Magnetization of grain-aligned HoBa2Cu3O7−δ

Physica C 162-164 (1989) 1629-1630 North-Holland

MAGNETIZATION OF GRAIN-ALIGNED HoBa2CU3OT.6*

M. WACENOVSKY+, H.W. WEBER+, O.B. H Y ~ ,

and D.K. F I N N ~

+&tomlnstitut der ~terraichlechon Univarsitaten, A-1020 glen, Austria °Ames Laboratory and Department of Physics, Iowa State University, Ales, IO 50011, U.S.A. Cylinders of g r a i n - a l i g n s d HoBa2Cu307_6 superconductors i n epoxy were i n v e s t i g a t e d using d i f f e r e n t i a l m a g n e t i z a t i o n and ac s u s o e p t i b l l i t y t e c h n i q u e s i n the temperature range 2 K ( T ( Tc. From both experimental t e c h n i q u e s , demagnetizing f a c t o r s of the g r a i n s could be deduced f o r H, ec and f o r He ea,b, which compare r e a s o n a b l y well with e s t i m a t e s baaed on the s i z e and shape of the g r a i n s . With t h i s i n f o r m a t i o n , a b s o l u t e values of the lower critical f i e l d Hcl ( f i r s t f l u x p e n e t r a t i o n f i e l d ) , i t s a n i s o t r o p y and temperature dependence were obf~insd. Furthermore, m a g n e t i z a t i o n c r i t i c a l c u r r e n t d e n s i t i e s ( i n t r a g r a i n J c ' s ) were c a l c u l a t e d from the l a g n s t i z a t i o n curves as well as from minor h y s t e r e s i s loops. Pronounced a n i s c t r o p i e s of Jc as well as s i g n i f i c a n t c o n t r i b u t i o n s of the Ho-mosenta t o the m a g n e t i z a t i o n , i n p a r t i c u l a r i n the ( a , b ) - p l a n e , have been observed. 2. DEMAGNETIZINGEFFECTS

i. INTRODUCTION Because of heavy twinning observed cc~nonly

From the d i f f e r e n t i a l

in the basal plane of (123)-superconductore,

in the

Meissnar s t a t e

graln-allgned materials as described first by

ratio

of

Farrell et al.l provide us with a prOLiSlng

same

ratio

alternative to single crystals for the study

diamagnetic

elopes is

m a g n e t i z a t i o n curves (Fig.l),

we o b t a i n a

(1-Da,b)/(1-Dc) also

saturation

1.67.

The

for

the

responsible signals

ac

case,

we

susceptibility

For the

find 1.50. Assuming Da l Db, the demagnetizing

present work, Ho-substltuted saxplea were pre-

factors for the field applied parallel to the

pared by d i s p e r s i n g p o l y c r y s t e l l i n e powder of HoBa2Cu3OT_5 s i n g l e g r a i n s i n commercial epoxy

a,b- and c-direction are obtained to be: D a,b -0.23 (0.25), Dc -0.54 (0.50). Since

anisctropy

effects

in

the

r e s i n and c u r i n g i n a magnetic f i e l d of 2 . 4 T.

these

From t h e o r i g i n a l sample, c y l i n d e r s were cut

those c a l c u l a t e d 3 f o r the a c t u a l s i z e of the

so t h a t t h e i r long a x i s was e i t h e r p a r a l l e l or

g r a i n s , we conclude t h a t i n d i v i d u a l decoupled

p e r p e n d i c u l a r t o the c r y s t a l l o g r a p h i c c - a x i s .

g r a i n s are indeed r e | p o s s i b l e f o r the magnetic

Hence,

response of our samples.

identical

"large"

saIples

of

exactly

geoMtrical

shape,

but

totally

two

results

compare

In this

an

magnetic

of

properties of these superconductors.

experiment.

in

r e a s o n a b l y well

with

different internal grain structure were available 2, which wore investigated by sc

3. FIRST PENETRATI(N FIELD

susceptibility techniques and by masuring the magnetization with a differential lowtemperature-chopper technlquo 2, which yields

t e c h n i q u e , any change of slope occuring i n the

t h e derivative Of the m a g n s t i , a t i o n , dX/d~0H , i n

slowly changing magnetic f i e l d s .

With

the

differential

~agnatization

m a g n e t i z a t i o n curve can be d e t e c t e d with high s e n s i t i v i t y ( c f . F i g . l ) and e v a l u a t e d with the demagnetizing f a c t o r s diac~m~d i n ~ t i o n 2. The results (Fig. 2) ohow a maooth

*Work supported i n p a r t by Fonde zur F~rderung der Wiseenachaftlichen Forschung, WiSh, under contract g 7098. 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

M. Wacenovsky et al. / Magnetization o f grain-aligned HoBa2Cuz07_ a

1630

lOO[

temperature

/

5~l

T- 4.2K

a

dependence

at

high

temperatures

and an upturn f o r T~O. The a n t s o t r o p y a t T'O amounts t o a f a c t o r

of ~4.

Since Bean model

c a l c u l a t i o n s 4 of t h e r e v e r s i b l e m a g n e t i z a t i o n 2

D

5

0 .

.

~ .

.

" .

350~ 0

.

,--1

I 10

•

[ 20

always

resulted

.Hcl. ,

we conclude

rather

than

the 30

much that

larger some

values

other

temperature

dependence

of

effects

f l u x p i n n i n g ar e r e s p o n s i b l e

observed

first

~O H (sT)

in

of

for the

penetration field.

4. CRITICAL CURRENT DENSITIES From ,OrTp.

the

magnetization

curve,

critical

current densities can be deduced in two ways

A m

which are essentially based model 4,5. The results may be

.&-

on the Bean ~1-marlzed as

follows. The critical current densities at low fields and low temperatures are very high (>1010 Am "2) and coaparable to the results obtained on single crystals. Bflc r 20

g 0

i 40

I 60

With increasing

field, Jc Initially drops by a factor of ~2,

I 80

I

I

i

1OO

120

140

160

T(K)

but then levels off for fields above ~1.5 T. The

decrease

drastic. FIGURE 1 Differential magnetizatlon and ac susceptibility of the saNples with different grain orientation.

of

Jc

with

temperature

is

Concerning the anisotropy of Jc, we

find, e.g., at 4.2 K and 2 T, that Jc(B,,a,b) is smaller by a factor of -20 than for B, Jc. 5. CONCLUSION In summary, we have presented evidence that

70

experiments 60

'T

H° 8a:t C°3 07°6

50 40

on

graln-aligned

(123)-materlale

in epoxy are very well suited to study the anlsotroplc

magnetic

superconductors

I

,Intergrain"

wlth

properties no

of

high-T c

interference

by

effects.

REFERENCES 10

,o

2o

30

,o

so

.o

,o

ao

oo

,(K)

Teaperature Hi ,a,b.

FIGURE 2 dependence of Hcl

for H, ,c

and

1. D.E. Farrell, B . S . Chandrasekhar, M.R. DeGulre, M.I(. Fang, V.G. KOgan, J.R. Clam, and D.K. Finneaore, Phys. Rev. B36 (1987) 4025. 2. M. Wacenov~ky, H.W. Weber, O.B. Hyun, D.K. Finnemore, and K. ](erelter, Physics C, submitted. 3. J.A. Osborn, Phys. Rev. 67 (1945) 351. 4. C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. 5. H.C. Freyhardt and P. Haasen, Z. Metallk. 58 (1967) 856.