Upper critical field anisotropy and dissipative flux motion in NdCeCuO single crystals

Phlsica C 185-189 (1991) 1913-1914 North-Holland

UPPER CRITICAL FIELD ANISOTROPY AND DISSIPATIVE FLUX MOTION IN Nd-Ce--Cu-O SING LE CRYSTALS

L.FABREGA, _~A.CRUSELLAS, J.FONTCUBERTA, X.OBRADORS, S.PIIOL, C.J.van der SEEK~, P.H.KES~, and J.BEILLE

T. GRENETb

ICNAS-CSlC, Campus de la UAB, 08193 Bellaterra, SPAIN ~Kamerlingh Onnes Lab., P.O.Box 9506, 2300RA Leiden, ~EDERLAND blab. Louis N6el, CNRS, 25 Av. des Martyrs, 38042 Grenoble, FRANCE

We r e p o r t r e s i s t i v i t y and AC s u s c e p t i b i l i t y measurements under e x t e r n a l m a g r ~ t i c f i e l d In NdCeCuO s i n g l e c r y s t a l s . From r e s i s t i v i t y data we have estimated the upper c r i t i c a l f i e l d s , coherence lengths and a n i s o t r o p y f a c t o r ; AC s u s c e p t i b i l i t y has allowed the e x t r a c t i o n o f the a c t i v a t i o n energy values, which t u r n out t o be h i g h e r than the p - t y p e HTSC ones.

low-frequency AC technique. Nagnetlc

1. INTRODUCTION n-type

HTSC

display

c o n t r a s t s respect t o t h e i r

some p-type

measurements o f the r e s i s t i v i t y

interesting counterparts:

under

external

to

6T

were

applied.

fields

AC s u s c e p t i b i l i t y

measured w i t h h =0.3G p a r a l l e l

t o the c - a x t s

up was of

0

the crystal; measuring frequencies ranged from

magnetic field have shown no broadening on

the

11 to 5565 Hz. Bias magnetic fields between O. IT

transition.

the

and 2T were applied also along the c - a x i s .

The

r e p o r t e d values

of

anisotropy factor for single crystals

of

compoundsi'z lie between the ones of

the

(F~30) and the BSCCO (F~3000) systems. thus

interesting

to

b e h a v i o r o f these new

study

the

HTSC, t o

nature of their pinning and flux

these YBCO It

is

irreversible determine

line

the

3. RESULTS

A paralle1 shift of the resistive transition is observed when applying increasing magnetic fields ( f i g . l ) ; this shift

lattice

susceptibility

resistivity

more pronounced

2.5 2.3 -:

and compare them to the p-type HTSC. In this paper we report

is

2.0-

and AC

H

H

1.8 ~

measurements under external

magnetic fields in Nd-Ce-Cu-O single crystals.

E

rll

0.8

2. EXPERIMENTAL The Nd-Ce-Cu-O single crystals used

a

in

this

,

0.0 0

study were grown by the self-flux method; the Ce

7

'

~'--.*'--...~ . _...J~

'~ '~

:,o.... ~'s.... 2o- ~z+s ' .~o -(K)

content is estimated3 to be x~0.15. Resistivity measurements were performed both along c-axis and the ab-plane of the crystal

the

FIGURE 1 p b(T,H) curves for and H!(c.

using a

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

1914

L Fabrega et al I Upper c "nticalfield anisotropy and dissipative flux motion

HII c. From the R(T,H) curves we

for

lower bounds

for

the

upper

estimated

crltica]

H ~¢(T) and H lie(T) (see ftg.~). c2

fields,

Ustng

the

a stronger plnning than the YBCO and BSCCO. I f , as tn the l a t t e r case, oxygen vacancies are dominant ptnnlng

centers,

we should

the

conc]ude

¢2

kTIH theory we ce]cu]ated H ¢~57 T, ¢2

these values give an antsotropy which ] t e s between NdCeCuO (1"~400)

the and

H 11~6.9 T; c2

factor4

reported

F~67,

va]ues

SmCeCuO (I'~29)

for

single

AC s u s c e p t i b i ] i t y provided an pi nntng

Ana]ysis o f

energles the

a]]owed

in

frequency

d i s s i p a t i o n peak, T , mode]5

this

n-type

to

by

evaluation

these

compounds.

dependence o f means o f

detemine

the

the

weak te~q~erature

The obtained U(H) va]ues,

range

the TAFF

activation

energies from a ]aw ] n ( f ) ~ U(H)/kT , that t.hey have a

of

assuming

dependence.

between 20meV

thus providing a para|]e]

shift

nature] of

the

explanation p(T,H)

indication o f the ro]e of

oxygen

However, a pre]iminary study 6 o f the by oxygen vacancies in t h i s system, framawork o f the 3D c o ] l e c t i v e does not reproduce neither ]ine nor the f i e ] d

the

(~20meV) and BSCCO (~8meV) systems, as expected. In f i g . 2 the

obtained

irreversibi]ity

|ine

the

theory,

the

pinning

Magnetic s c a t t e r i n g and/or

vacancies, together with a possib]e

defects

within

irreversibi]ity

dependence o f

energy at low f i e l d s .

pinning

pinning

for

YBCO

is

not yet avai]ab]e.

than

the

~(T,H)

vacancies

extended

ones

the

experimanta]

somewhat hlgher

corresponding

for

and

and 14OBEY. These energtes are the

HTSC

may be at the basis of the htgher U(H,T) va]ues,

t r a n s i t i o n s . However, a conc]usive

c r y s t a l s t'z .

the

that the lower anisotropy of

re]ated

to

the 3D f ] u x ] i n e ] a t t i c e , shou]d

the

oxygen

me|tin9

be

of

considered

to i n t e r p r e t the experimenta] r e s u ] t s .

from the AC s u s c e p t i b l ] i t y is shown. ACKNOWLEDGENENTS 5.0

We acknowledge NIDAS-ClCYT

4.0

~"3.0

SCI-O389-N(A).

=:2.0

REFERENCES

1.0t 0.0

fi nanci al

support

NAT88-0163-C03-02

1. Hidaka et a l . ,

from and

the the

Nature 338 (1989), 635.

2. Dalichaouch et a l . , Phys.Rev.Let.64 (1990),

' 00

b~Y.

T/Tc

FIGURE 2 I r r e v e r s i b i l i t y lines from the R=O ( c i r c ] e s ) and f=5565 Hz ( t r i a n g l e s ) c r i t e r i o n s

Physica C165 (1990), 265.

4. N.A.Crusellas et a l . , submitted t o Physica 5. P.H.Kes et a l . , 242.

4. CONCLUSIONS We have found that the Nd-Ce-Cu-O system

3. S.PiSol et a l . ,

Sup. Sci. and Tech. 1 (1989),

has [6] L.F~brega et a l . , to be pub|ished.