Journal of Low Temperature Physics, Vol. 105, Nos. 3/4, 1996

Macroscopic Magnetic Properties in TIBa2Ca3Cu4Ol l .~ Single Crys ta ls

Lu Zhang, J.Z. Liu*, M.D. Lan**, C. Collins, and R.N. Shelton*

Department of Physics, California State University, Stanislaus, Turlock, CA 95382, USA.

*Department of Physics, University of California, Davis, CA 95616, USA. **Department of Physics, National Chung Hsing University, Taiwan.

The four-CuO2-layer TlBa2Ca3Cu4011.~ compound has a high superconducting transition temperature of 128 K and strong interlayer coupling. We have performed magnetic experiments on single crystals of TlBa2Ca3Cu4011-8, which include hysteresis loop, irreversibility line and time relaxation measurements. From thesemeasurements, we observed the large critical current density, high irreversibility line and strongly temperature dependent flux creep rate in the TlBa2Ca3Cu4011_~ single crystals.

PACS numbers: 74.60.-w, 74.60.Ge, and 74.30.Ci

1. INTRODUCTION

One significant observation in high temperature superconducting (HTS) materials is that superconducting transition temperature (Tc) increases with increasing number of CuO2 layers (up to three layers). Single layer 2-1-4 compounds have Tc above 20 K, the double-layer YBa2Cu307& (YBCO) and Bi2Sr2CaCu208 (BSCCO) have Tc in the order of 90 K, and triple- or four- layer compounds have Tc exceeding 100 K. The thallium-based superconductors attracted attention at an early stage, mainly due to their high Tc. Discovered in early 1988,1 the thallium-based superconductors encompass the broad homologous series, T1Ba2Can-lCunO2n+3+6 (n=l-5), T12Ba2Can-lCUO2n+4+8 (n=l-4), (TI,Pb)Sr2Can-lCunO2n+3+8 (n=l-3) and various substituted phases. 2 In the application arena, the low resistivity values and high transition temperatures (up to 128 K) observed in the thallium-based superconductors are particular encouraging and make them more promising candidates for fabrication of high- frequency devices and high-speed, thin-film radiation detectors.

However, few attempts have been made to study the superconducting properties in the thallium-based superconducting crystals (except T1Ba2CaCu207+~, Tc = 90 K and T12Ba2CaCu2010+8, T = 110 K)

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0022-2291/96/1100-0999509.50/0 �9 1996 Plenum Publishing Corporation

1000 Lu Zhang et al.

because of the complex crystal structures, toxic and volatile material, and the difficulty in obtaining good quality single crystals.

We have successfully grown TIBa2Ca3Cu4011-5 crystals with Tc of 128 K. The x-ray analysis confirmed that the crystals contained four Cu t2 layers within a unit cell and had a structure of T1Ba2Ca3Cu4011-5. In the present study, the magnetic measurements including hysteresis measurement, irreversibility line, and flux creep of T1Ba2Ca3Cu4Oll_g crystals.

2. E X P E R I M E N T A L

A self-flux growth technique was used to prepare single crystals of T1Ba2Ca3Cu4Oll.8. 3 The crystal structure was characterized using x-ray diffraction and the superconducting transition. The x-ray analysis showed that the crystals had a tetragonal structure with a space group of P4/mmm and lattice parameters of a = b = 0.384 nm and c = 1.873 nm. This confirmed the crystal structure with four Cu t2 layers within a unit cell. 2 The superconducting transition temperature of 128 K was measured by dc susceptibility (Fig. 1).

0

-10

-20

-30 0

TIBa2Ca3Cu4Oll.,5' c r ~

I ~ o o . o . . I 1

FC~ --. Tc=128K O

Jo �9 �9 �9 �9 �9 �9 ~176176176176 I t = 5 0 e

Z F C ~ H ]1 c

20 40 60 80 100 120 140 160

T (K) Fig. 1 Shielding (ZFC) and Meissner (FC) measurements of T1B a2Ca3Cu4011-5 single crystals.

The magnetic properties of the sample were measured using a superconducting quantum interference device (SQUID) magnetometer. Magnetic hysteresis experiments are the measurements of the magnetization

Magnetic Properties in T1Ba2Ca3Cu4011-6 1001

versus field at a fixed temperature. During the measurements, field was increasing from zero to a maximum value of 5.5 T, then decreasing, reversing in sign to a maximum negative value of -5.5 T, and finally increasing back to 5.5 T.

The irreversibility line experiment was performed by cooling the sample to 5 K in zero field, applying a magnetic field, and measuring magnetization at different temperatures by warming up the sample to 150 K (ZFC), and then cooling to 5 K with the same field (FC). Thus, irreversible temperature, Tirr, is determined by a point of no observable deviation between ZFC and FC curves in M versus T measurement.

The magnetic relaxation experiments were carried out at temperatures above 2 K and in external magnetic fields up to 5 T. The magnetization at fixed temperatures and fields was measured as a function of time, M(t). In all the relaxation measurements, the field applied was chosen to be greater than the penetration field (H > H*) to assure a well- defined fully penetrated state for a field parallel to the c-axis of the crystal.

3. RESULTS AND DISCUSSION

The critical current density can be estimated by analysis of magnetic hysteresis measurements, 4 Jc -- 30 AM/d; where d is the dimension of in the ab plane for a square shape single crystal and AM the width of the hysteresis loop. The critical current density is estimated to be 2.5x106 A/cm 2 at 5 K. This is higher than 1.4x106 A/cm 2 obtained in detwinned YBa2Cu307_~ crystals 5 and lx 105 A/cm 2 in Bil.7Pbo.3Sr2CaCu208 single crystals 6 at the same temperature. Furthermore, the critical current density measured in T1Ba2Ca3Cu4Oll_~ crystals has a strong temperature dependence between 5 and 30 K (see Fig.2) and reaches a plateau on the order of 105 A/cm 2 up to 50 K. The strong temperature dependence of the critical current density below 30 K may be caused by the large thermally activated flux motion in T1Ba2Ca3Cu4011~ system.

The irreversibility field measured in T1B a2Ca3Cu4011-8 crystal s i s plotted in Fig.3 using H versus 1-Tirr/Tc. The data follow a power dependence H~(1-Tirr/Tc) m (see solid line in Fig.3) with m=5. This power law dependence was also observed in other HTS materials where m was varied from 4/3 to 2. 7 The large value for m found in the presen t studies implies that the irreversibility line has a strong temperature dependence in T1Ba2Ca3Cu4Oll. 8 system. This may lead to large flux creep due to thermal activation.

Some previous experiments 8 showed that the Tirr is 20-30 K higher for YBa2Cu307_S than for Bi2Sr2CaCu208 for HIIc. Our data in T1Ba2Ca3Cu4Oll~ crystals indicate the irreversibility line in is 35-40 K higher than our previous measurements on Bil .7Pbo.3Sr2CaCu208 crystals 9. The high irreversibility line shows that flux pinning may be more

looz Lu Zhang et al.

effective in a large range of temperature and field for T1Ba2Ca3Cu4011-8 system than other HTS materials.

. 0 - - ~ . - - , - - - - - -

t,,t E

<

2.5

2 .0

1.5

1.0

0 .5

0 .0

O TIBa2Ca3Cu4Oll~3 crystals

0

O n i l e

�9 �9 �9 �9 �9 |

10 20 30 40 50 60

T (K) Fig. 2. Critical current density at zero field as a function of temperature.

5 0

4 0

30

20

10

0

TIBa2Ca3Cu4Oll .5 crystals

n l l c

A

Fig. 3. Irreversible field as a function of the reduced temperature. Solid line is a power-law fit.

-10 ' ' ' ' 0 0.1 0 . 2 0.3 0 .4 0 .5 0 .6 0 .7 0 .8

1-T /T i r r c

Magnetic Properties in T1Ba2Ca3Cu4011-8 1003

Figure 4 shows the relaxation rates, IdlnM/dlntl, observed in TIBa2Ca3Cu4Oll& crystals. The relaxation rates increase linearly with increasing temperature and change significantly from 0.007 at 2 K to 0.1 at 30 K for H = 1 T. A temperature independent activation energy of 0.024 eV can be estimated by a collective pinning model as dlnM/dlntl = kBT/U, 10 which is larger than 0.01 eV observed in BSCCO crystals. 6

0.12

~- 0.09

0 .06

0.03

f TIBa2Ca3Cu4011.s crystals I I !

0 10 20 30 40

T(K) Fig. 4. Magnetic relaxation rate versus temperature of T1Ba2Ca3Cu4Oll.5 single crystals.

All the previous relaxation experiments yielded a finite intercept of the flux creep rate by extrapolating linearly to zero temperature. This indicates that the relaxation rate does not vanish when T --, 0, which corresponds to a quantum creep of vortices. The zero temperature quantum creep rates were estimated to be 0.01 in YBa2Cu307.8 I1 and 0.02 in Bi2Sr2CaCu20812, which were comparable with the flux creep rate due to thermal activation at 5K. However, the relaxation rate is shown to vanish when the temperature extrapolates linearly to zero in T1Ba2Ca3Cu4Oll-8 crystals (Fig.4), which indicates the flux creep is only thermally assisted. The unobservable flux creep rate in T1Ba2Ca3Cu4Oll_~ crystals at low temperatures (quantum creep) may be related to the observed high irreversibility line and large critical current density in this system.

1004 Lu Zhang et al.

4. CONCLUSION

The four-CuO2-1ayer T1Ba2Ca3Cu4Oll-8 compound has a high transition temperature, strong interlayer coupling~ Large hysteresis was observed in T1Ba2Ca3Cu4011-~ crystals and the critical current density was estimated to be 2.5x106 A/cm 2 at 5 K. The h-reversibility line experiments show a power dependence H ~ (1-Tirr/Tc) 5. The time-dependent magnetic relaxation experiments indicate that the relaxation rate increased linearly with increasing temperature below 30 K, which leads to a temperature independent intrinsic activation energy of 0.024 eV. The relaxation rate was also shown to vanish when the temperature approaches zero, which reflects quantum creep is unmeasurable above 2 K in T1Ba2Ca3Cu4011-~ crystals, Large critical current density and high irreversibility line exhibit that the flux pinning is relatively effective over a large range of temperatures in the TIBa2Ca3Cu4Oll,5 compound. The strong temperature dependence of the critical current density and unobservable relaxation rate at low temperatures reflect that flux creep due to thermal activation is predominant.

ACKNOWLEDGMENTS

This work is supported by the National Science Foundation grants DMR-95-32-085 and DMR-94-03895.

REFERENCES

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edited by Allen M. Hermann et al., (Marcel Dekker, Inc. 1994). 3. J.Z. Liu et al., J. Cyst. Growth 109, 436 (1991). 4. E.M. Gyory et al., Appl. Phys. Lett. 55, 283 (1972). 5. Lu Zhang et al., Phys. Rev. B 50, 7092 (1994). 6. Lu Zhang et al., Phys. Rev. B 44, 10190 (1991). 7. A.P. Malozemoff et al., Strong Corrections and Superconductivity,

edited by H. Ful~uyama et al., (Springer, Heidelberg, 1989), p. 349. 8. D.C. Larbalestier and M.P. Maley, MRS Bulltin 18, 50 (1993). 9. Lu Zhang et al., Phys. Rev. B 45 4978 (1992). 10. V.M. Vinokur et al., Phys. Rev. Lett. 67, 915 (1991). 11. L. Fruchter et al., Phys. Rev B 43, 8709 (1991). 12. A.C. Mota et al., J. of Low Tem. Phys. 95, 377 (1994).