Physica B 194-196 (1994) 1855-1856 North-Holland PflYSICA Cri t ica l C u r r e n t o f H i g h Qua l i ty N b T a Th in F i l m s and H e t e r o s t m c t u r e s

D. Kumar, M.G. Blamire, and A.M. Campbell

IRC in Superconductivity, Cambridge University, Madingley Road, Cambridge CB3 0HE, U.K.

We report results from experiments aimed at controlling the pinning properties of a model thin film system. In general, pinning in thin films (even epitaxially grown) is dominated by the surfaces, both through intrinsic surface pinning, and as a result of growth nuclei and defects at the surface. However, by grading the surfaces of NbTa films to pure Ta in a heteroepitaxial system we have eliminated surface pinning, and the grain boundary pinning present in polycrystalline films. Measurements of critical current as a function of magnetic field angle have shown that the surface pinning contribution follows a (sin~) 1/2 dependence identical to that attributed to intrinsic pinning in high Tc materials.

1 I N T R O D U C T I O N has shown that surface pinning in bulk materials can be reduced by plating the superconductor with a normal metal[5,6]. High resolution SEM of the NbTa polycrystalline films showed a columnar grannular structure with an average grain size of approximately 10-20 nm (Fig. 1).

Sample areas of 10 [am wide and lmm long were patterned using photolithographic techniques. The critical current Ic was measured as a function of field and angle using a standard four terminal method at 4.2 K with a criterion of l~tV/mm.

12 1 • Polycrystalline

10 ,~ . . . . Epitaxlal

8 B [ T : 4 . 2 K ~ , , ~

2 ak Ik

o J ~ . . . . . . "i" -20 35 90 145 200

Angle (~)

Fig. 2. A plot of Jc vs angle (O) between an applied magnetic field and the film surface for polycrystalline and epitaxial thin film.

Since the discovery of the high-temperature superconductors, intense efforts have been made to understand their properties in terms of coupled two- dimensional systems[I-3]. Many research workers have modelled high Tc superconductors with multi layer structures of low Tc materials[4]. However, for such experiments to be meaningful, samples are required with a well defined geometry and distribution of flux pinning sites. In the present work we have deposited high ~, NbTa thin films and found that pinning in these films is dominated by the columnar grain boundaries and film surfaces. The concept of pinning due to surfaces is well established and tends to be the dominant source of pinning when the field is applied parallel to the surface. It has attributed to mechanisms such as the Bean-Livingston surface barrier, surface sheath and surface defects[5]. In this paper, we demonstrate that surface and bulk pinning can be controlled and so permit high Tc modelling experiments.

1 2 0 n m

Fig I SEM photographs of Polycrystalline NbTa thin films (a) Surface morphology (b) Cross-section

2 E X P E R I M E N T A L

We have deposited a series of Nb0.7Ta0. 3 thin film structures onto R-plane sapphire substrates using dc magnetron sputtering. Polycrystalline samples were deposited cold, whilst for the epitaxial films the substrates were heated to a temperature of -600°C. Samples with the structure Ta/NbTa/Ta were also deposited in an attempt to reduce the surface pinning, by grading from Ta to NbTa over -80rim to avoid a sharp interface. Previous work

3 R E S U L T S AND D I S C U S S I O N

The sharp peak in Fig. 2 at ~ = 0 ° & 180 ° observed for polycrystalline and epitaxial NbTa thin films indicates that the very smooth surface makes the strongest contribution to flux pinning. In a high Tc material such as YBCO similar sharp peaks for qb=0 ° & 180 ° are also observed [7] and are interpreted as intrinsic pinning due to CuO 2 planes. In the perpendicular direction (0=90 ° ) the columnar grain boundaries are responsible for higher Jc in polycrystalline NbTa thin films. These grain boundaries have been eliminated in epitaxial thin films and the small peak in Jc for the perpendicular direction may be due to poor edge definition as a result of patterning the film and pinning by the edge surfaces. Fig. 3 shows clearly that the Jc due to the surface pinning contribution varies linearly with

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(sin@) 1/2 for both polycrystalline and epitaxial thin films for lower @ (0.50-8 °) values. However in polycrystalline thin films with increasing angle @ the columnar grain boundary contribution begins to dominate. The behaviour of high T c thin films in this configuration fits a similar relation [7].

0.60

0.50

0 . 4 0

'~.~ 0.30

-.--. 0.20

0.10

I I I

• - P o l y c r y s t a l l i n e J - -~ - - Epitaxial

0.000 t i t 0.l 0.2 0.3 0.4

(sin ~),/2

Fig. 3. Inverse of Jc vs (sin@) 1/2 for @ values between 0.5 °& 8 ° showing linear fits to the data.

Data for the Ta/NbTa/Ta structure is plotted in Fig. 4; the strong peak at @ = 0 ° (180 °) of Fig. 2 has diminished to a very low value due to the reduction in the surface pinning. The peak at @=90 ° is still present due to the columnar grain boundaries.

4 I I I I

3.5 t ; T a / N b T a / T a 3 ~ T = 4 . 2 K ~ t rack

>~ ~', B = 0.45 T A b, -d 2.5 " 5

~7" 2 / X / \ / \ ; . 3

~ 0.5 ~ ~ " - - . , , , . . . . ~ l

' ' ' ' ; 8 0-50 0 50 100 150 2 OO (In degrees)

Fig. 4. A plot of Jc vs angle @(same as in Fig.l) for graded structure of polycrystalline and epitaxial Ta/NbTa/Ta (note the difference in scale ).

The residual source of pinning in epitaxial materials is growth defects present in the substrate- film interface; in the TaJNbTa/Ta system these occur in the epitaxially grown base Ta layer rather than the NbTa. The great reduction in total pinning in the epitaxial graded structure is shown in Fig. 5.

7

5

4

E

10 6 I I I I

~ " - . Epitaxial Ta/NbTa/Ta 105 \ " , , T = 4 . 2 K

1 0 4 ~ " ' o - o . " q D = 0 °

- • - ° ' ° " " - o _

10 3 "- ~ - - , . .

102

10 i i t i 0 100 200 300 400 500

Field (mT)

Fig. 5. A plot of Jc vs field for parallel & perpendicular direction to film surface of epitaxial Ta/NbTa/Ta graded structure.

4. C O N C L U S I O N S

By grading the surfaces of NbTa films to pure Ta over a distance of the order of the penetration depth in heteroepitaxial systems surface pinning has been reduced to a low value; this al lows the unambiguous identification of the surface pinning component in samples with ungraded surfaces. The critical current density in samples with full surface pinning has been found to follow an identical angular dependence to that in high T c thin films. This type of model system will permit a greater understanding the flux pinning in high Tc superconductors where the bulk pinning is considered mainly responsible for the behaviour of transport current.

5. A C K N O W L E D G E M E N T

We are grateful to Prof. H. Ahmed for allowing the use of the SEM-900, and to Mr. P. Hunneyball for assistance in its operation.

6 R E F E R E N C E S

[1] A.A. Abrikosov, Physica C, 201,413 (1992) [2] W. K. Kwok, U. Welp, V. M. Vinokur, S.

Fleshier, J. Downey & G. W. Crabtree, Phys. Rev. Lett., 67, 3 (199l)

[3] R. M. Silver, A.L. de Lozanne & M. Thompson, IEEE Trans Supercon. (in press).

[4] K.E . Gray, R. T. Kampwirth, D. J. Miller, J. M. Murduck, D. Hampshire, R. Herzog & H. W. Weber, Physica C, 174, 340 (1991)

[5] A. D. Gupta & E. J. Kramer, Phil. Mag., 26,779 (1972), & 26, 769 (1972)

[6] A . M . Campbell and J. E. Evetts, Adv. Phy. 21, 199 (1972)

[7] M. Tachiki and S. Takahashi, Sol. St. Comm., 70, 291 (1989)