Flux pinning in single Tl-layer 1223 superconductors

Physica C 183 ( 1991 ) 67-72 North-Holland

Flux pinning in single TMayer 1223 superconductors T o s h i y a Doi, M i c h i y a O k a d a , A t s u k o Soeta, T o y o t a k a Yuasa, K a t s u z o A i h a r a , T o m o i c h i K a m o a n d Shin-Pei M a t s u d a Hitachi Research Lab. Hitachi Ltd., 4026 Kuji-cho, Hitachi City, Ibaraki 319-12, Japan

Received 2 August 1991 Revised manuscript received 29 August 1991

The presence of pinning centers in high temperature superconductors (HTSCs) is a prerequisite for obtaining a high critical current density (J¢) in a magnetic field. We have discovered that, perhaps more generally, HTSCs having only a single layer between the superconducting copper layers (for example the Y-Ba-Cu-O or TI-Sr-Ca-Cu-O systems) can possess effective pinning centers that are absent in the double layer cases. Several samples prepared by a partial melt process, with the nominal composition (Tlo.sPbo.5)~(Sr0.sBao.2)2Ca2Cu3Og,showed the high intragrain Jc, by the magnetization method, greater than 2 × 104 A/cm2 at 77 K and 1 T. This result suggests that more effective pinning centers exist in the (1223) phase of the modified TI-SrCa-Cu-O system (ref. [1 ] ). We propose an explanation why both YBa2Cu307and (1223) of TSCCO contain useful pinning centers, whereas both Bi- and Tl-double-layersuperconductors do not. Effectivepinning, in this case greater for single layers rather than double layers, is related to the strength of the superconductive order-parameter in these layers intervening the Cu-O layers.

1. Introduction For many applications o f superconductors, critical current densities o f 104-106 A / c m 2 in the presence o f significant magnetic fields are required. At present, YBa2Cu307 bulk samples prepared by melt textured growth [2], quench and melt growth [3] or melt powder melt growth [4] methods achieved Jc values exceeding 104 A / c m 2 at 77 K and 1 T. In the case o f B i - S r - C a - C u - O (BSCCO) or T 1 - B a - C a C u - O ( T B C C O ) thin films, Jc has been found to be 10 5-10 6 A / c m : at 77 K in the absence o f a magnetic field, but decreases drastically in the presence o f a magnetic field applied perpendicular to the film surface [ 5 - 7 ] . The Jc o f tape-shaped wires, in the absence o f a magnetic field, is also high, above 104 A / cm 2 at 77 K; however, when the magnetic field is applied perpendicular to the tape surfaces, the J~ decreased drastically [8,9 ]. The decreases o f Jc were ascribed to flux creep in the absence o f pinning centers [ 10 ]. Attempts to introduce pinning centers into TI2Ba2Ca._ iCUnO2n+4 or Bi2Sr2Can_ iCunO2n+4 superconductors at 77 K, have so far been unsuccessful. This is in accord with the point [ I 1 ] that a smaller pinning effect is to be expected when the co-

herence length is shorter along the c-axis. Both Tl2Ba2Can_ ICUnO2n+4 and Bi2Sr2Ca~_ ICUnO2n÷4 superconductors have double T1-O or B i - O layers in the crystal structures, which are thought to be rather insulating regions [ 12 ], therefore the superconductive order-parameter between the superconducting C u - O layers is thought to be much smaller [ 13 ]. Recently, these phenomena have been theoretically anticipated by Kes et al. [ 14], Clem [ 15 ] and, after this work was done, checked experimentally by Kim et al. [ 16 ] focussing on the twodimensional pancake vortex model and its relationship with the distance between the C u - O sheets. In order to exemplify this supposition, we have attempted to introduce pinning centers into the less known single Tl-layer T 1 - S r - C a - C u - O ( T S C C O ) [ 17 ] superconductor, as opposed to the double layer type. The TSCCO system gives rise to two main superconducting phases, T1 ~Sr2Ca2Cu309 (" 1223") and TIISr2CaICu207 ( " 1 2 1 2 " ) [ 1 8 - 2 0 ] . Although the ( 1223 ) and the ( 1212) phases of T S C C O system are difficult to synthesize in pure form [21 ], the substitution o f Pb into the TI site [21 ], N d into Sr site [22], Y into Ca site [23], Ba into Sr site [19] is

0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

68

T. Doi et al. ~Flux pinning in single Tl-layer 1223

sometimes effective for improving the purity levels. In this paper we report that TI-1223 did have some pinning centers in it at 77 K, and also we propose an explanation why only YBa2Cu307 had been able to have effective pinning centers whereas double layer TBCCO (hereafter called DL-TBCCO) and BSCCO had not. With the above in mind, we present the composition of (Tlo.sPbo.5)l(Sro.sBao.2)2Ca2Cu309 as a prototype (1223) sample.

2. Experimental Appropriate amounts of BaO, SrO, CaO and CuO were mixed and calcined at 870°C for 20 h and the resultant B a - S r - C a - C u - O powder was then further mixed with T1203 and PbO, and pressed into pellets ( ~ 1 mm in thickness and 30 mm diam.). The pellets were sintered at 800-900°C for l0 h and cooled to room temperature in the furnace. The resultant (TI,Pb)- (Sr, B a ) - C a - C u - O was reground and again pressed into pellets. The pellets were heated at 9501100°C (partial m e l t ) f o r 10-120 rain, cooled to 880°C at 1°C/s, then further annealed at 850-880°C for 10-50 h; this procedure is summarized in fig. 1. Electrical resistivity was measured by the standard four-probe method with soldered indium contacts. AC susceptibility was measured in the field of 50 G. Powder X-ray diffraction data was obtained using a

Mixing T Calcination T Mixing T Pelletizing T Sintering T Grinding Pelletizing T Partial Melt T Annealing

Philips PWI700, with a curved graphite diffractedbeam-monochromator, using Cu Kct radiation. Microstructures were studied using Hitachi S-800 SEM. The DC magnetization was measured using a vibrating sample magnetometer at a sweep rate of l0 Oe/s. Jc was deduced from the hysteresis AM in M H curves using Bean's model.

3. Results and discussion Figure 2 shows a DTA curve of the sintered sample of (Tlo.sPbo.5) ~(Sro.sBao.2) 2Ca2Cu309; there are three peaks under 1000°C: at 876°C (peak 1), at 906°C (peak 2) and at 938°C (peak 3). It is believed that above the temperature of the deep peak 3, (i.e. at our sintering temperature above 950°C) there is enough liquid for rapid crystal growth, lower level partial melting seems to occur at the two lower peaks and would, therefore play an important role in the annealing. (Clearly a great deal of work is required to elucidate the exact mechanisms but some more details will be published elsewhere.) Figure 3 shows the X-ray diffraction patterns of the (Tlo.sPbo.5) l (Sro.sBao.2)2CaECU309bulk sample, (a) after sintering at 880°C and (b) after partial melt at > 950°C and after annealing at 880°C. In fig. 3(a) most of the peaks were indexed as the (1223) structure, with tetragonal symmetry, a=0.384 and c = 1.54 nm, and relatively small peaks ( < 10%) of BaPbO3. When the bulk sample was quenched from partial

SrO, BaO, CaO, CuO

10

870°C 20 hours addition of T1203, PbO

800 - 900"C 10 hours

;> ...t

~

PeakTemperatures

-25

850 - 880"C 10 - 50 hours

Fig. 1. Preparation of (Tlo,sPbo.s) l (Sro.sBao.2)2Ca2Cu309sample.

"'-..

(~)906 °C ~)938 °C

-60 950- I100"C 10- 120min.

\

(D 876 °C

L 0

I

I

I

",. "~'r I " I

~

I

1

I

I

500 1000 Heating Temperature ( °C )

Fig. 2. DTA curves of the (Tlo.sPbo.s) 1(Sro.sBao.z)2Ca2Cu309.

sintcring sample of

71. Doi et al. / Flux pinning in single Tl-layer 1223

(a)

69

• (1223) ohase o (1212) phase •

• BaPbO3

4-J

"--~ "~ (". "~ D O00~ • •(~-

(b)

b•

.i

;

o

iv

•

0°

•

20

30

40

50

60

70

2 0 (degree) Fig. 3. X-ray diffraction patterns of (Tlo.sPbo.5), (Sro.sBao.2)2Ca2Cu309, (a) sintering sample, (b) after partial melt and after annealing sample.

Fig. 4. SEM image of the sample after partial melt and after annealing. Gray part lighter part: 1212 phase, darker part: 1223 phase; black part: (Ca, Sr)2CuO3, white part: BaPbO3.

melt region ( > 9 5 0 ° C ) , it was found to be not superconducting above 77 K. In associated work, it has been shown by XRD and SEM that the 1223 completely decomposes at this temperature. The superconducting (1223) and ( 1212) phases are formed from liquid reacting with remaining solid during the slow cooling period or during the post annealing. As shown in fig. 3(b), the sample after partial melting and after annealing obviously contains the (1223) phase, the ( 1212) phase, but now more BaPbO3, and unknown phases which have appeared as there was some TI loss. The ( 1212 ) phase also has tetragonal symmetry, a = 0 . 3 8 4 and c = 1.21 nm. Figure 4 shows a SEM image of the sample after partial melt and annealing. Many of the grains were found to be the dark gray 1223 and light gray 1212 phases; the volume fraction of these superconducting phase is estimated to be about 75%. Also shown are black grains of (Ca, S r ) 2 C u O 3 (evidently one of the unknowns above) and white grains of BaPbO3. The grain size of (1223)

and (1212) are about 20-60 Ilm and the grains of (Ca,Sr)2CuO3 or BaPbO3 are 1-30 p.m. Figure 5(a) shows the temperature dependences of the electrical resistivities of the sample after partial melt and annealing; fig. 5(b) shows the susceptibility change of the bulk sample. The transition is very sharp for both the resistivity drop and the susceptibility change. Here we do not attempt to estimate the volume fraction of the superconducting phases from susceptibility measurements as this is fraught with danger as has been pointed out [24]. Figure 6 shows the M - H loop at 77 K for the sample after partial melt and after annealing, comparing with TI2 (Sro.zBao.a)2Ca2Cu30]o by a high speed zone melt method [25]. The sample dimension was 1 × 1 × 10 m m 3, and a magnetic field was applied parallel to the longitudinal direction of the specimen. Magnetization hysteresis AM of the sample after partial melt and after annealing was at a level similar to that YBa2Cu307 [ 3 ], contrasting with the AM of

70

T. Doi et al. / Flux pinning in single Tlolayer 1223

20

I

(a)

I

tO

0

I i I 100 200 Temperature (K)

0

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(b)

300

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v

-1.0

Jc = 3 0 A M ~ L ,

?

i

g~

......

~-...

-2.0 I

0

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100 200 Temperature (K)

300

Fig. 5. Critical temperature of ( Tlo.sPbo.5), ( Sro.sBao.2) 2 C a 2 C u 3 0 9 sample after partial melt and after annealing. (a) Temperature dependences of electrical resistivity, (b) Temperature dependences of AC susceptibility.

100

'

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TI2 ( Sro.2Bao.s) 2Ca2Cu301o (T1-2223) which was very small; above the applied field of 0.5 T it decreased almost to zero, notwithstanding that the sample was prepared by a similar melt process. AM was found to be almost independent of the sample direction as expected for a more or less random polycrystalline sample. From these measurements we cannot know to what degree the supercurrent is inter- or intra-granular, but we may, nevertheless, easily determine the lower bound Jc by assuming the worst case where the sample size is taken to be the current loop size and use the following relation [9 ],

-

::"'~:~. ~.:.'-y_.;. . . . . . . . . . . . .

I

I

I

-0.5

0

0.5

1.0

Magnetic Field (T) Fig. 6. Magnetization loop for the sample after partial melt and after annealingat 77 K. (a) TI-1223prepared by the partial melt process, (b) TI-2223 prepared by the high speed zone melt method.

where L is the current loop size. It is difficult to estimate the size of the current loop. Usually, L is assumed to be equal to the grain size in such a case, but here we conservatively assume L = 1 ram, the sample thickness, (the current loop must surely be less than 1 mm). Jc is calculated then to be at least 2)< l04 A/cm 2 at 77 K and 1 T, many pinning centers thus exist in the sample. In the SEM photograph of fig. 4, there are many particles of BaPbO3 and (Ca, Sr)eCuO3, and, although they may play some role as pinning centers, considering the coherence length of the high-To superconductor, these second phase particle sizes seem to be too large to be making a major contribution. Defects such as vacancies, dislocations, stacking faults, etc., being more on the correct scale, around the impurity particles are more likely to be contributing pinning centers [26]. Although the exact nature of the pinning centers is as yet unresolved, the important thing is that a TI-O single layer TI-1223 sample does manifest pinning at 77 K. Figure 7 shows the crystal structures of the superconductors considered here. Both DL-TBCCO and BSCCO, into which pinning centers could not be introduced, have the double T1--O or Bi-O layers which are thought to be rather thick "insulating" regions [ 12]. On the other hand, T1-1223 and YBa2Cu307 into which pinning centers could be introduced, have single layers seemingly operating in a different way. In case of DL-TBCCO and BSCCO, in the thick "insulating" region, the superconductive order-parameter is thought to be weaker [ 13 ], and the vortex "disk" between adjacent layers thought to be more weakly coupled magnetically than for the single layer

71

T. Doi et al. / Flux pinning in single Tl-layer 1223

O Sr or Ba Ca

Bi or TI

B a - - ~

Insulating layer

0 ""-..

Sr-- o-0.... ° 1 :

Cu

Y ! 0t~"

?e

YBazCu307

Bi2 Sr2 Ca2Cu3 O10 or Ti 2Ba2 Ca2 Cu3(ho

TISr2 Ca 2Cu3 09

Fig. 7. Crystal structures of high-Tcsuperconductors considered. case. When an magnetic field is applied parallel to the c-axis, there is little interaction between two vortex "disks" on either side of the more insulating TIO (or Bi-O) double layer region [ 14,15]. In such a case, an introduced pinning center can pin only a half unit-cell-length part of the flux. We should introduce pinning centers into every unit cell in order to get a higher Jc value in a magnetic field which is applied parallel to the c-axis, but this seems to be almost impossible. In the TI-1223 and YBa2Cu307case, the insulating region is much thinner than in both DLTBCCO and BSCCO. So here the vortex "discs" can couple more strongly between superconducting layers, enabling a longer flux line to form over a larger region. Each 3D fluxon becomes strongly attached to each single pinning center as a result; i.e. each pinning center becomes much more effective. In conclusion, and in accord with some earlier theoretical expectations put forward for the YBa2Cu307 case [ 11,13-15 ], we have achieved samples in which effective pinning exists in the TI-1223 superconductor which has single TMayers. The intragrain Jc was

> 2 × 1 0 4 A / c m ~ at 77 K and 1 T. We propose this to be an example of where the C u - O layers, being closer together and thus more strongly magnetically interacting, enhance the efficiency of flux pinning in high-To superconductors.

Acknowledgements We would like to thank N. Inoue, K. Tanaka and T. Ozawa for their assistance in the experiments, T. Kanai and T. Nabatame for their discussion. We also than P.E.D. Morgan, Rockwell International Science Center, for stimulating discussion.

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T. Doi et al. / Flux pinning in single Tl-layer 1223

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