ELSEWER Physica C 282-287 (1997) 2177-2178

Fishtail effect and its mechanism in PMP Y-Ho-Ba-Cu-0 superconductors

Yong Feng+b, Lian Zhou”, Jianguo Wenb and Naoki Koshizukab

“Northwest Institute for Nonferrous Metal Research, P.O.Box 71 Baoji Shaanxi 721014 P.R.China

b Superconductivity Research Laboratory, ISTEC, l-lo-13 Shinonome, 1-chome Koto-ku, Tokyo 135 Japan

The magnetic properties of Y-Ho-Ba-Cu-0 samples, which were prepared by the PMP method, have been

investigated. The fishtail effect is found below 60K in Y0.&Io0.4B~C~30y, while it is absent in the

Y,,.sHoO.zBazCu~O, sample until 30K. The origin of the fishtail effect in our sample is discussed.

1. INTRODUCTION same oxygen annealing as the pre-sintered one.

The critical current density (Jc) in most Y-Ba-Cu-

0 single crystals and in melt processed samples

increases anomalously with increasing field in the

mixed state, in a field parallel to the c-axis direction.

This fishtail effect is also observed in other high-Tc

compounds such as Bi-Sr-Ca-Cu-0, Tl-Ba-Cu-0.

Although several models based on oxygen deficient

phases, lattice melting, collective pinning of vortices,

and dimensional crossover have been proposed,

the exact mechanism has not been fully understood

[l-3] In this paper, Y-Ho-Ba-Cu-0 samples have

been fabricated through the PMP method and their

properties have been investigated.

3. RESULTS AND DISCUSSION

2. EXPERIMENTAL

The Y0.8H00.2Ba2Cu3Qy (HO2) and

Y0.6H00.4Ba2Cu30y (HO4) samples were prepared by

the PMP technique. Details about the process were

reported previously [4]. The magnetic hysteresis

loops are performed on a SQUID magnetometer.

After the measurement, the HO4 sample is re-

sintered at 900°C for 30 hours and is followed the

Jc values are calculated by using Bean’s critical

state model. As shown in Figure 1, Jc is

comparatively high, which indicates that there are

strong pinning centers in these sample. In addition,

the fishtail effect can be observed in HO4 sample.

The peak field corresponding to the fishtail

phenomenon increases with decreasing temperature.

However, the fishtail effect disappears when

temperature is above 77K in field parallel to c-axis ,

which is different from the ordinary fishtail in

YBCO. This result can not be explained in the

framework of oxygen-deficient pinning. Recently,

Groot et al provided the evidence that the fishtail

patterns continue to exist in the YBCO samples

subjected to prolonged oxygenation. The fishtail

effect is almost the same in YBCO subjected to 40h

and 90h oxygenation [5]. Moreover, no fishtail can

be found in the HO2 sample until 30K. Based on the

above discussion, it can be concluded that the fishtail

phenomenon may be related to the Ho addition.

0921-4534/97/%17.00 0 Elsevier Science B.V. All rights reserved. PII SO921-4534(97)01218-S

2178 Y Feng et al. /Physica C 282-287 (1997) 2177-2178

0 1 2 3 4 5 6 7

H(T)

60

50

40

30

20

10

0 0 , 2 3 4 5 6 7

H(T)

Fig. 1. Jc vs.magnetic field for HO2 and HO4 samples.

In order to further study this problem, the

magnetization is also measured for the after-sintered

HO4 sample. It can be seen from Figure 2 that the

fishtail effect is pronounced and is found in both

field directions, whereas no fishtail pattern exists in

the pre-sintered one in I-UC. It is thought that this

fishtail in our samples may be due to the

paramagnetic moment of holmium. TEM

observations show that there are a lot of collective,

stair-like stacking faults in the pre-sintered sample.

These stacking faults are strong pinning centers so

that the pinning effect created by the Ho addition is

masked. After the sample is re-sintered at 9OO”C, the

density of stacking faults is remarkably reduced. As

a result, the pinning effect induced by the Ho

addition becomes more important. Therefore, the

fishtail pattern is pronounced in the after-sintered

sample. Moreover, the cation disorder created by the

Ho addition may also be responsible for the fishtail

effect in our samples.

01234567

P,WT)

?? pre-smtered. H 11 C ?? pre-sintered, H 1 C A after-sintered. H II C

moo~“‘~~~~“~“-~ v after-sintered, H 1 C

01234567

@f(T)

Fig.2. Field dependence of Jc for the HO4 sample.

4. CONCLUSION

The fishtail effect is observed below 60K in the

HO4 sample and is pronounced after this sample

was re-sintered at 900°C. It is thought that the

paramagnetic moment and cation disorder induced

by Ho doping. may be responsible for this fishtail

phenomenon.

REFERENCES:

1. M.Daeumling et al, Nature 346 (1990) 332.

2. L.Elbaum, et al, Phys.Rev.Lett. 69 (1992) 2280.

3. F.Zuo, et al, Phys.Rev.B 49 (1994) 12326.

4. Yong Feng, et al, J.Appl.Phys. 74 (1993) 5096.

5. P.Groot, et al, Physica C 185-189 (1991) 2471.