Enhanced flux line pinning and irreversibility in Pb doped Bi2Sr2CaCu2O8+y

Physiea C 185-189 (1991) 2209-2210 North-Holland

ENHANCED FLUX L I N E P I N N I N G AND I R R E V E R S I B I L I T Y IN P b DOPED Bi2Sr2CaCu2Os+y P.L. Paulose, Sujata Patil and G. Gfintherodt

2 Physikalisches Institut, RWTH Aachen, W-5100 Aachen, Germany

The magnetic properties of polycrystalline Bi2.xTxSr2CaCu2Oe+y (T=Pb, Nb) have been investigated. It is found that flux pinning in Pb0.3 and Pb0.4 has been improved over Bi2212. However, Pb0,2 and Nb0.2 samples show rather weak flux pinning. The irreversibility temperature (Tirt) scale with extrinsic property such as flux pinning and not with Tc.

1.

INTRODUCTION

field. The Tirr was determined as the temperature at which

The existevee of the irreversibility line in the magnetic

field cooled (FC) and zero field cooled (ZFC) magnetization

field H versus temperature phase diagram of high Tc

curves merge. The temperature was varied by 1K steps in the

superconductors (HTSC) is interesting from an applications

vicinity of Tirr.

point of view and since its origin/cause has been subject of great debate. Several models have been proposed to explain

3.

RESULTS AND DISCUSSION

this feature thought to be unique to HTSC. These models mainly fall into two categories:

The superconducting transitions (mid point) for Pb.2,

First, considering extrinsic

Pb0.4 have been shifted by about 16K towards higher

features (e.g., defects) of the superconductors1 and second,

temperatures compared to Bi2212 while those of Nb0.2 and

taking into account intrinsic properties such as coherence

Pb0.3 samples increased by 6K (see Fig.l). Fig.2 displays the

length, Tc etc 2. There are expe:imental evidences in favour of

hysteresis curves at 50K of Bi2212, Nb0.2, Pb0.2 and Pb0.4

the first 1.3

samples.

as well as second category2'4'5. We have

undertaken systematic studies of Bi2_xPb×Sr2CaCu2Os+y ( x

The behaviour of Pb0.3 is similar to that of the

Pb0.4 sample and hence not shown in figure. The hysteresis

= 0, 0.2, 0.3, 0.4). Our aim was to examine 1) whether Pb

loop width (AM, which is ~ measure of pruning) of Bi2212

substitution introduces pinning centers

and Pb doped samples with x=0.3 and x=0.4 are similar at

and enhances the

critical current density Jc and 2) whether the irreversibility line correlates with Tc or with the pinning feature of the

0.4.

sample. For the sake of comparison we have studied Nb 1CCCO Ai2212

substituted Bi2212 samples.

..... o.o

2.

EXPERIMENTAL The samples were prepared

by normal solid state

pkn .

.

BiLTPbo.~Sr2CaCu2Oe~ _ ~ B!I.sPbo,4Sr2CGCu2Oa+y ~.~g~--/~T ' BIl.ePbo.2Sr2CaCu20

-0.4

reaction of the appropriate amounts of Hi20 3, SrCO3, CuO, .

~

,( u')

Bit.aN bo.2Sr2CaCu2Oa.y

t--

~,,a

.

.

.

Nb205. The X-ray .

.

.

.

.

.

.

.

.

.

.

.

a;w,~,.t;,~,, . . . . •~YRD~ . . . . . ! ,. r.

.

Nh d,,p~a .

.

.

.

.

.

samples showed impurity p"ases which were reduced after 10 days annealing in air at 860°C. The XRD confirmed that the impurity phases of all the samples amounted to less than 5% and showed no signature of the Bi2223 phase. A SQUID magnetometer ( Quantum Design) was used for measurements of magnetic properties.

0

20

40

60

80

100

T(K)

The magnetic

relaxation measurements were performed after cooling the

Fig.1. Normalised dc X as a function of temperature (H=5Oe).

sample in zero field and then applying the chosen magnetic

0921-4534/91/503.50 © 1991 - Elsevier Science Publishers B.V.

All rights reserved.

P.L Paulose et at / Enhanced flux line pinning and irreversibility

2210

12

°'I

T

-0.1 ~ ~ ~" ~

E--

-0.7

I

I0

'

- .

'-.

.

.

|lls,,,l,.l,,,(l=l¢,;*illl,

1

2

--

~qp

--~'~--"~-Bil.sPba=Sr2CaCu=O~ 4-f4-H- Bil~Nb~=SrzCoCu=0~ ~ B;2212 ~ Bil ?Pb~SrzCoCu=0~ ¢¢0~A'~ Bis:ePba,SrzCoCu=0e~

|t

Q) 0

|~ ~ I' I1

~,~ 8

. •

i..111',,,i

0

50K

: " : : : Bi,~Nb~$rzCoCu=O~ I ~ Bi~jPbo~Sr=CoCu=0e~ [ ~ Bi~Pba~SrzCoCu=0~.y

~

.o-o., lL

=

t

- -

.

3

i i2a........~'6.......;~,....... 6 .~.......~ .......~ .......~o.......~o Tirr(K)

,,l|,i.||lll

4

Magnetic Field (kOe)

Fig.2. Magnetization hysteresis loops at 50K.

Fig.3. The irreversibility temperature as a function of H. The solid lines are guides to the eye.

5K. This could be due to a similar defect structure of the

found to have a power law dependence on magnetic field with

samples in the absence of thermal excitations at low

the exponent in the range 0.5-0.7. From the figure, it is clear

temperatures. At temperature above 20K, AM of Pb doped

that the irreversibility lines of samples Pb0.3 and Pb0.4 which

samples with x -- 0.3, 0.4 is larger than that of Bi2212, Nb0.2

have relatively

larger

pinning are extended

to

higher

and Pb0.2 samples, implying that pinning in PbO.3 and Pb0.4

temperatures and fields compared to those of Bi2212, Pb0.2

samples has been improved above 20K probably due to defects

and

introduced by the substitution of Pb. The XRD revealed the

irreversibility lines do not scale systematically with Tc but

presence of Ca2PbO4 (,',3%) as impurity phase in these

rather are related to the

samples. The pinning has not been improved in Nb0.2 and

superconductor.

Nb0.2

samples.

Thus,

these

results

show

that

pinning strength of the high Tc

PbO.2 samples probably because of the insufficient amount of defects introduced. At 50K, AM in Fig.2 is by an order of

ACKNOWLEDGEMENTS This work was supported by BMFT and DFG/SFB 341.

magnitude larger in the case of Pb0.3 and Pb9.4 samples compared to Bi2212, Nb0.2 and Pb0.2 samples. The magnetization measured as a function of time shows

One of us (PLP) would like to thank the A.v.Humboldt Foundation for financial support.

a logarithmic time dependence. The pinning energies have been calculated by applying the modified thermally activated

REFERENCES

flux creep model6. The pinning energies of Pb0.3 and Pb0.4

1.

Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60

those of Bi2212 are 20-60meV in the temperature range 5-

2.

A. Houghton et al., Phys. Rev. B 40 (1989) 6763.

30K. Above 30K, the magnetic relaxation in Bi2212 is too fast

3.

W. Kritsch et al., Europhys. Left. 12 (1990) 179.

to be observed, implying that pinning is very weak. Thus the

4.

P.L. Gamme| et al., Phys. Rev. Lett. 61 (1988) 1666.

pinning is improved in these Pb doped samples.

5.

P.H. Koch et al., Phys~ Rev. Lett. 63 (1989) 1511.

6.

Y.Xu et al., Phys. Rev. B40 (1989) 10882.

(1988) 2202.

samples are 20-120meV in the temperature range 5-60K while

The irreversibility lines are shown in Fig.3. They are