Trapped flux, persistent current and diamagnetic shielding in a ring and a disk of Y1Ba2Cu3O7

Physics C 162-164 (1989) 1185-1186 North-Holland

TRAPPED FLUX, PERSISTENT CURRENT AND DIAMAGNETIC SHIELDING IN A RING AND A DISK OF YIBa2Cu307.

M.A.-K. MOHAMED, J. JUNG AND J.P. FRANC]( Department of Physics, University of Alberta, Edmonton, Canada T6G 2Jl

Trapped flux, persistent current, diamagnetic shielding and Meissner effect in a ring and a disk of YIBa2Cu307 have been studied by the measurement of the distribution of magnetic field across the sample at 77K in applied magnetic field up to 1000 G. It was observed that penetration of magnetic flux into a disk depends on applied field only. The penetration of flux into the ring depends on both applied magnetic field and time. The dependence of the decay rate of trapped field on an initial trapped field revealed the vortex-persistent current interaction in the ring. The vortex motion in the ring is the combination of flux-creep and flux-flow processes.

In

this

magnetic ring

paper

flux

we

report

penetration

of Y I B a 2 C u 3 0 7 ,

current

interaction

these

probe

expulsion

the in a

vortex-persistent

The

measurements

of

were

77K with an axial cryogenic Hall

(sensitivity

along

and

the

of

and the time-dependence

phenomena.

performed at

studies

the

± 30 mG) which was scanned

diameter

of

the

ring.

The

that

remained

in

the

field was reduced

ring

after

the external

to zero at that time.

decay of shielding is logarithmic in time.

The The

decay rate depends on the applied field (Pig. 2) and shows a maximum at 15-20 G.

The motion of

vortices is responsible for the decay of shielding

with

time.

The

magnitude

of

the

trapped

field depends on the applied field (Pig. 3).

distribution of a magnetic field measured across the zero-field cooled ring for an applied field 21 G is presented in Fig. i. show

the

distribution

of

23.6

'I .... I .... I .... I ' ' ' ' [ ' '

The upper curves

the

magnetic

- ~",,,,,

field

ZFC; 77K

.~,;~""-

measured in the presence of an external magnetic field.

These

shielding. magnetic

curves

represent

The lower curves field measured

the external

diamagnetic

show the trapped

across

the ring after

field was reduced to zero.

At an

(.9 .---

11.8

"10 (D . m

applied field of 20 G the flux lines enter the central

hole.

diamagnetic generation circulating

This

causes

the

shielding

around

the hole

and the

of a macroscopic persistent

current

around

the

ring

when

field is decreased down to zero. no

time-dependent

shielding hole

was

results

shielding. shielding

decay

observed. in

of

the

Cutting

time-dependent

reduction

the

of

applied

For the disk diamagnetic the

u-

-10

-5

0

5

10

Distance (mm)

central

decay

of

The heavy curves in Fig. 1 show the (upper curve) measured after the time

6 x 104 sec and the trapped field (lower curve)

0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

FIGURE 1 The distributions of the magnetic field. Upper curves: field on, lower curves: field off. The ring edges: -8 and +8 mm.

M.A.-K. Mohamed et al. / Trapped flux, persistent current and diamagnetic shielding

1186

3.0

0) 10 o~

i

[

i

i

I

i

i

]:

i

20

I

I

'

ZFC; 77K

'

'

'

I

'

'

'

'

~tl-. A

/

5" ov 1.5

-%

"

~:.

"10

10 I I I

O.

o

%

Ii

-r -o L.~.-.f~

0

,

,

i

"$~ ............................I. i

I

0

i

i

i

ZFC; 77K ~

60

120

0

FIGURE 2 The decay rate of shielding vs. the applied field.

reaches

its saturation value at a field above

350 G.

The field trapped in the ring shows a

current

In this case a persistent

and vortices

trapped

field.

give the contribution The

trapped

logarithmically with time.

the

disk

this

For

the

curve). initial linear

trapped function

with

can a

is

linear

be

of

flux

(dashed

jump

described to

creep*

gives

by

higher

rates at a trapped field of about i0 G. theory

3.0|'

the

disk.

persistent

In

current

the

place.

vortices

ring

modifies

the

activation

a

macroscopic

steepens

the

current

large

the persistent

vortex-denslty

flow will

enough occur.

i

i

1400

'

'

i

i

I

i

i

i

!

i

i

ZFC; 77K

O 5oV-v

1.5

"-6

vortex-density

I1" 10

0

0

2O

I0 Trapped Field (G)

FIGURE 4 The decay rate of trapped field vs. the initial trapped field. The solid line: the ring, the dashed line: the disk.

vs.trapped takes

field

is

apparently

caused by

gradient

and

gradient

flux

The jump in the decay rate

the

flux-flow like motion of vortices close to the central hole of the ring.

current

increases the drivin E force acting on vortices. For

i

creep.

At higher fields the interaction between and

I

decay

the vortex

and persistent

the vortex-lattlce

,

Hagen's

At trapped fields below i0 G no interaction between

F

a

energy 1.6 ± 0.3 eV at 77K for the vortex creep in

I

4).

the decay rate vs the

field

,

The decay rate is a

function ring

to

field decays

function of the initial trapped field (Fig. For

,

FIGURE 3 The trapped field vs. the applied field.

The trapped field for the zero-field cooled disk

maximum at 150-200 G.

~

7017 Applied Magnetic Field (G)

Applied Magnetic Field (G)

the

~ a

10

REFERENCES I.C.W. Hagen, R.P. Griessen and E. Salomons, Physica C157 (1989) 199.