5x1014

0.00

5K

-0.05

M (emu/gm)

H // C (b)

Unirradiated

0.1

1x1013 5x1013 1x1014 5x1014

0.0

-0.1

-60

-40

-20

0

20

40

60

Field (Oe) Fig. 1. Minor hysteresis loops at a field of 50 Oe for: (a) Bi2212 and (b) Bi-2223 at different doses at 5 K.

where a is considered to be 2 assuming grain boundary junctions to be SNS type [11]. The intergrain pinning potential U0 ð0Þ and dislocation density ðqÞ are plotted in Fig. 2a and b respectively for Bi-2212 and Bi-2223 at different doses. In Bi-2212, U0 (4.36 eV in the unirradiated stage) is found to decrease monotonically up to the dose of 5 · 1013 Oþ5 /cm2 (0.70 eV) and afterwards, it decreases very slowly. In contrast, a sharp rise in U0 up to 8.51 eV is observed at the dose of 5 · 1013 Oþ5 /cm2 in Bi-2223. Dislocation density is generally much higher in the high energy intergranular region compared to that in the grain. Thus, during irradiation with heavy ions like oxygen, the high energy intergranular regions are at first getting more affected than the intragranular regions. A large increase in dislocation density is therefore observed at the initial stage of irradiation in Bi-2212 (Fig. 2a). The

15

9

Bi-2212 Bi-2223 (b)

(a)

10

6

5

3

0

0.00

2.5x1014

5x1014

ρ ( x 1010 / cm2 )

0.05

high value of dislocation density may be responsible for sharp decrease in U0 due to unfavorable complex configuration formed at the grain boundary. These may be due to irradiation induced increase of grain boundary thickness. We had earlier observed in case of proton irradiation that the increase of grain boundary thickness affects Jc more than other grain boundary parameters [12]. Above 1 · 1013 Oþ5 /cm2 , the probability of dislocation density as well as vacancy type defect increases simultaneously which thus further restricts U0 to a lower value. On the other hand, Bi-2223 with more nonstoichiometric composition contains more point defects (5.26 ppm) and dislocation density (4.975 · 1010 /cm2 ) at the unirradiated stage itself as compared to Bi-2212 (2.66 ppm as vacancy and 2.647 · 1010 /cm2 as dislocation density). Hence, low dose of irradiation does not contribute further to a significant change in dislocation density of Bi2223 at the initial stage of irradiation. Above 1 · 1013 Oþ5 /cm2 , both dislocations and vacancy type defects are increasing simultaneously resulting in decrease of intergranular pinning potential U0 with dose except at 5 · 1013 and 1 · 1014 Oþ5 / cm2 , where U0 increases quite significantly (Fig. 2). This anomalous behavior is not fully understood. Perhaps, the dislocation density q (5.117– 6.885 · 1010 /cm2 ) at the irradiation dose of 5 · 1013 and 1 · 1014 /cm2 may give rise to an optimum

U0 (0) (eV)

Unirradiated 1x1013

(a)

57

0

Dose ( O+5 / cm2 )

Fig. 2. (a) Intergranular pinning potential U0 ð0Þ and (b) dislocation density ðqÞ (represented as dotted lines) of Bi-2212 and Bi-2223 as a function of dose.

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P. Sen et al. / Physica C 407 (2004) 55–61

stable defect configuration at the grain boundary resulting in effective pinning with high value of U0 . This phenomenon is not observed in Bi-2212 as the optimum value of dislocation density like Bi-2223 is not seen in Bi-2212 (Fig. 2).

shows JcH versus H for Bi-2212 and Bi-2223 at different doses. Jc0 defined as the critical current density at zero magnetic field, has been evaluated using empirical relation

3.2. Intragranular pinning behavior

where Jc0 and H0 are the fitting parameters [14]. Jc0 values have been determined to extract the effect of irradiation induced defects on Jc0 in absence of magnetic field. Jc0 and concentration of vacancy type defects determined through PALS, have been plotted in Fig. 5a and b for Bi-2212 and Bi-2223 respectively. In Bi-2212, intragranular Jc0 increases with dose and it is maximum (1.2 · 105 A/cm2 ) at 5 · 1013 Oþ5 /cm2 whereas in Bi-2223 Jc0 is maximum (1.92 · 105 A/cm2 ) at 1 · 1014 Oþ5 /cm2 (Fig. 5). In Bi-2212, at the initial stage of oxygen irradiation

Fig. 3a and b show some typical magnetization curves at high field (10,000 Oe) for Bi-2212 and Bi2223 respectively. At the field higher than Hc1 , flux enters the grain and hence the intragranular critical current density JcH can be evaluated from Clem–Bean formula [13] JcH ¼ ½30DM =a

ð4Þ

where M is magnetization and a is the average grain size ð2Rg Þ of the samples taking into account of the granularity in polycrystalline samples. Fig. 4

0.4

JcH ¼ Jc0 expH =H0

Unirradiated 1x1013 5x1013 1x1014 5x1014

(a)

0.2

(a) 1.0

0.0

5K H // C

-0.4 0.8

Unirradiated 1x1013 5x1013 1x1014 5x1014

0.5

Unirradiated 1x1013 5x1013 1x1014 5x1014

(b)

0.4

JcH (X 105 A /cm2 )

M (emu / gm)

-0.2

ð5Þ

5K H//C 0.0 2.0

(b)

1x1013 5x1013 1x1014 5x1014

1.5

0.0

1.0

-0.4

0.5

-0.8

0.0 0

-10000

-5000

0

5000

10000

Field (Oe) Fig. 3. Major hysteresis loops at a field of 10,000 Oe for: (a) Bi2212 and (b) Bi-2223 at different doses at 5 K.

Unirradiated

1000 2000 3000 4000 5000 6000 7000 8000 9000

Field (Oe) Fig. 4. Field dependence of intragranular critical current density ðJc Þ of: (a) Bi-2212 and (b) Bi-2223 as a function of dose (The dotted lines represent the fits to the exponential relation.).

P. Sen et al. / Physica C 407 (2004) 55–61 2.0

Bi-2212 Bi-2223

12.5

1.5 10.0

(b)

1.0

7.5

Conc. (ppm )

Jc0 (x 105 A / cm2 )

(a)

15.0

5.0

2.5

0.5

0.00

2.5x1014

5x1014

Dose ( O+5 / cm2 )

Fig. 5. (a) Intragranular Jc0 and (b) vacancy type defect concentration (represented as dotted lines) of Bi-2212 and Bi-2223 as a function of dose.

(i.e. up to 1 · 1013 Oþ5 /cm2 ), the concentration of defect as divacancies remains almost constant, but it increases with dose due to nonionising energy loss (NIEL) of the oxygen ions through elastic collisions with different atoms. Intragranular Jc0 is seen to increase with the concentration of vacancy type defects. This is in contrast to intergranular Jcint and pinning potential U0 , which decrease with increasing dose where dislocation density has increased. Above 5 · 1013 Oþ5 /cm2 , the size of defect increases significantly in Bi-2212 due to agglomeration of vacancies accompanied with the reduction in the defect concentration, which can be explained in the light of availability of loosely bound excess oxygen ðdÞ, needed for structural stability in the Bi–O layer [15]. These excess oxygen are loosely bound (BE ¼ 0.073 eV) and are highly vulnerable to be knocked out by energetic oxygen ions [16]. With increasing dose of irradiation, a large number of oxygen vacancies are thus produced. The probability of agglomeration of the vacancy is high when the concentration and size of the vacancy are large. In Bi-2212, the concentration of vacancy is high only at 5 · 1013 Oþ5 /cm2 . Thus a large number of oxygen vacancies undergo agglomeration with the existing vacancies leading to an effective increase in their size from divacancy to trivacancy at the cost of reduction in defect concentration and we observe a decrease in intra-

59

granular Jc0 . As mentioned earlier, in oxygen irradiation, intraganular Jc0 increases with increasing the concentration of vacancy type defects rather than its size in contrast to proton irradiation, where size of defects was the governing factor for enhancing Jc0 [2,3]. In Bi-2223, the vacancy type defect concentration (5.6 ppm) as monovacancy is high even in the unirradiated stage as compared to Bi-2212 (2.6 ppm). Moreover, oxygen ion irradiation generates large number of vacancies. These defects combine with those already present and thereby form large vacancy cluster as divacancy with higher concentration in Bi-2223 up to the dose of 5 · 1013 Oþ5 / cm2 in contrast to proton irradiation where defect size remained constant with dose. Increasing size and concentration of defects with dose thus cause increasing Jc0 in oxygen irradiated Bi-2223 up to the dose of 5 · 1013 Oþ5 /cm2 . At 1 · 1014 Oþ5 /cm2 , the defect concentration increases enormously with reduction of size from divacancy to monovacancy. The probability of cationic displacement at large dose is high as the value of d.p.a for oxygen irradiation is quite significant. The irradiation induced changes can be assessed from d.p.a based on the calculation using TRIM-98 [17]. Occupancy of cationic atom at the site of defects (leaving its original lattice site as monovacancy) reduces the size of vacancy cluster but increases the concentration of defects. In case of Pb-doped Bi-2223, the positron trapping sites are essentially monovacancies. The availability of loosely bound oxygen in the Bi–O layer of Bi-2223 is less due to partial substitution of smaller Biþ3
by larger Pbþ2 ion (1.20 A)
needed for ion (0.93 A) attaining structural stability as compared to Bi2212 [15]. Owing to unavailability of these loosely bound excess oxygen atoms in Bi-2223, occupancy of cationic atoms at the site of irradiation induced oxygen defects dominates over agglomeration leading to an effective decrease in defect size with increasing concentration (13.50 ppm) and shows maxima in Jc0 . However, above 1 · 1014 Oþ5 /cm2 , the high defect concentration may be responsible for agglomeration of vacancies with the existing ones resulting in further increase of defect size at the cost of reduction in defect concentration. This may

P. Sen et al. / Physica C 407 (2004) 55–61

lead to finally decrease in Jc0 . The extent of size increment in Bi-2223 is restricted only to divacancy as it is deficient in loosely bound excess oxygen. On investigating the variation of Jc with applied magnetic field, we notice that there is a power law dependence as Jc / Bb . There are various models depending on defect size, shape and concentration. The simplest is the summation model valid at low field (i.e Bapp  Bc1  100 Oe), which incorporates only the summing of elementary pinning forces ðfp Þ. Jc can be written as [18] Jc ¼ ½Nd fp =B

ð6Þ

In real flux lattice with the applied field B > Bc1 (i.e. the region of our interest), there is a net pinning force causing distortion of the vortex lattice and hence summation model is not valid here. The distortion is parameterized by u. It is a measure of the strength of pinning [19] and is expressed by u=a0 , where a0 is the intervortex spacing dependent 0:5 on the external magnetic field as a0 / ð/0 =BÞ , where /0 is the quantum of flux. Here Jc can be expressed as [20] Jc ¼ ½Nd fp2 ðu=fp Þðd=a20 Þ =B

ð7Þ

The range of pinning force (d) is typically of the order of n. At high field, pinning is highly correlated and the collective pinning approach is valid in the weak pinning scenario [21]. The most modified form of Jc in this case is 2 Jc ¼ ½0:0039Nd2 fp4 =ðd2 C66 C44 Þ =B

ð8Þ

where C66 and C44 are elastic moduli of flux line lattice for shear and tilt respectively. The dependence of Jc on B is quite different in these three different models. The direct summation model in Eq. (6) follows the power law Jc / Bb where b is equal to )1. In the statistical dilute pinning model given in Eq. (7), b  0:5. Collective pinning approach given in Eq. (8) roughly describes the exponent b  3:75. In our systems, the results are in good agreement with prediction of statistical dilution model with b  0:4. In order to investigate the effect of defect morphology on pinning, the distortion parameter u has been determined based on statistical dilute pinning

9.0

Bi-2212 Bi-2223

U / a0 (%)

60

5K H // C 7.5

6.0

0.00

2.5x1014

5x1014

Dose (O +5 /cm2 ) Fig. 6. Distortion parameter U =a0 of Bi-2212 and Bi-2223 as a function of dose at 5 K under magnetic field of 1000 Oe.

model. The values of u=a0 have been plotted in Fig. 6 for Bi-2212 and Bi-2223 at different doses. The more the distortion, the more is the probability of gaining strain energy by the system, which may be responsible for depinning of flux line lattice (FLL) resulting in reduction of Jc . In Bi-2212, this is in fact in agreement with our prediction and Jc is maximum at the dose of 5 · 1013 Oþ5 /cm2 where u=a0 is minimum. There is a correlation between concentration of defect and the distortion parameter. As the concentration of defect increases the pinning sites, the vortex thus experiences less strain by the application of magnetic field, causing thereby reduction in distortion parameter u=a0 . In Bi-2223, intragranular Jc also increases with dose with reduction of u=a0 . Jc is maximum at 1 · 1014 Oþ5 /cm2 with the minimum value of u=a0 . At higher dose, i.e. above 1 · 1014 Oþ5 /cm2 , intragranular Jc is quite low compared to unirradiated one, although it has a sufficient low value of u=a0 determined on the basis of simple vacancy type defects. We have analysed these defects through PALS. But in reality, these defects may be more complex in nature consisting of dislocation, voids, clusters etc. which may not be revealed completely by PALS. Thus the determination of the distortion parameter determined on the calculation from Nd based in PALS for the dose higher than 1 · 1014

P. Sen et al. / Physica C 407 (2004) 55–61

Oþ5 /cm2 should be considered as qualitative indication only.

61

with Vibrating sample magnetometer. The authors would also like to thank Professors Bikash Sinha and Prasanta Sen for their constant encouragement and support.

4. Conclusion We have carried out dc magnetization studies at low field (50 Oe) and high field (I T) for the evaluation of intergranular and intragranular pinning potentials in the 116 MeV oxygen irradiated textured polycrystalline Bi-2212 and Bi-2223 superconductors. There has been a general decrease of intergranular pinning potential ðU0 Þ with dose as in the case of proton irradiation studied earlier. The decrease was associated with increase in dislocation density. But, in case of Bi-2223, an anomalous increase in U0 particularly at the dose of 5 · 1013 and 1 · 1014 Oþ5 /cm2 has been observed. A remarkable difference in intragranular pinning potential has been observed in oxygen irradiated samples as compared to 15 MeV proton irradiation. In Bi-2223, there is a sharp rise in intragranular critical current density at a particular dose of irradiation, which was absent in case of proton irradiation. Moreover, in oxygen irradiation intragranular Jc has increased with increasing the concentration of vacancy type defect in contrast to the size which was the governing factor for enhancing intragranular Jc in case of proton irradiation. We have studied the variation of Jc with magnetic field and the nature of pinning can be explained from statistical dilute pinning model with exponent b  0:4, indicative of strong pinning scenario.

Acknowledgements The authors would like to express their gratitude to Dr J.V. Yakhmi of Bhaba Atomic Research Centre for magenetization measurements

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