Thermally activated flux creep velocity in Hg0.69Pb0.31Ba2Ca2Cu3O8+δ

2386

M.1 Qin et al. /Physica

in Fig.2 are the hysteresis loops of the film at several temperatures indicated by different symbols.

C 282-287

(1997) 2385-2386

tion of the hysteresis loop and the virgin magnetic moment

W-0 =

r/R

(b’

:i :: ( : :: /

*, :

:, : 8 :

i

I

i

-‘--; .--~_._ ._,,, _,_ ~_,_,,.....,) _.,.. ..._... -r.:.;.r.:--‘ I

,

I

.I*‘-.:_ ,..

A_._<._.

;_._,_._,;

_._..

_I ..-.-..~,.‘~:‘;‘,._. ,,,,_.x.C

-...;.; ,_,.,. r,-,y;::_...-~ ,.a.. -:._,_.‘..y‘,‘....

.I 0.25 .,,I,

0.50

:‘:‘...,..-2; =::= ;____,I...

‘,‘J’__-

0.75

,_ _ ~,

1

r/R

Fig.3 (a)Calculated z-component of the flux density profiles of the film in the film plane(z=O) at several low applied magnetic fields. (b)The corresponding current density profiles. It is found that using the Kim type expression [6] I,(H) = Lo/(1 + [HI/Ho) can not fit the virgin magnetic moment and the hysteresis loops well simultaneously, therefore, we choose the two current model as discussed by Senoussi [7] for the calcula-

IhO

1+

IHI/Hho

+

110

1 + IWHIO

(1)

where IhOand 11~are independent of the magnetic field, HhOand Hto are characteristic fields. The first term on the right hand side would describe the high field behavior whereas the second one would correspond to the low field domain. The model of Mikheenko et al. and Zhu et al. can be used only at low applied fields, i.e., vortices have not penetrated into the center of the disk. However, with the fitting procedure used in this paper (fix the location of the critical state region and treat the external field as unknown), it is easy to extend their model to fit the experimental data at high fields. The fitting results are shown as solid lines in Fig.1 and Fig.2, the coincides are evident. Plotted in Fig.3a and Fig.3b are the axis flux density profiles in the film plane and the current density profiles in the film at T=51K for several low applied magnetic fields, respectively. It can be seen from Fig.3 that the field is strongly enhanced at the edge of the film, although the field has not penetrated into the center of the disk, current flows over the entire of the disk. These features are different from those of a cylinder magnetized parallel to its axis. In summary, we have extended the approach of Mikbeenko et al. and Zhu et al. to calculate the critical state behavior, including the current density profiles, the flux density profiles, the virgin magnetic moment and the hysteresis loops, of a disk shaped superconducting thin film. The experimentally measured virgin magnetic moment as a hmction of the applied magnetic field and the hysteresis loops of the film can be well fitted by the approach described. REFERENCES 1. P.N.Mikheenko et al., Physica C 204(1993) 229. 2. J.Zhu et al., Physica C 212(1993)216. 3. J.R.Clem et al., Phys.Rev.B 50( 1994)9355. 4. J.McDonald et al., Phys.Rev.B 53(1996)8643. 5. HZhang et al., J.Vac.Sci.Technol. All(1993) 390. 6. Y.B.Kim et al., Phys.Rev.Lett. 9( 1962)306. 7. S.Senoussi, J.Phys.111,2(1992)1041.

PHYSICA E Physica C 282-287

(1997) 2385-2386

Magnetic response of YBa2Cu30,-6 thin film in a perpendicular magnetic field M.J.Qin”, X.N.XuO, A.M.Suna, X.X.Yaoa, J.P.Zuob and S.Z.Yangb aDepartment of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing 210093, P.R.China bDepartment of E lectronic Science and Engineering, Nanjing University, Nanjing 210093, P.R.China Magnetic measurements have been performed on a disk shaped YBa&usOr_6 thin film with an applied magnetic field normal to its surface. By extending the critical state model developed by Mikheenko et al. and Zhu et al., the demagnetizing effects on the magnetic response of the film are discussed. The current density profiles and the flux density profiles have been obtained and discussed.

Recently, Mikheenko et al.[l] and Zhu et a1.[2] presented an exact critical state model for a thin superconducting disk in a perpendicular magnetic field, resulting in many studies on related subjects [3,4]. However, it should be noted that relative to the theoretical works, there lack systematical experimental data on thin film to relate the magnetic moment to its critical current density according to the procedure of Mikheenko et al. and Zhu et al. In this paper, we try to take this into account by performing experiments on a disk shaped YBa$us07_6 thin film. All experiments reported here have been perfor-

med on a disk shaped YBazCu307_6 thin film with radius R=4mm, thickness d=2000~ and T, = 87K, deposited on single crystal SrTiOa substrate using offaxis dc magnetron sputtering [5]. The film was c-axis oriented, with the c-axis perpendicular to the disk’s plane. Magnetic measurements have been carried out on a homemade vibrating sample magnetometer, with the applied magnetic field normal to the surface of the film using zero field cooled process. Shown in Fig.1 are the virgin magnetic moment of the film as a function of the applied field at several temperatures indicated by different symbols. Plotted

1

0.20

51K

0.2

. . 0

I 2

0.15

Q)

rl

56.2K 1

if

F 0 f 6 0.05

w-----v+

72.5K]

3 f 0.1 . aI

W

9 -0.2

;77.2K o.m,

cnu

5 g 0.0 . E 0 f6 -0.1.

E 0.10

P

5lK

56.2K

lm

150

2w

250

Applied Field (Gs)

-m

v

am

-loo0

0

looo

2ml

77.2K1 xlm

Applied Field (Gs)

Fig.1 Virgin magnetic moment of the film at temperatures indicated by different symbols. Solid lines are fitting results. 0921-4534/97/%17.00 0 Elsevicr Science B.V PII SO92 l-4534(97)01 357-9

-

All rights reserved.

Fig.2 Hysteresis loops of the film at temperatures indicated by different symbols. Solid lines are fitting results.