Macroscopic persistent currents in laser ablated YBa2Cu3O7-x films

HACROSCOPIC PERSISTENT CURRENTS IN M S E R ABLATED YBz12Cu307_, FILMS

B. S T R I T ~ ~ I + , P. LEIDERER*, R. FEILE+*, _{U .} RRUGER+ ANI)

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Thin f i h a of YBa2Cu307-, have h e n produced in shtu by laser ablation. Ae the condensation of the superconducting f i l m 1s done onto hat substrates (600

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persistent current could be obeerved during one hour.

Measurements of t h e same fllm a f t e r 8 months i n normal air showed a 4 0 $ decrease of jc and a stronger decrease of jc w i t h an externed magnetic f i e l d , suggesting that in the meantime the macroscopic r i n g current density is dominated by intergranular weak links.

Thin _{f i l m} preparation by laser ablation

The commonly used set-up for laser ablation I11 as described e a r l i e r [ 2 ] wee m d i f i e d i n order t o achieve hamogenous f i l m a growth over a largar area. As can be seen from F i g . 1 we took advantage of the fact that t h e laaer causes ablation perpandicular ta t h e surface o f the target.

Thu6 B cylindrical, rotating target is used. The curved target s u r f a c e causes ablation into e wider angle. The rectangular laser h a m i s focuased by a cylindrical. lens into

a Line-aha@ area p a r a l l e l to the axes of the cylinder. ^Thus the broadness of the beam aesures homclgenous ablation a l e 0 in t h e other Lateral dimension of the sobatrate. The substrates themselvee are lying on a heating stage which is located in t h e m i d d l e of t h e homogenous a b l a t i o n c l o u d . The oven can be h e a t e d u p to 8OO0C. T h e surrounding oxygen pressure amounts to several mbar.

Mat. RH. 80c. Symp. P m . Yol.

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F i g . 1: E x p e r i m e n t a l set-up Fig. 2: Sketch of the sample for laser ablation. w i t h the vibrating

coil geometry.

Cylindrical pellets ( @ = 15 mm) made from stochiornetric sintered YBa2Cu307 were uaed as t a r g e t s . In some experiments we employed targets prepared by plasma spraying. The resulting films did not yield as good results as from sintered and more dense targets. In most cases t h e t h i n films were condensed onto (001) SrTiOj or ccystalline ZrO The pulsed excirner laaer used f o r t h e ablation has the fol?owing parameters: wavelength 248 n m , pulse durati n

f

.

Persistent current measurements

Pecsistent currents in t h i s sample w e r e determined v i a the spatial distribution of their magnetic field, a s described earlier [ 3 ] . The detector was a small, rectangular pick-up coil (length 1 = 10 nun, width w = 1 . 5 mnt, thickness = 0 . 0 mm), which was vibrated by a piezoelectric bimorph in such a mode that mainly t h e and, A , facing the sample was in motion and the opposite side was nearly at rest (see Fig.

2 ) . In this way the coil acts a s a local detector, registerrng essentially only the magnetic field perpendicular to the sample p l a n e a t position A .

F o r t h e persiatent current measurements a f i l m of 4 5 0 nm thickneas was evaporated

onto

mm2

RESULTS AND DISCUSSION

I n our opinion the preparation of high quality supercon-

ducting films requires in-situ processing at elevated sub- strate temperatures and high O2 partial pressure in the vacu- um chamber. Our best results were o b t a i n e d at substrate sur- face temperatures of about 7 0 0 ° C (measured by a t h e m o - c o u p l e glued to the top of t h e substrate surface) and a base pres- sure of 1 mbar 02. Films with T ( R = O ) - 9 0 K, j C ( 7 7 - K ) > 2 x 1 0 ~

~ / c m * and thickness o f about 2 6 0 nm can be o b t a ~ n e d within 10 minutes including h e a t i n g , ventilation and unmounting.

Composition was always found to be within 10 % of the nominal one by RBS-measurements. Good c - a x i s preferential orientation normal to the substrate plane could be obtained on single crystalline (001) SrTi03 ,and (random) ZrO with optimum temperatures during deposltlon

.

S r T i O m i n i m u m yield values a s lovi a s 5 . 5 8 could be measured.

$he overall quality of the superconducting films over a lacge area was checked by the persistent current measurement.

A crucial question is whether the persistent currents truly do follow a closed p a t h around the ring or w h e t h e r they are localized to certain portions of the film. The spatial distribution of the magnetic field of a ring-shaped sample allows to distinguish, unequivocally, between these possibilities [4]. The results prove that the measured field distribution is the expected one for a macroscopic ring current. These persistent current8 exist in the textured films a t 77 K u n d e r different conditions for field-cooled (fc) and zero-field-cooled ( z f c ) samples. In all cases the following is observed :

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9 .

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F i g . 3a (open circles) shows the ahielding current,

I which is induced in t h e z f c ring when an external ffk&'%e is applied. In small fields a linear relation between

HBhield

I <

Fig. 3a: The ahielding current I,

i,

b t The ring current at 77 K and zero external field,

plotted vs. BeXt to which the sample had been exposed before t h e measurement. ( o 3 / B 9 , 11/89).

t h e sample was stared for 8 months in normal laboratory air (closed circles). The critical current density decreased by about 40 % to 0.35 A, decreased w i t h increasing field and vanished at 16.5 G .

F i g . 3b shows the current in the zfc ring in zero external field after t h e ring had been exposed briefly to BeXt. Thus a flux was trapped in the ring if BeXt ,had exceeded a critical v a l u e . This ring current shows a slmllar field dependence as in Fig. 3a.

Whereaa the initial macroscopfc current density ( a b o u t 2 weeks after preparation) does not depend on the presence of B ,t

5

( 5

From these measurements w e believe that only one region of weak links occurred in the film which is responsible for the diminished and field dependent critical current values.

These spatially resolved measurements yielded another interesting result. Fig. 4 showa the magnetic field distribu- tion across the center of a hamogenoue square film (not a ring) on Z r 0 2 ( 1 0 x 1 0 mm2) after the ring had been e x p o s e d to 1.8 G. A pronounced periodic s t r u c t u r e of the trapped magnetic flux is observed. The positians of the different peaks depend on the magnitude of the external field. An explanation for this behavior is still lacking.

F i g . 4 : Pick-up signal of the local detector (Fig. 2) v5rsus position across an YBCO film on ZrOZ (10 x 10 m ^{) .} ACKNOWLEDGEMENT

Technical assistance of Th.Klumpp is gratefully acknowledged.

REFERENCES

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N. J i s r a w i , Y.H. Min-Lee, W.L. Mc Lean and M. Croft, Appl. Phys. Lett,

2 ,

[ 2 ] J. Fsohlingadorf, W. Zander and B. Stritzker, $01. State Comm. 67, 9 6 5 ( 1 9 8 8 ) .

[ 3 J J. Frbhlingsdoxf, W. Zander, s. Stritzker, R. F e i l e and P. Leiderer, Physica C J59, 513 (1989).

[ 4 J P. Leiderer and R. Feile. 2 . Phys. B

x,