Irreversibility line and critical current densities of ‘1223’ cuprate TlBa2Ca2Cu3O9: a single crystal study

Irreversibility line and critical current densities of '1223' cuprate T1Ba2Ca2Cu309: a single crystal study A. Wahl, V. Hardy, A. Maignan, C. Martin and B. Raveau Laboratoire Crismat, CNRS URA 1318, ISMRA, Universite de Caen, Boulevard du Mar6chal Juin, 14050 Caen Cedex, France

Received 5 May 1994 This paper reports for the first time on the pinning properties of a TIBa2Ca2CuaO9single crystal. The irreversibility line and the critical current densities have been determined by magnetic measurements in the configuration H//c. The location of the irreversibility line in the (H, T/Tc) plane and the values of critical current densities have been found to be higher than in previous studies on single crystals of all the other phases belonging to the same TI-Ba-Ca-Cu-O system. The irreversibility lines of the TI-monolayer cuprates are located above those of the TI-bilayer cuprates. Moreover, a systematic steepening of the irreversibility line as the thickness of the CuO2 multilayer is increased is in evidence. These results are discussed on the basis of the Josephson decoupling model proposed by Kim et al.

Keywords: flux pinning; critical currents; high T¢ superconductors

The layered thallium and bismuth cuprates are susceptible to showing higher critical temperatures than YBaECU307, but their pinning properties have been found to be weaker than those of the '123' phase. It is now widely acknowledged that this feature is related to their peculiar structure, in which the CuO2 multilayers are separated by non-superconducting, normal layers. Such structures can be described as intergrowths of rn oxygen-deficient perovskite type layers with n+l rock salt type layersL The Tl-based compounds, formulated TlnBa2Cam_lCUmOE+.+Emaccording to the above description, form a rich family, since n and m can vary, respectively, from 1 to 2 and from 1 to 4. From the viewpoint of the physical properties, an important distinction can be made between the Tl-monolayer (n = 1 ) and the Tl-bilayer (n -- 2) cuprates. In particular, a study on ceramic materials 2 has shown that the irreversibility line steepens as n is lowered from 2 to 1. Although magnetic investigations of unorientated ceramics can be considered as representative of the H / / c properties, better reliability can be achieved with studies on single crystals. Moreover, among the various studies of the thallium cuprates, several cationic species are often involved2-4 (e.g. Pb or Bi substitutions on the T1 sites) that make comparisons difficult since these species may influence the pinning properties of the materials. This is, for instance, the case for the '1223' cuprate Tlo.sPbo.sSr2CaECU309, which exhibits the best pinning properties 3 and appears to be the most promising for high current applications. Nevertheless, the role played by the structure and by the presence of lead on the thallium site in determining such

0011-2275/94/110941-05 © 1994Butterworth-HeinemannLtd

properties is, to date, not known. Therefore, to investigate the role of the structure by itself, a comparative study of cuprates involving the same cations is imperative. Within the basic family T1-Ba-Ca-Cu-O, results on single crystals are available5-13 only for T1-2212 (T1EBaECaCu2Os)and T12223 (T1EBa2CaECU3Olo). The aim of the present paper is to report for the first time determinations of the irreversibility line (IL) and critical current densities (Jc) of the T11223 cuprate T1BaECaECU309 performed on a single crystal. First, the crystal growth as well as the basic structural and physical characterizations are presented briefly. Then, the Jc values and the IL location derived from magnetic hysteresis loops are presented and compared to other members of the T1-Ba-Ca-Cu-O system.

Experimental details Single crystals have been grown from melts of BaCuO2: CaO:T1203 in the ratio 2:2:1/2. The thermal treatment and the method involving sealed silica tubes have been previously described for the synthesis of T12Ba2Ca2Cu3Olo crystals 11. Several crystals were extracted from the preparation, whose Weissenberg patterns correspond to the reflections of a unique crystal with cell parameters a 3.85/~ and c ~ 15.8 ~ (space group P4/mmm), characteristic 14 of T1BazCazCu309. One of these crystals, with dimensions 0.81 x 0.48 x 0.13 mm 3, was retained for the magnetic study. The magnetic measurements were carried out by means of a 5.5 T SQUID magnetometer in the configuration H / / c.

Cryogenics 1994 Volume 34, Number 11 941

Irreversibility line and critical current densities: A. Wahl et al.

Figure 1 shows the ZFC curve obtained under 10 Oe. The determination of the critical temperature from such a shielding curve is not straightforward: on the low T side, the transition begins early due to the large demagnetizing factor while, for the high T side, there is always a small tail which can be, at least partially, ascribed to fluctuation effects. Assuming extrapolation to zero of the tangent at the midpoint of the transition gives a reliable estimate of To, one obtains T¢ = 114 K. This value is in perfect agreement with the results previously obtained for as-synthesized polycrystalline ceramics of this phase ~s, which exhibit a Tc of 115 K. According to the initial slope of the virgin magnetization curves at low T and with the demagnetizing factor corresponding to the inscribed ellipsoid (0.74), the superconducting volume is found to be equal to 100%.

I

i . . . .

i

. . . .

i . . . .

i . . . .

.

I

.

.

.

.

I

.

.

.

.

I

.

.

.

.

-,or,.<.. .......

~

IlJ

o -20 -40 -60 I

.

.

.

.

I

0

(a)

.

.

,

,

5000

I

.

.

.

.

I

10000

.

.

.

15000

.

I

20000

H(0e/ . . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

'

4OO

Half hysteresis loops have been recorded between 0 and 5 T for 13 temperatures ranging from 5 K up to l l 0 K. The shape of these curves depends strongly on the temperature as shown in Figure 2. Above ~ I 0 0 K, the loops exhibit the standard shape expected for a continuously decreasing Jc(H) function (inset of Figure 2a). In particular, a single sharp peak followed by a continuous decrease of 1~ is encountered on the increasing branch of the loops. For lower temperatures, roughly between 90 and 60 K, a very marked fishtail shape is observed with two well defined peaks on the increasing branch M(H) (Figure 2a). For still lower temperatures, ranging from 50 K down to 30 K, the first peak on the increasing branch is no longer visible and appears as just a kink (Figure 2b). However, the double peak structure can still be observed on the decreasing branch, indicating a great similarity with the previous temperature range. On the contrary, for temperatures lower than 20 K, there is no more evidence of a fishtail feature on the hysteresis loops (inset of Figure 2b). In order to carry the analysis of the hysteresis loops further, a determination of the field of complete penetration Hp(T) is needed. This value corresponds to the merging point of the virgin magnetization curve in the complete hysteresis loop. Enlarged plots of this merging region are displayed in Figure 3 for 40 and 70 K, which correspond to the two characteristic shapes of the fishtailed loops. At . . . .

.

20

0

and discussion

- ' - i

.

4O

03

500

Results

.

60

i

. . . .

i . . . .

i

. . . .

i . . . .

i

. . . .

i

. . . .

i

"~ 2~ 0 -100 -200 -300 ,

.

.

0

.

10000

20000

30000

Co)

40000

I

.

.-

50000

H(Oe)

Figure2 Magnetic hysteresis loops in configuration H//c for different temperatures: (a) 70, 80 and 90 K, with 100 K in the inset; (b) 40, 50 and 60 K, with 20 K in the inset

'

'

'

'

1

'

'

'

'

1

'

'

'

'

1

'

40K -100

-200

-3OO

. . . .

E~

--400

,

I

rJ

) 1 1 1

I , , , ,

0

I , , i , I I

5000

10000

15000

H(Oe) ¢..)

%.J

-1

. . . .

I

. . . .

I

'

'

'

'

I

'

'

'

'

I

. . . .

o

I

'

70K

-20 -2 v

-40 -3 -60 *

10

20

30

40

50

60

70

80

90 100 110 120 130

T(K) Figure I ZFC curve of TIBa2Ca2Cu309 single crystal (10 Oe applied along the c-axis)

942

Cryogenics 1994 Volume 34, Number 11

I

0

. . . .

I

. . . .

1000

I

|

2000

,

,

,

I

,

•

i

3000

,

I

*

4000

*

,

,

|

5000

H(o ) Figure 3 Merging region of virgin magnetization curve ( 0 ) in complete hysteresis loop (e) for two temperatures

Irreversibility line and critical current densities: A. Wahl et al. . ' ' ' ' 1

70 K, Hp is located just slightly above the first peak, as is commonly observed when only one peak is present. This indicates that the 'anomaly' on this kind of loop is the second peak and not the first one. At 40 K, Hp is found to be clearly lower than the field corresponding to the IMI maximum. It can be noticed that, for 40 K as well as for 70 K, Hp is located at the IMI minimum of the complete loop. For 40 K, this field corresponds to just a kink on the virgin magnetization curve, simply because the different temperature dependences of the two peaks present at 70 K make them merge for lower temperatures. The above determinations of Hp provide evidence that the fishtail effect actually corresponds to a peak in the J~(H) dependence. This problem is, at the present time, the subject of intense study9J6-18, and a discussion of this specific point in the case of the Tl-based compounds will be addressed elsewhere. The critical current densities have been evaluated from the hysteresis loops using an extended Bean model for rectangular shaped samples ~9 J¢ (A cm -z) = [20 AM (emu cm-3)]/[b(1-3b/a) (cm)]

(1) where AM is the width of the loop, and a and b are, respectively, the larger and the smaller lateral dimensions of the crystal. This relation has been applied for the half loops within the field range Hp < H < 5 T - 2Hp to ensure the validity of the Bean model. This range can be extended to 0 < H < 5 T - 2Hp for complete loops, bearing in mind that the self field is predominant below Hp. The obtained Jc(H, T) curves are displayed in Figure 4 with logarithmic scales. The field dependence of Jc is strongly marked by the fishtail phenomenon. It must be noted that these curves cannot be scaled due to the smoother temperature dependence of the peak field compared to the irreversible field. For temperatures not too close to Tc (----- 105 K), the determination of H~(T) using the closing points of the hysteresis loops is quite definite, as shown by the abrupt disappearance of irreversibility (much better defined than in the case of ceramics) illustrated in the inset of Figure 5 with the enlargement of a Jc(H) curve around Hirr. The IL reported on Figure 5 corresponds to a criterion J~ < 50 A cm -2. In order to compare the behaviour of the T1-1223 phase with that of other thallium cuprates, the IL values of • --i

. . . . . . . .

i

. . . . . . . .

I

. . . . . . . .

. . . .

I . . . .

I

. . . .

I . . . .

I . . . .

I . . . .

I . . . .

I

. . . .

I . . . .

I . . . .

40000

q)

30000

¢J

0 20000

o

10000

...i .... i

0

.7,..!o.!7 ..............

lO 20 30 40 50 60 70 80 90 100 110

o

T(K) Figure 5 Irreversibility line of TIBa2Ca2Cu30g derived from the closing points of isothermal hysteresis loops (H//c). The reliability of this method is illustrated in the inset

the T1-2212 and T1-2223 phases have been determined for single crystals prepared in the same conditions as our T11223 cuprate. They were found to be similar to those previously reported for these Tl-bilayer compounds H. The IL values of these three thallium cuprates and that of the T11212 cuprate (T1BazCaCu2OT) previously obtained for a single crystal2° are reported on a reduced temperature scale in Figure 6. This semi-logarithmic plot shows the exponential-like temperature dependence of the T1-1223 IL for t < 0.9. Note that the other monolayer cuprate T1-1212 displays the same trend, whereas the two bilayer cuprates T1-2212 and T1-2223 exhibit an S-shaped curve similar to the one of Bi-2212. Before comparing the IL location of the different compounds, it must be recalled that this line is not an intrinsic property of a phase. Among the relevant extrinsic parameters are the characteristic time of the measuring technique, the native structure of defects and the geometry of the sample2L In our case, the method used for the IL determination was the same for all the single crystals and their dimensions are comparable. Moreover, since their methods of synthesis were similar, the structure of defects (nature and density) can be expected to be not too different; but the potential influence of this last parameter cannot be cornlos

'"" • i . . . . i . . . . i . . . . i . . . . i . . . . , . . . . i . . . .

i

10 s

c~

~

3

0

K

~)

105

104

O

104 .~

10 3

~

10 3

102

101

90K

80K 70K 10

10 °

.... i 10 2

. . . . . . . .

i

. . . . . . . .

10 3

i 10 4

. . . . . . . .

i

2

,..i

0.2

10 5

i ....

0.4

I ....

0.5

I ....

0.6

i ....

0.7

i...li

0.8

....

0.9

.0

T/%

H(o ) Figure4 Magnetic field dependence of critical current densities extracted from hysteresis loops using relation (1), for different temperatures

....

0.3

Figure6 Irreversibility lines determined on single crystals by the closing point of hysteresis loops (H//c) for different phases in the TI-Ba-Ca-Cu-O system: A, TI2Ba2CaCu2Oe; 0 , TI2Ba2Ca2Cua010; II, TIBa2CaCu207; 0 , TIBa=Ca2Cu309

Cryogenics

1994 Volume

34, Number

11

943

Irreversibility line and critical current densities: A. Wahl pletely discarded. Nevertheless, it can be assumed that the noticeable differences between the phases observed in Figure 6 are significant. The IL values are found to lie in the (H, t = TITc) plane in the following order, from lowest to highest: TI-2212, T1-2223, T1-1212, T1-1223, One can also compare the temperature dependence of Jc at low fields for the different phases, using the remanent magnetization Mr of the isothermal loops (when 2Hp < 5T) and a formula derived from (1) 4(T) ~ [40 Mr(T)]/[b(1-b/3a)]

(2)

The hierarchy between the different phases displayed in Figure 7 is the same as that previously observed for the IL, except for the T1-1212 phase which is here very close to TI-2212. A striking feature concerns the fact that the Jc values of T1-1223 are higher than those of TI-2212 by an order of magnitude over the major part of the temperature range. Otherwise, the Jc values of TI-2223 are also found to be higher than those in T1-2212, in agreement with previous results on aligned samples 22. The relatively low values of Jc in T1-1212 are rather surprising if one considers the high location of its IL in the (H, t) plane. However it must be pointed out that the structure of defects, which is sample dependent, can play a very different role with respect to the Jc values at low fields and the IL location. In our cases, the pinning centres present in the single crystals may be essentially point defects hardly detectable by HREM investigations, which should have a greater influence on Jc than on IL, according to the results of proton irradiation experiments 23. Let us return to the differences between the phases observed concerning the location of their IL in the (H, t) plane. A few series of results dealing with this topic have been reported for ceramics 2 and thin films 24. The model based on Josephson decoupling proposed by Kim et al. 24 has been found to account quite well for the results of these two studies. In this model the IL, i.e. the appearance of a highly dissipative regime, is related to the motion of pancake-like vortices. The proposed mechanism involves first a Josephson decoupling of the line vortices followed by a melting or a depinning process within the resulting 2D vortex lattice. As a first approximation for the analysis of the 10 7

'''1''''1''''1''''1''''1''''1

....

I''''1''''1

....

10 6

o

10 4. 10 3 10 2 101

,,1

....

I ....

I,.,,I

....

I ....

I ....

I ....

I,.,,1,~.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

T/Te Figure 7 Temperature dependence of critical current densities at low fields for different phases in the TI-Ba-Ca-Cu-O system. These values have been determined on single crystals using relation (2). Key to symbols as for Figure 6

944

Hi~

oc ( p c d c ) -1

(3)

where Pc is the T-axis resistivity, which is assumed to increase exponentially with the distance di separating the copper multilayers, and dc is the repeat distance of the structure along the T-axis, i.e. dc = (di + ds), where ds is the thickness of the copper multilayers. Presland et at. 2 found that the IL values of their ceramic materials formed two groups in the (H, t) plane; the T1monolayer cuprates lying clearly above the Tl-bilayer cuprates. For the monolayer phases, the lines of T1-1212 and T1-1223 are nearly superimposed, while among the bilayer phases, the lines steepen as the thickness of the CuO2 sheets decreases, Presland et al. have indicated that these features can be qualitatively accounted for by relation (3): the difference between the Tl-monolayer and the Tl-bilayer cuprates can be ascribed to an effect of di, while the influence of m in the bilayers can be related to the variation of dc via ds. In a previous study carried out on thin films 24, Kim et al. have reported similar results: firstly, almost a merging of the IL values for T1-1212 and T1-1223; and, secondly, a lower location of the line for the only bilayer cuprate, T12212, that was considered in this study. The present study is only in partial agreement with the above results. On the one hand, one observes in our case the same distinction between the mono- and the bilayer compounds: the line of T1-1212 is above the line of T12212 and the line of T1-1223 is above that of T1-2223. But, on the other hand, the influence of the parameter ds is completely different in our case. One in fact observes that the IL values steepen as the thickness of the copper multilayer increases (Figure 6): the line of T1-2223 is above the line of T1-2212, and the line of T1-1223 is above that of T1-1212. Within the framework of the simplified Josephson decoupling model mentioned above [relation (3)], our results could imply ads dependence of the parameter Pc- In magnetoresistive studies of Tl-based single crystals8"12, a systematic increase of ~c with m has been evidenced. Such a variation of the out-of-plane coherence length could induce a decrease in the tunnelling resistivity Pc as m, and thus d~, are increased.

Conclusions

10 5

r,.)

differences between phases 2, the Josephson decoupling field has been considered to be the main parameter in the above mechanism, yielding the relation

.

cq

.<:

e t al.

C r y o g e n i c s 1994 V o l u m e 34, N u m b e r

11

The irreversibility line and the critical current densities of a TIBa2CaECU309 single crystal have been determined by magnetic measurements ( H / / c ) and compared to the available data for single crystals of the other phases belonging to the same system: Tl2Ba2CaCu20 8, TIEBa2Ca2Cu3Olo and T1BaECaCu207. The location of the irreversibility line in the (H, T/Tc) plane and the values of critical current densities are the highest for T1BaECa2Cu309. Moreover, it is confirmed that the irreversibility lines of the phases with a single T10 layer are located above those with a double T10 layer2,24. However, a systematic steepening of this line as the thickness of the CuO2 multilayer is increased is in evidence, contrary to previous studies 2. Within the framework of the Josephson decoupling model proposed by Kim et al. 24, this last result would imply a relation between Pc and the structural parameters which is more complex than was

Irreversibility line and critical current densities: A. Wahl et al. first predicted. A direct determination of Pc by resistive measurements on single crystals should shed light on this point.

References 1 Raveau, B., Michel, C., Hervleu, M. and Groult, D. Crystal Chemistry of HTS Oxides, Vol 15, Springer, Berlin. Germany (1991) 207 2 Presland, M.IL, Tallon, J.L., Flower, N.E., Buckley, R.G. et al. Cryogenics (1993) 33 502 3 Zheng, D.N., Campbell, A.M., Liu, R.S. and Edwards, P.P. Cryogenics (1993) 33 46 4 Nabatame, T., Sato, J., Saito, Y., Aihara, K. et al. Physica C (1992) 193 390 5 Kopflov,V.N., Schegolev, I.F. and Togonidze, T.G. Physica C (1989) 1621164 1143 6 Giordanengo, B., Genicon, J.L., Sulpice, A., Chanssy, J. et al. Physica B (1990) 165 1147 7 Andrii, W., Hergt, IL, Hiergeist, R., Taubert, J. et al. Physica C (1993) 213 471 8 Mulmida, H., Kawaguchi, K., Nakao, M., Kumakura, H. et al~ Phys Rev B (1990) 42 2659 9 Kopilov, V.N., Koshelev, A.E., Schegolev, I.F. and Togonidze, T.G. Physica C (1990) 170 291 10 Hardy, V., Provost, J., Groult, D., Hervieu, M. et al. Physica C (1992) 191 85

11 Maignan, A., Martin, C., Hardy, V., Simon Ch. et al. Physica C (1994) 219 407 12 Hardy, V., Maignan, A., Goupil, Ch., Provost, J. et al. Supercond Sci Technol (1994) 7 126 13 Oussena, M., Porter, S., Volkozub, A.V., de Groot, P.AJ. et aL Phys Rev B (1993) 48 10575 14 Subramanian, M.A., Parise, J.B., Calabrese, J.C., Torardi, C.C. et al. J Solid State Chem (1988) 77 192 15 Martin, C., Maignan, A., Provost, J., Michel, C. et al. Physica C (1990) 168 8 16 Tamegai, T., Iye, Y., Oguro I. and IOshio, K. Physica C (1993) 213 33 17 Yang, G., Shang, P., Sutton, S.D., Jones I.P. et al. Phys Rev B (1993) 48 4054 18 Yeshurun, Y., Bontemps, N., Burlachkov, L. and Kapitulnik A. Phys Rev B (1994) 49 1548 19 Wiesinger, H.P. Sanerzopf, F.M. and Weber, H.W. Physica C (1992) 203 121 20 Hardy, V., Provost, J., Groult, D., Ravean, B. et al. Mat Sci Eng B (1992) 13 279 21 Li, Q., Suenaga, M., Li, Q. and FreltoR, T. Appl Phys Lett(1994) 64 250 22 Fang, M.M., Finnemore, D.K., Farrell, D.E. and Bansal, N.R. Cryogenics (1989) 29 347 23 Civale,L., Marwick, A.D., McEIfresh, M.W., Worthington, T.K. et al. Phys Rev Left (1990) 65 1164 24 Kim, D.H., Gray, K.E., Kampwirth, R.T., Smith, J.C. et al. Physica C (1991) 177 431

Cryogenics 1994 Volume 34, Number 11

945