Magnetic irreversibility in granular superconductors: AC susceptibility study

mU

Physica C 185-189 (1991) 1843-1844 North-Holland

MAGNETIC I R R E V E R S I B I L I T Y IN GRANULAR SUPERCONDUCTORS: AC S U S C E P T I B I L I T Y STUDY. F . Pe r e z , X. O b r a d o r s , J. Fontcuberta. ICNAB, C S I C , C a m p u s UAB, 0 8 1 9 3 B e l l a t e r r a , t4. V a l l e t , J. Gonz&lez-Calbet. Lab. Magnet.ismo Aplicado, RENFE-U.C.Madrid,

Barcelona, 28290

SPAIN.

Las

Matas,

SPAIN.

Ac s u s c e p t i b i l i t y measurements o f a c e r a m i c weak-coupled s u p e r c o n d u c t o r in v e r y low ac f i e l d s (2mG, 111 Hz) a r e r e p o r t e d . We p r e s e n t evidence for the observation of the magnetic irreversibility f o l l o w i n g a ZFC-FC t h e r m a l c y c l i n g by means o f a c s u s c e p t i b i l i t y m e a s u r e m e n t s . I t i s shown t h a t this technique a l s o r e f l e c t l o c a l m a g n e t i c f i e l d e f f e c t s in g r a n u l a r s u p e r c o n d u c t o r s , a s p r e v i o u s l y s u g g e s t e d in microwave s u r f a c e r e s i s t a n c e and I-V c h a r a c t e r i s t i c s . The d i s s i p a t i o n superconductors several

processes

in

polycrystalline

resistivity

have been r e c e n t l y r e a n a l y z e d by

a u t h o r s by means o f c r i t i c a l

c u r r e n t 2'a,

microwave s u r f a c e r e s i s t a n c e a and V-I

and in t h e d i a m a g n e t i c s c r e e n i n g .

a lower t e m p e r a t u r e Tc t h e r e i s a phase between t h e g r a i n s ,

characte-

coherence,

and hence long

zero resistance

se of the screening capability.

that

among

p l a y an e s s e n c i a l

character role

in

of the

these

samples

understandig

of

t h e microwave s u r f a c e smaller after

was

resistance

a ZFC p r o c e d u r e ,

that

whasboard

potencial

indeed

activated

while i t would be

expected that the dissipation the total

was

higher

when

magnetic f l u x in t h e sample i n c r e a s e s .

T h i s anomalous b e h a v i o r basis

be

of

local

was

field

explained

effects,

on

range

is

The

separation

determined

of

the

describing

the

more

tilt

thermally

t h e g r a i n boundary Josephson j u n c t i o n s

d

in

.

We have measured the complex ac susceptibility

the

is

the

phase s l i p p a g e p r o c e s s e s o c c u r r i n g

2.P

2

the

by

so-called

of a polycrystalline sample (YBa Cu

i.e.

phase

J o s e p h s o n c o u p l i n g e n e r g y which accounts f o r t h e barrier

shown

may

transition

potential

the magnetic hystory dependent properties. In t h e r e c e n t work o f J i a i t

both

locking

and a f u r t h e r i n c r e a -

r i s t i c s 4 . The main c o n c l u s i o n s o f t h e s e works i s the granular

At

,

..-;

•

. . . .

,

'

,

L

Fe

0. t

'L

O7.6 )

i

/

- -

0.000 II1"

distribution relevant In t h i s

of

the

magnetic

than t h e t o t a l

flux

,-O~ -0.005-

i| |

amount of t r a p p e d f l u x .

work we e x t e n d t h e a n a l y s i s

B

presented

-0.010'.

• ,,itllll !|lll°

,,..............°...,,.,,......*..."'"'"" e

by t h e s e a u t h o r s t o t h e ac s u s c e p t i b i l i t y nique

(~

= k ' + i ~ " ) 5. A c l e a r

ac magnetic irreversibility

tech-

identification

of

....... .::.::::::.-." .....-'A •,,:,:.:':,~::I:'::~::~:'::=:..: ......... •.... !

effects

associated

to

-O.020J-" ~

first

10

have

two

stage

t e m p e r a t u r e Tco t h e g r a i n s

c o n d u c t i n g and t h u s t h e r e

The s u p e r c o n d u c process. become

is a first

At

a

super-

d r o p in t h e

!

20

f l u x may be o b t a i n e d when a weakly

c o u p l e d sample i s i n v e s t i g a t e d . tor transition

,, .

.-",,°'FC ..

-o.o~5

0

intergranular

.," ,," .,',, • •

ZFC " "

•

~

30

~

40

~

50

60

7O

80

90

O0

T (K)

Fi~.l:

I n - p h a s e component o f

measured

at

~c

different ac (h~) and dc {H) magnetic fields. A) H=0 and h:,=2mO, B} H=0 and ho=10G, ZFC} b~=2mG and H=10G after ZFC, FC) bo=2mG and H=I00 after FC.

0921-4534Dl/$03.5q © 1991 - Elsevier Science Publishers B.V. All rights reserved.

F. P~rez et aL / Magnetic ~ i l i ~

1844

where a clear separation exists a l o n g both

tem-

out of phase component of ~

, also shows ac divergent behavior among the ZFC and FC.

peratures T and T (Fig. l). It is also import CO rant to note that the phase locking temperature observed when measuring in very

low

ac

in gn~u1~r superconductors

The interpretation of

ripple

these

results

this

may

be

given in complete analogy with that proposed

by

fields (ho=2 m(]) is very near to the zero resis? tance temperature . Further evidence of the weak

junctions occurring at the grain boundaries

are

grain coupling nature of this

strongly reduced with the application of a

mag-

sample

t h r o u g an s t u d y o f t h e a c f i e l d of the diam~etic

screening.

is

(ho)

As i t

gained

dependence is

field

I~=10

w a s h e s o u t any

of

a

phase

coherent

h~=10 G i s s t r i c t l y

the

curve

coincident

crushing it,

b~=2 mG. Moreover, ceptibility

even

with that

at

measu-

the FC process because of the

i.e.

lowest

field,

ZFC process where the grains

an ac m a g n e t i c f i e l d o f 10

completely

the

superconducting

allowing the magnetic flux to penetrate

our

among

and FC curves (H=lO G) measured at the field of ho=2 mG. First,

it

is

clear

dissipation

grains

within

a

ceramic

of the granular effects.

comple-

volume. ob-

the

ZFC

same

ac

that

a

applied

associated

the grain coupling process, as reflected by

distribution

test

magnetic field H. When look for the

Our work show that the experimental selectivi-

ceptibility and may also serve as a useful

and, also, it i s observed that the phase-locking the

completely

de-

G

higher screening is obtained in the FC processes

temperature Tc strongly depends on

appear

the

sus-

The main result in Fig. 1 is, however, the servation of a clear divergence

vortex

superconductors is well determinated by ac

ve vaIue of Z' at 0 K in both cases m a y b e relasuperconducting

existence

in

then

tely within the intergranular space. The relati-

ted to the fraction of

lower

intergranular space.

ty of the flux

of these results appear

in

diamagnetic and thus concentrate the flux in the

o f ho=10 G.

The s i g n i f i c a n c e

A

pinning within the grains, at variance with

field,

a t l e a s t up t o

field

intergranular magnetic induction is obtained

c u r v e a p p e a r s t o be c o m p l e t e l y i n d e -

maximum f i e l d

couple

our

local

the intergranular space in both cases.

i n t h e powdered form t h e s u s -

pendent of the ripple

very clearly:

at

G

measured

r e d w i t h t h e same s a m p l e i n a powder form, after

and

must be related to a different

a

Actually,

screening

dissipation observed in the ZFC and FC processes

of

transition.

netic field. Hence the different

observed

in Fig.1 the application evidence

ref. 3,4. The critical currents of the Josephson

to the

ACKNOWLEGENENTS: This research was supported by MIDAS-CICYT: MAT88101631CA)3 and SCI-0389-M(A). REFERENCES lli J.E.Evetts, B.A.Glowacki; Cryogenics 2_88, 641

(1988). 12/ M.E.Mcllenry, M.P.Maley, J.O.Xillis; Phys.Rev.B 4_2, 4838 (1990). 13/ L.Ji, M.S.Rzchowski, M.Tinkham; Phys.Rev.B 4_2, 4838 (1990). /4/ D.L~pez, F.de la Cruz; (submitted for publ ication). 15/ R.B.~-~--L~-~, ~z u, H . '......., ~c~a~ C.A.Thompson; in "Magnetic susceptibility of superconductors and other spin systems". R.A.Hein, T.L.Francoville, D.H.Licbenber~, eds, Plenum Press (in press). /6/ R.Gross, P.Chaudhari, D.Dimos, A.Gupta, O.Eore; Phys.Rev. Lett. 64, 228 (1990). /7/ X. Obr~dors, F. P~_f e z , J.Jurado, M. A.Crusei ~&s, J. Fontcuberta, M.Vallet, J. Gonz~ ~ez-CaJbet, E.Garcia; in "Transport properties of superconductors". R.Nicolsky ed., World S c i e n t i f i c vo].23 (]990).