Journal of Magnetism and Magnetic Materials 104-107 (1992) 521-522 North-Holland

Enhanced flux line pinning in Pb doped Bi2SrzCaCu2Os+y Sujata Patil, P.L. Paulose, C. Quitmann and G. Giintherodt 2 Physikalisches Institut, RWTH Aachen, W-MOO Aachen, Germany

The magnetization measurements of Pb doped Bi2212 show an enhancement in hysteresis loop widths above 20 K compared to that of Bi2212. This implies improved critical current densities (Jc) and pinning in Pb doped Bi2212 probably due to the presence of impurity phases. The values of the apparent pinning potential U 0 calculated from magnetic relaxation measurements are larger for Pb doped Bi2212 up to a factor of two at 60 K.

In High-To Superconductors (HTSC) magnetic flux lines have a relatively high degree of mobility, which is considered to be due to weak pinning potentials. From an applications point of view, it is desirable to have HTSC with pinning centers which provide efficient pinning at 77 K. It was shown that pinning could be improved for Bi2212 by introduction of cation defects into the sample [1]. Here we report on magnetic prop- erties of Pb doped Bi2212 in comparison to Bi2212, studied with the aim of introducing pinning centers which are effective at temperatures as close as possible to 77 K.

The samples were prepared by normal solid state reaction of the appropriate amounts of Bi20 3, SrCO 3, CaCO~, CuO and PbO at 820 °C for 20 h. The final sintering of Bi2212 was performed in air at 860°C for 48 h. For Pb doped Bi2212 (Biz_xPbxSrzCaCuzOs+y with x =0.3, 0.4) the final sintering was carried out in flowing N 2 at 820 o C for 24 h as air annealing results in a significant amount of impurity phases [2]. The samples were characterized by differential thermal analysis, energy dispersive X-ray analysis and X-Ray Diffraction (XRD). The XRD measurements confirmed that the impurity phases amount to less than 5%. The Pb substituted samples exhibited orthorhombic splittings which increased with Pb concentration. XRD and dc electrical resistivity measurements did not show any signature of the Bi2223 phase.

A SQUID magnetometer (Quantum Design) was used for measurements of magnetic properties. The magnetic relaxation measurements were performed af- ter cooling the sample in zero field and then applying the desired magnetic field. The data were taken in an approximate time window of 100 to 7000 s and at H = 0 . 1 and 1T.

Figure 1 shows hysteresis curves of Bi2212 and Bl.6Pb0.4SrzCaCuzOs+y at 50K. The hysteresis curve of the sample with x = 0.3 is similar to that of x = 0.4 and hence not shown in the figure. The hysteresis loop widths (AM) of all the samples are similar at 5 K. This points to a similarity in the intrinsic defect structure in Bi2212 and Pb doped Bi2212 as intrinsic defects are

more effective at low temperatures in the absence of thermal and other excitations. At higher temperatures, however, the loop widths of the Pb substituted samples are larger than that of Bi2212 (see fig. 1). The Jc is calculated using Bean's model [3] and its field depen- dence at 50 K is shown in fig. 2. The Jr of Pb doped samples (x = 0.3, 0.4) at 50 K is by roughly an order of magnitude larger than that of Bi2212 and obviously the pinning in Pb doped samples has been improved. This could be due to defects introduced by substitution of Pb in Bi2212. XRD measurements show the presence of Ca2PbO 4 (= 3%) as impurity phase. It is possible that this impurity phase or the disorder caused by the substitution act as pinning centers. There have been reports of improved Jr in substituted as well as nonsto- ichiometric HTSC samples [4,1]. But this improved Jc is achieved at the cost of T c. However, in our Pb substituted samples T c shows a marginal increase.

The magnetization measured as a function of time shows a logarithmic time dependence. Therefore, our data could be well described by Anderson's thermally activated flux creep model [5]. According to a modified

0 . 1

"S

E

c- O - 0 , 3

N

~ - 0 . 5

CJ

- 0 . 7 0

T = 50K

===== B[2212

2 4 6 8 10

Magnetic Field (kOe)

Fig. l. Magnetization hysteresis loops of Bi2212 and Bil.6Pb0.aSrzCaCu2Os+y at 50 K.

0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

522 S. Pat# et al. / Enhanced flux line pinning in Pb doped Bi2212

50K

CCO¢O Bi2212 .-..-..-..-..-. Bil.rpbo.aSraCaCu=Os.~

100000 - ; I I I I BiLaPbo.4Sr2CaCu2Os+y g--. E

< o

10000

, , , ; , , , , , , , ,

1000 10000 H(Oe)

Fig. 2. Calculated Jc (using Bean's model) as a function of H for Bi2212 and Bil_~,PbxSr2CaCu2Os+y (x = 0.3, 0.4) at

50 K.

version of this model the decay of magnet iza t ion is given by [6],

d M A M ~ 2

d In t = l n ( t B / r ) - U o / k B T '

where U 0 is the activation energy, tn is the typical t ime at which the m e a s u r e m e n t starts and r is a re laxat ion t ime ( taken to be 10-12 s in this work). The appa ren t p inning potent ia l U 0 calculated from the above equa- t ion by using the measu red values of A M has been shown in fig. 3 as a funct ion of t em pe r a t u r e for fields of 1 and 0.1 T. U 0 of Pb subs t i tu ted Bi2212 increases almost l inearly with t empe r a t u r e and exceeds the maxi- mum value of Bi2212 at 30 K for H = 1 T by a factor of 2 at 60 K for H = 0.1 T. The magnet ic relaxat ion is very fast above 30 K for Bi2212 even at a field of 0.1 T and the value of U 0 could not be est imated. This combined with the observed rapid decrease in A M with t empe ra tu r e points to a very weak pinning in Bi2212 above 30 K. The l inear increase of U o with t empera tu re could be due to the following reasons: U 0 is an accurate measure of the flux p inning potent ia l provided it has a l inear d e p e n d e n c e on Jc [7]. How- ever, it has been repor ted that U 0 shows a non l inear d e p e n d e n c e on Jc [8] and the increase in U 0 with t empera tu re could be an artifact of this nonl inear i ty [6]. Al ternat ively the increase of U 0 with t empera tu re has been explained on the basis of a collective flux p inning model [9], a dis t r ibut ion of activation energies [10] or the field d e p e n d e n c e of Jc [11].

In conclusion, despite the fact tha t we get only es t imated values of the p inning potent ials , it is clear

120 ;4-

/

// 100 / ,t •

/

~ 60-

40-

i2212 ~ ¢vAO¢O BI1.TP bo.~Sr2C o Cu20e~-y

20 1 ~ I I I I t Bil.sPbo.4Sr2CaCu208+y

0 / ' 111 'U l l l , l lH , t l [ l l l l l l u HIIIIIZ,,]IIIIII1,1111,1111, ,111,,11H,M 0 10 20 30 40 50 60 70

T(K) Fig. 3. Calculated pinning potential (see text) for Bi2212 and Pb doped Bi2212 as a function of temperature for H = 1 T (solid lines through data points) and for H = 0.1 T (dashed

line through data points).

tha t the t empe ra tu r e range over which p inning is effec- tive has been enhanced up to 60 K in Pb subst i tu ted samples compared to tha t of Bi2212, which is l imited up to 30 K only.

The authors wish to t hank Prof. V,V. Moschalkov for helpful discussions. This work was suppor ted by B M F T and D F G / S F B 341 / . One of us (PLP) would like to thank the A. v. Humbold t Founda t ion for f inan- cial support .

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