Study of grain boundary characteristics of proton irradiated textured polycrystalline Bi2Sr2CaCu2O8+x and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+x superconductors

PERGAMON

Solid State Communications 120 (2001) 201±204

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Study of grain boundary characteristics of proton irradiated textured polycrystalline Bi2Sr2CaCu2O81x and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O101x superconductors P. Sen*, S.K. Bandyopadhyay, P. Barat, P. Mukherjee Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Calcutta 700 064, India Received 9 July 2001; accepted 10 August 2001 by C.N.R. Rao

Abstract Intergranular critical current density …Jcint † has been measured for 15 MeV proton irradiated textured polycrystalline Bi2Sr2CaCu2O81x (Bi-2212) and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O101x (Bi-2223) superconductors. The effects of irradiation on the grain boundary characteristics, in particular junction barrier thickness (d) and junction width (w), have been studied as a function of dose. At higher dose (i.e. .5 £ 1015 protons/cm 2), Jcint is affected primarily by d rather than by w which is more prominent in Bi-2223 as compared to Bi-2212. q 2001 Published by Elsevier Science Ltd. PACS: 61.80J; 74.60 Keywords: A. Superconductors; C. Grain boundaries; D. Tunneling

1. Introduction

2. Experimental

High temperature superconductors (HTSC) are granular in nature and the transport critical current density depends on both intragranular and the intergranular network. We had earlier observed a substantial increase in intragranular critical current density and pinning potential of irradiated BSCCO systems through magnetization and magnetoresistance studies at high ®eld (up to 7 T) [1,2]. Intergranular current density …Jcint † depends primarily on the grain boundary characteristics such as junction barrier thickness (d), width (w) and length (l). These parameters can be affected during particle irradiation. Thus it has become important to study the grain boundary characteristics as a function of dose. For the purpose of understanding the grain junction characteristics, magnetization at very low ®eld (30 Oe) i.e. far below Hc1 (,100 Oe) has been measured, where the magnetization current ¯ows essentially through the grain boundary [3,4]. In this paper, we have evaluated these grain boundary parameters as a function of dose from magnetization and analyzed their manifestations in Jcint :

Textured polycrystalline Bi-2212 and Bi-2223 samples, identical with respect to Tc (variation within ^0.25 K), remanent magnetization [0.0085 (^0.0005) emu/gm for Bi-2212 and 0.0175 (^0.0005) emu/gm for Bi-2223 at temperature of 20 K and ®eld of 30 Oe] were chosen for irradiation with 15 MeV protons available from Variable Energy Cyclotron Centre. DC magnetization were performed at a temperature of 20 K in DMS-1600 vibrating sample magnetometer having sensitivity of 1 £ 1024 emu. The demagnetization factor (0.716) has been incorporated on the basis of oblate spheroid model [5]. 3. Results and discussions Hysteresis loops corresponding to the applied ®eld of 30 Oe at 20 K for Bi-2212 and Bi-2223 systems are shown in Fig. 1(a) and (b), respectively. The intergranular Jcint for low ®eld can be expressed as Jcint ˆ ‰H=2pŠf …D†

* Corresponding author. Fax: 191-33-3346871. E-mail address: [email protected] (P. Sen).

…1†

where D is the demagnetization factor and H is the magnitude of ®eld. For practical purpose, H has been compared

0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00378-7

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Fig. 2. Field dependence of intergranular critical current density …Jcint † as a function of dose for Bi-2212 and Bi-2223.

Fig. 1. Hysteresis loops at a ®eld of 30 Oe for (a) Bi-2212 and (b) Bi-2223 at different doses at 20 K. Field applied parallel to c-axis.

with the remanent magnetization. The term p is the average range of intergranular pinning. It has been seen that for an isolated Josephson vortex in two-dimensional array of grains, p is equal to the grain radius Rg [6]. Jcint of Bi-2212 and Bi-2223 are plotted in Fig. 2(a) and (b), respectively. In general, intergrain regions are of high energy and are being primarily affected by irradiation. The insigni®cant changes in Jcint values for both Bi-2212 and Bi-2223 up to the dose of 5 £ 1015 protons/cm 2 are indicative of insigni®cant damage occurred in the intergranular region during irradiation. This may be due to low displacement per atom [d.p.a] (,6 £ 1025 for 5 £ 1015 protons/cm 2 in BSCCO systems based on calculation using modi®ed TRIM [7]) which is low enough to cause any substantial damage in the grain boundary region [8]. Above 5 £ 1015 protons/cm 2, Jcint falls drastically in Bi-2223 as compared to Bi-2212 (Fig. 2) which is the manifestation of the change of grain boundary parameters (i.e. w and d) with dose as will be evident later. In the polycrystalline sample, the shielding current is presumed to ¯ow through a network of Josephson junction at grain boundary. ThePoverall current (for a single loop) can be written as I ˆ nk ick ; where nk is the number of Josephson junction in the kth area and ick is the critical

current of any junction. ick follows the Fraunhoffer relation as ick ˆ ic…0† usin…f†=fu where f is the ¯ux entered through the junction. In a junction having junction barrier thickness (d) and width (w), f can be written as f ˆ ‰pm0 …2l 1 d†wHi Š where Hi is the internal ®eld and l is the penetration Ê for BSCCO). depth (l , 3000A The current density pro®le for each sample was calculated by evaluating a single current loop (from selfconsistent summation of Josephson currents) across the network of grain boundaries over the whole thickness (Fig. 3(a)) and then by determining the series of such current loops starting from the surface of the sample to inward, until the total shielding ®eld …H i † produced by such loops is equal to the applied ®eld (H). The boundary condition for such a pro®le at the surface can be expressed as Hi ˆ H and Jc ˆ Jc …H† obtained from intergranular Jc. This two-dimensional problem can be reduced to a one-dimension by considering symmetry [9]. For a ®nite thickness t, the shielding current can be written as Iˆ

t X t0

sin pm0 …2l 1 d†wHi =f0 DxDz Jc0 pm0 …2l 1 d†wHi =f0

…2†

where t0 is the minimum sample thickness of the order of 1 mm. In the case of ascending loop (Fig. 3(b)), the local magnetization is given by 8 > < 2H Mi ˆ Zb=2 > J0 …x† dx : x0

0 , x , x0 x0 , x , b=2

…3†

For a descending loop (Fig. 3(c)), from a certain

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Fig. 3. (a) Direction of shielding current loop for a rectangular sample under an applied ®eld in z-direction. Length of the sample is a and width is b. (b) Internal ®eld and current density distribution for the ascending branch of the M±H loop: x0 is the length measured from the origin which shows the extent of the sample free from ®eld and current: J0 is the maximum shielding current density corresponding to minimum internal ®eld (Hi). (c) Internal ®eld and current density distribution when applied ®eld just starts decreasing from Hm (maximum ®eld): xm is the length measured from the origin which shows the extent of the sample free from the ®eld and current: …b=2 2 x1 † is the extent of the sample from the surface through which positive J1 supercurrent ¯ows; Jm is the maximum shielding current density.

maximum ®eld (Hm), the local magnetization is given by 8 > 2H 0 , x , x0 > > > Zx1 > > < …H 2 H† 1 Jm …x† dx xm , x , x1 m Mi ˆ …4† xm > > > Z b=2 > > > J1 …x† dx x1 , x , b=2 : x1

The parameters x0, x1, xm, b, J0, and J1 have been explained in Fig. 3. Junction barrier thickness (d), width (w), length (l) and zero-®eld current density of a junction [Jc(0)] have been considered as input variables for the calculation of M due to current loops while simulating the experimentally observed values of DM as a function of doses. Fig. 4(a) and (b) shows the variation of d and w with dose. In both systems, the junction width (w) and thickness (d) change insigni®cantly up to the dose of 5 £ 1015 protons/ cm 2 (Fig. 4) and thus Jcint remains constant (Fig. 2). Above

5 £ 1015 protons/cm 2, they follow a power law as w , ‰doseŠa and d , ‰doseŠb : The exponent a is of the same order (,0.50) for both the systems. But the exponent b in Bi-2223 (1.08) is quite high as compared to Bi-2212 (0.53). In our earlier PAL studies, we have seen that the defect concentration in the unirradiated Bi-2223 (5.10 ppm) is more than that of unirradiated Bi-2212 (2.63 ppm) [1]. Bi-2223 with large concentration of defects is vulnerable to generate irradiation induced defects which accommodate in the grain boundary region causing a signi®cant increase in junction barrier thickness d (in the nm range) compared to width w (in the mm range). Thus the grain boundary thickness is more affected due to proton irradiation than the width and this is more prominent in Bi-2223 containing more defects. This is re¯ected in the variation Jc with d and w as a function of dose. The variation of Jcint with d and w can also be expressed by power law relation as Jcint , w2a1 and Jcint , d 2b1 . It is seen from the variation of Jcint with w that the exponent a 1 for

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Jc. Thus proton irradiation helps to increase the Hc2 in Bi-2212 contrast to Bi-2223 up to a certain value of dose. 4. Conclusion We have determined the intergranular critical current density of proton irradiated Bi-2212 and Bi-2223 as a function of dose by measuring magnetization at low ®eld (30 Oe). We have also estimated the variation of the junction parameters with dose. Jcint are not being affected during irradiation up to the dose of 5 £ 1015 protons/cm 2 as the junction barrier thickness and width change insigni®cantly. Above this dose, there is a drastic change in barrier thickness compared to width with dose, particularly in case of Bi-2223. This has a manifestation of Jc depending more strongly on d rather than w as re¯ected in higher values of the exponent b 1 compared to the exponent a 1. Acknowledgements Fig. 4. (a) Variation of junction barrier thickness d and (b) junction width w as a function of dose for Bi-2212 and Bi-2223.

Bi-2212 and Bi-2223 are 0.73 and 0.18, respectively. The variation of Jcint with d shows that the exponent b 1 is high as compared to a 1 (e.g. b 1 for Bi-2212 and Bi-2223 are 0.93 and 0.76, respectively) especially in Bi-2223. The granular polycrystalline superconductors can be thought of as SNS type junctions with the grain boundary being normal regions connecting superconducting grains. The current can be conceived as arising from tunneling across the junction barrier thickness. The junction barrier thickness d is in the range of nm which is ideal for tunneling. Thus Jcint depends more drastically on d than on w and this is more prominent in Bi-2223 where the exponent b 1 is much higher than a 1 due to large concentration of defects at the unirradiated stage itself. But in case of Bi-2212, b 1 and a 1 are comparable as the variation of d and w with dose are almost same. The present study reveals that there is no signi®cant change in the grain boundary properties up to the dose of 5 £ 1015 protons/cm 2. Earlier we had seen that the intragranular pinning potential and hence intragranular Jc are maximum at 5 £ 1015 protons/cm 2 in Bi-2212. Upper critical ®eld Hc2 manifests the enhancement of intragranular

The authors would like to thank Professors Bikash Sinha and J.N. De for their constant encouragement and support. The authors also like to express their gratitude to Dr A. Sen of Central Glass and Ceramic Research Institute for rendering the facility of DMS-1660 Vibrating Sample Magnetometer. References [1] P. Sen, P.M.G. Nambissan, S.K. Bandyopadhyay, P. Barat, M. Ghosh, A. Barman, P. Mukherjee, S.K. De, Physica C 303 (1998) 108. [2] P. Sen, S.K. Bandyopadhyay, P.M.G. Nambissan, P. Barat, P. Mukherjee, Int. J. Inorg. Mater. (2001) in press. [3] G.C. Kim, M.Y. Cheon, Y.C. Kim, Physica C 300 (1998) 105. [4] N. Murayama, Y. Kodama, S. Sakaguchi, F. Wakai, K. Michishita, Y. Ikuhara, Physica C 185±189 (1991) 2213. [5] J.A. Osborn, Phys. Rev. 67 (1945) 351. [6] K.-H. MuÈller, M. Nikolo, R. Driver, Phys. Rev. B 43 (1991) 7976. [7] J.P. Biersack, L.G. Haggmark, Nucl. Instrum. Meth. 174 (1980) 257. [8] O. Meyer, in: A.V. Narlikar (Ed.), Studies of High Temperature Superconductors, vol. 1, Nova Publishers, New York, 1989, p. 139. [9] D.-X. Chen, R.B. Goldfarb, J. Appl. Phys. 66 (1989) 2489.