The effect of a thin silver layer on the critical current of epitaxial YBCO films

PHYSICA

Physica C 197 (1992) 1-8 North-Holland

The effect of a thin silver layer on the critical current of epitaxial YBCO films Emil Polturak, Gad Koren, Daniel Cohen, David Cohen and Igor Snapiro Physics Department and Crown Centerfor Superconductivity, The Technion-lsraelInstitute of Technology, Haifa 32000, Israel

Received 5 May 1992

We compare measurements of the critical current density of an epitaxial YBCO film with that of an identical film overlaid by a thin silver layer. We find that the presence of the silver lowers T¢ of the film by about 1.5 K, which is two orders of magnitude larger than predicted by the theory of the proximity effect for our experimental conditions. In addition, J¢ of the Ag/YBCO film near Tc is also significantly lower than that of the bare YBCO film. We propose two alternate interpretations of this effect, one in terms of destabilization of the flux distribution in the film and the other making use of the effect of the silver on the BeanLivingston surface barrier for the initial penetration of flux. The latter seems the more plausible explanation of our results.

1. Introduction Silver is one o f the materials c o m m o n l y used in conjuction with high-T~ superconductors. The addition of Ag to YBCO ceramics enhances the critical current o f the composite material over that o f YBCO ceramics [ 1,2 ]. It makes low resistance contacts to the superconductor [ 3 ], and can also be used as a barrier material in the fabrication o f Josephson junctions [4,5]. The proximity effect between Ag and YBCO was investigated by several groups, mainly by transport measurements done at low temperatures [ 6,7 ]. Studying Josephson junctions containing Ag as a barrier, we became aware o f the weakening effect it has on the properties o f YBCO in contact with it. At the time, we attributed this p h e n o m e n o n to interdiffusion of the Ag and YBCO which can be significant at the high temperature at which the junctions are prepared [4 ]. To discriminate between the effect o f interdiffusion and that induced by proximity coupling, we decided to carry out systematic experiments on the properties o f A g / Y B C O bilayers in which the silver was evaporated at low temperature, thus eliminating interdiffusion o f the two materials. In order to prevent damage to the surface of the films, we chose a contactless method o f measurement, namely AC susceptibility. Similar measurements were performed on N b N / A I bilayers in

the linear response regime [8 ]. Our technique is somewhat different, in the sense that we emphasize the regime near To, where the response is nonlinear, providing information on Jc, the critical current density in the film. To our surprise, we found that the presence o f a silver layer on top o f an YBCO film lowers J¢ way and beyond what the static theory of the proximity effect would predict. In the following, we describe the experiments leading to this conclusion and offer a qualitative explanation o f the effect based on the difference between the pinning strength of flux in YBCO and in the silver layer.

2. Experimental setup Our AC susceptibility method employs a self-inductance bridge, which compares two identical planar spiral coils in a bridge configuration. A detailed description of the bridge has been published elsewhere [9,10]. Briefly, a thin film sample is laid fiat on top of one of the planar coils. Below the transition temperature the film acts as a ground plane, reducing the self-inductance of the coil and throwing the bridge out o f balance. There are several advantages o f the self-inductance method, [ 9,10 ] over the mutual inductance method [ 11 ], the main one being that the

0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

2

E. Polturak et al. / Effect o f thin silver layer

use of a single coil greatly simplifies the mounting and positioning of the sample. Another major difference between our setup and other published designs [ 11 ], which use coils much smaller than the diameter of the film is in the geometry of the field. A comparison between these two methods is shown in fig. 1. In the case of a coil with a diameter much smaller than that of the film, the field is localized near the center of the film, as shown in fig. 1 (b). Then, for small enough excitation field, one probes the response in the limit of a small induced current, i.e. the linear response regime. In this regime, one measures the kinetic inductance and

hence the penetration depth [ 11 ]. In our case, the planar coil generates an approximately constant field over an area which is larger than that of the film (fig. 1 ( b ) ) . Since a thin film perpendicular to the field direction has a very large demagnetization factor, the field lines tend to concentrate near the edge of the film, inducing sizable screening currents which increase radially. If the current density at a distance r from the centre reaches Jc, as in fig. 1 (c), the field will penetrate from the edge of the film down to this radial distance (fig. 1 ( d ) ) . The screening ability of the film will be reduced, and so will be the diamagnetic signal. As the temperature is lowered, Jc in-

LARGE COIL

SMALL COIL

(o)

/FILM

I

I l *°* I °*%,COIL

I ....

(b)

I

t °°°°°°'°

He

I I

/

J °°°°'°°°°

He

f

J (c)

~Jd

3d

~r

(d)

/

I

L;r

Ii B

• r

Fig. 1. A comparison between measurements of AC susceptibility using small and large coils shown are (a) coil geometries, (b) field geometries, (c) shielding current distribution In the film, and (d) field geometry within the film.

E. Polturak et al. /Effect of thin silver layer

creases, and eventually the field will be totally excluded from the film. Therefore, what we probe is the temperature dependence of Jc. We call the interval from Tc down to the temperature at which the shielding reaches its maximal value the transition width. Obviously, the transition width will increase with the amplitude of the external field. We now turn to the AC losses, which determine the imaginary part of the susceptibility. The main loss mechanism in the film just below Tc is due to hysteretic motion of flux lines. Since the magnetic field throughout most of the film exceeds H ~ in this temperature range, the field will penetrate the film in a vortex structure. As a result of pinning, flux lines are not free to reach equilibrium positions after field changes. In this situation, the magnetic moment of the film will lag behind the external field, and thus hysteresis losses can be expected. In the case of a superconducting disk with diameter 2R much larger than its thickness ds placed in a magnetic field perpendicular to its surface we have high sample aspect ratio (demagnetization factor N = 1 - n d s / R i.e. N ~ 1 ), and therefore even weak magnetic fields will result in strong fields on the edges of the disk. This means that the transition width of a susceptibility measurement will be considerably wider for a thin film than for a long cylindrical sample.

we could compare two identical films differing only by the presence of the silver layer. In figs. 2 and 3 we show the susceptibility data obtained with a l ~tm thick film of YBCO on SrTiO3 substrate contrasted with data from an identical film overlayed with 300 A of silver. These data were obtained with a I G amplitude of the AC field. The most notable feature seen in these figures is a lower transition temperature for the film overlayed with silver. In addition, the transition width for the silver coated film is larger than for the bare YBCO film. Finally, the magnitude of the dissipation peak in fig. 3 is smaller for the silver coated film. Since the magnitude of this peak is proportional to Jc. this is the first indication that the presence of silver decreases the critical current density. Similar results were obtained for films deposited on MgO, in this case the 0.004 0.0035: 0.003 YBaCuO film ut Ag

~" 0.0025 -i 0.002 Io 0.0015 (,9 co 0.001 O < 0.0005

3. Experimental

Our samples are thin films of YBCO, typically a few thousand A thick deposited by a laser ablation technique [ 12,13 ] on (100) MgO o r S r T i O 3 substrates. The films deposited by this method are of high quality and grow epitaxially with the c-axis perpendicular to the surface of the film. After growing the YBCO at 750°C, the films are cooled in an O2 atmosphere down to 100°C, where an overlaying silver film was deposited on half of the substrate. The reason for the low temperature deposition was to prevent diffusion of the silver into the film, which becomes significant at temperatures of 400°C or higher. The film was patterned by laser ablation into two identical disks, 4 m m in diameter, one with silver coating and one without. We chose laser ablation lithography in this step to prevent any contact of the surface of the film with chemical agents. In this way,

3

80

82

84

86 T(K)

88

90

92

Fig. 2. Real part of the AC susceptibilityfor the two films under comparison. 0.0040.0035-

0.003~m~ 0.00250.002-

YBaCuO film with Ag k

t~ 0.0015

YBaCuO film

0.001(rJ

0.0005-

0~ -0.0005 80

82

84

86 T(K)

88

90

92

Fig. 3. Imaginary part of the AC susceptibility for the two films

under comparison.

4

E. Polturak et aL / Effect of thin silver layer

thickness was 1 ~tm for the YBCO and 500 A for the silver. These data were presented elsewhere [ 10 ]. The reasons dictating this particular choice of film thickness for the YBCO and Ag layers are discussed below. Here, we only remark that the choice was made in order to ensure optimal coupling via the proximity effect. We carried out numerical simulations of the current distribution in the film using the model of Sun et al. [ 14 ], which is an adaption of the Bean model to the case of a thin film in a perpendicular field. We found that the model describes our data well. The calculated current distribution in the film [ 15 ], obtained in a self-consistent calculation similar to that by Fiory et al. [ 11 ], is history dependent and can be quite complicated. In the simplest case, the current density per unit area increases with the radius, and saturates at the critical current density. This case is illustrated in fig. 1 (c). The field penetrates inward from the edge of the disk down to the value of the radius at which J = J c (fig. 1 ( d ) ) . Near To, this value of the radius shrinks to zero, while at low temperature it is almost equal to R. Therefore, the low temperature magnitude of the diamagnetic signal, which is proportional to the area of the film from which the field is excluded, will reach its limiting value. At a higher temperature, the magnitude of the signal can be used to determine Jc. Note that the calculated field profile within the film, shown in fig. 1 (d), shows enhancement near the edges, which is due to the large demagnetizing factor. By comparing the results of the simulations with the experimental data, we conclude that Jc of the silver coated films is lower than that of the bare YBCO [ 15 ].

4. Discussion

According to our understanding of the experimental system, the films overlayed with silver have a lower Jc near T¢. This is a surprising result, since the addition of silver to YBCO is usually thought to enhance the critical current [ 1,2 ]. However, this enhancement is usually obtained for ceramic, granular samples, where the silver fills in the intergranular spaces, thus increasing the contact area between the grains. This in itself would only serve to lower the resistivity of the sample, so one additionally needs

to invoke a proximity effect between the YBCO and the silver in order to explain the larger J¢. However, even with the addition of silver, J¢ of bulk ceramics is several orders of magnitude smaller than that of high quality epitaxial films such as the ones used in this work. Therefore, such effects should be marginal in our experiment. Before attempting to explain this effect, we discuss several possible causes that would yield a spurious result of this sort. First, one has to consider th possibility that the film was damaged during the silver deposition process, for example due to loss of oxygen while the vacuum chamber was evacuated. This, however, should equally affect the bare YBCO film present in the chamber. In fact, we turned to the method of simultaneous preparation of the coated and uncoated films to rule out this effect. Another possibility is that eddy currents in the silver layer induced by the AC field heat up the film, in which case the temperature of the film will be higher, and J¢ lower. Our estimation of the heating, based on the measured resistivity of silver films made in our vacuum chamber, show that it is negligible for a 300 /k thick film. To prove this, we repeated the measurements at several frequencies of the AC field between 5 kHz and 40 kHz. Since eddy current heating depends on o~2, the square of the frequency of the AC field, one would expect Tc to decrease and the transition width to increase with co. We found no such effect, and therefore we are confident that eddy currents are not the source of the decrease in J¢ As a final step in the corroboration of our results, we carried out a DC transport measurement of J¢ after patterning the films used in the susceptibility measurement into microbridges. This time, we used wet lithography with a PMMA photoresist. The baking temperature of the resist was kept below 100°C, to prevent diffusion of the silver into the YBCO. The data, presented in fig. 4, clearly shows that both Tc and Jc are indeed lower for the film overlayed with silver. We propose an interpretation of our experiment based on the proximity effect between YBCO and silver. Such an effect was shown to exist when the silver was in contact with the a - b plane of YBCO [7 ]. Our films are c-axis oriented. To provide the coupling to the a - b planes, we grew the YBCO films sufficiently thick, so that their surface roughness is several hundred A. Thus, a large portion of the top

E. tJolturak et al. / Effect of thin silver layer

A

~'~ 3 E

A Ag/YBC0 A FILM A A A

~2

O

,~,

[]

×

YBC0 FILM Q

A

O

A A

0

I 84

I

I 86

o

^ .~,~

I 88

I DD 90

T(K)

Fig. 4. J¢ for the two films as measured by transport.

surface makes contact with the silver along some inplane direction. With thinner films, the roughness is smaller, leading to a smaller in-plane contact area. Consequently, we found that the size of the effects discussed here is diminished for thinner YBCO films [ 15 ]. Regarding the thickness dependence of the Ag layer, it should be larger than ~N, the coherence length of the normal material. For a clean metal, ~N= hVF/ 2rckBT, which amounts to about 150/k for Ag at 90 K. Our Ag layers have a higher resistivity than that of the pure metal, and therefore ~N would be smaller. Since the order parameter decays exponentially with ~N, there is no point in investigating Ag layers thicker than 2--3~N. Hence, all our experiments were done with Ag layers in the thickness range of 300 to 500

A. The de-Gennes theory of the proximity effect [ 16 ] predicts that for an SN sandwich, T¢ should be lower than T~o, the transition temperature of S. This is due to the leakage of normal electrons into the S side, leading to a depression of the order parameter therein. We now estimate the size of this effect. Taking the temperature dependence of ~s the coherence length, as (s (T) = 0.74(o ( I - T! T¢)-w2, which is appropriate to the clean limit, we obtain [ 16 ] T~ - T¢o = - (0.74rQ z ~°2 Tc • -ST ds

(1)

Taking T~=90 K, ~o=20 ,~, and d~= 1 lam, we find that for our samples the predicted depression of T~ is less than l0 -2 K, which is two orders of magnitude smaller than the measured shift of about 1.5 K. We

5

carried out a similar estimate using the equations appropriate to the dirty limit, and found that the discrepancy is about the same. We have also used a recent version of this theory [ 17 ], which takes into account the reflection of quasiparticles at the SN boundary, and found a similar disagreement. Thus, the straightforward application of de Gennes theory cannot explain our data. The reason, in our opinion, is that the effect here is dynamic, having to do with the pinning of flux rather than with the thermodynamics of the transition. It is possible to derive a simple formula linking the shift of Tc with the critical current in the sample. Suppose that the film is located in a field Ho, perpendicular to its surface, at a temperature very close to To. The value of the areal shielding current, Jcd,, is assumed to be small enough so that the field penetrates freely. Under these conditions, there is no shielding and the sample appears normal in a AC susceptibility measurement. According to the critical state model, we can take the current density everywhere in the film as fixed at J¢. Then, the calculation of the screening field generated at the center of the disk is elementary. Taking the uniform current density per unit area as Jcds, we obtain

UoJ~d,

B(r=0) = - 2

ln(R/~),

(2)

where R is the radius of the disk and 5 is the radius of the smallest region around the center which will generate a detectable signal once the field is excluded from it. The ratio R/~, being the ratio of the largest signal to the smallest detectable signal, is essentially the signal to noise ratio of our experimental AC susceptibility setup. The value of this ratio is on the order of 102-103, depending on the frequency and AC field amplitude. Near To, the experimental temperature dependence of Jc as measured by transport can be described by J¢=J~o( 1 - TITs) ~ where o~~ 2. On the other hand, the value of Tc determined in the AC susceptibility measurement will be the temperature at which the shielding field Bo =#oHo. Denoting this temperature by TBo, and substituting the form of J~ into eq. (2) we find

T_TBo=T( c

•

2Bo -

-

-

"~'/"

\ltoJ¢od~ In (R/6).]

"

( 3 )

6

E. Polturak et al. /Effect of thin silver layer

Substituting our experimental values of Jco into eq. (3), we obtain T~-TB0 = 1.6 K for the YBCO film and 4 K for the film overlaid with Ag. These numbers are of the correct order, and should be compared with the measured numbers of 1.4 K for the YBCO film and 2.6 K for the Ag/YBCO film. Thus, our simple model shows that the transport and susceptibility data are consistent with each other for the YBCO film. There is some discrepancy regarding the Ag/YBCO film, which will become rather important later on, when we consider the effect of the different field geometries present in the susceptibility and transport measurements on Jc. First, though, we have to explain why in principle Jc of the Ag/YBCO film is lower than that of the YBCO film. In the following, we present two qualitative arguments, both of which can lead to the effect described in this paper. The first argument has to do with the stability of the critical state. We first recall that J~ is determined by the strength of the pinning. Suppose that the film is located in a field perpendicular to its surface, as in the AC susceptibility setup of fig. 1. The field penetrates some distance into the film from its edge. The gradient of the flux distribution in the film determines J~. In the critical state, the Lorentz force on the flux line, J × 40, is exactly balanced by the pinning forces. The pinning force is proportional to the condensation energy multiplied by the volume of the pinning center. Obviously, as the gap induced in the silver is much smaller than that of YBCO, the pinning in the silver will be very weak. According to theory [ 16 ], the penetration depth in the normal metal is defined locally, since the order parameter depends strongly on the coordinates. However, it is always larger than that of the real superconductor, and therefore much greater than ~N. It is therefore permissible to treat the Ag layer as a type II superconductor having a very weak pinning. Suppose now that the external field is increasing. As the shielding currents increase with the increasing field, the Lorentz force will attempt to push the lines towards the center of the film. The sequence of events arising from the increase of the field is shown in fig. 5. Since the line is only weakly pinned inside the silver, it will bend and lengthen, and move from its original position, denoted as 1 in fig. 5, to position 2. This additional length of the line within the silver costs energy, and a state of lower energy can be

I )T EDGE OF FILM -.~

/

(2)/~J Ag

YBCO

SUBSTRATE

Fig. 5. A schematic representation of the process leading to the destabilization of the flux distribution in the film by the silver layer. For clarity, the thickness of the silver layer is grosslyexaggerated in the figure. The steps involvingthe motion of the flux line through its sequential positions are discussed in the text.

reached by moving the line to position 3 in fig. 5. As the field increases further, the process will repeat itself until the field has penetrated fully into the superconductor. Although the energy cost due to the extra length of the line within the silver is small due to the smallness of the condensation energy there, it can lead to an observable effect as the critical state distribution is only marginally stable. In his book on superconductivity, de Gennes pointed out an analogy between the critical state distribution of flux and a sandpile, which redistributes itself continously with every little perturbation. This analogy was investigated in a recent numerical study of the response of the flux distribution to a small driving force [ 18]. The study shows that a significant redistribution of the flux takes place even for a perturbing force much smaller than the pinning force. Furthermore, the redistribution seems remarkably independent of the size of the initial perturbation, as befits a marginally stable situation. A redistribution of flux will necessarily lead to a smaller gradient of the field within the film, and therefore to a lower Jc. The range of perturbing forces used in this study goes down to 0.1 of the pinning force of a single flux line. Writing this force as Jc X ~od, we find that in our case the ratio of the perturbing force to the maximal pinning force will be JcNdN/Jcsds, where the subscript refers to the local values in the normal and superconducting layers. This ratio corresponds to a force two to three orders of magnitude smaller than the force used in the

E. i~olturak et al. /Effect of thin silver layer

numerical study [ 18 ]. However, since the response o f the flux in this study seems independent o f the magnitude of the perturbing force, it may well be that the mechanism described by the authors, namely that o f self-organized criticality, can account for our results. Obviously, the extension o f this study [ 18 ] to smaller driving forces would help to clarify this point. The other mechanism leading to a smaller Jc that we discuss has to do with the presence o f a B e a n Livingston surface barrier to flux penetration [ 19 ]. Intuitively, any explanation involving a surface effect seems appealing in the present context, since it is only the surface o f the film which is modified by the silver overlayer. The presence of a surface barrier near T~ was suggested by Konczykowski et al. [ 20 ], following their investigation of field penetration into single crystals o f YBCO. It was found that the value o f the field at which flux first penetrates into the crystal was a factor 2-3 larger than the thermodynamic value of H~1. The value was lowered dramatically after the surface o f the crystal was damaged by electron irradiation. For a field parallel to the surface, flux penetration will occur after the external field reaches the value H s ~ H c l ( 2 / ~ s ) ~ ( q ~ o / A ~ s ) [ 19 ]. The reason for the dependence on 2 is due to the screening range beyond which there is no appreciable interaction between the flux line inside the superconductor and its image outside, while the reason for the dependence on ~s comes from the minimal distance from the interface at which the flux line becomes a well defined entity. In our case, the silver layer can be thought o f as a weakly superconducting surface o f the film. Then, the surface barrier will be lower than that for a uncoated film, since the effective penetration depth in the normal metal is much larger[8,16]. In addition, the minimal distance o f the flux line from the interface will be ~s or ~N, whichever is larger. In total, the surface barrier will be significantly lower at the silver coated surface o f the film. From its very definition, the lowering o f this surface barrier should become manifest when the field is parallel to the surface coated with silver. In the AC susceptibility measurement, the field is largely perpendicular to the surface, as discussed above (fig. 1 ). In contrast, the parallel component of the field will be large in the transport measurement, where the field generated by the transport current is circum-

7

ferential to the surface. Therefore, the effect o f the lowering o f the surface barrier should be more pronounced in the transport measurement. If indeed this mechanism is responsible for the decrease o f Jc, then Jc as measured by transport should be lower than that measured by AC susceptibility. This, in fact, can explain the discrepancy between the measured and calculated shift of T~. Since if one plugs too small a value of J~o into eq. (3), the calculated shift will be larger than the one measured directly by the AC susceptibility. For that reason, we feel that the lowering of the surface barrier by the normal layer offers the most plausible explanation o f our results. In conclusion, we presented a rather dramatic effect o f an Ag overlayer on the critical current o f epitaxial films near To. We advanced two qualitative arguments as to the source of this effect. In our view, our results can be best accounted for through the lowering o f the Bean-Livingston surface barrier due to the presence of a normal metal.

Acknowledgements We are grateful to R. Mints and Y. Yeshurun for useful discussions, and to M. Ayalon for technical assistance. This work was supported in part by the Fund for Promotion o f Research at the Technion, and by the US-Israel Binational Science Foundation.

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E. Polturak et al. / Effect of thin silver layer

[ 10 ] E. Polturak, Daniel Cohen, David Cohen and G. Koren, in: Proc. of ONR workshop on AC susceptibility of Superconductors and other Spin Systems, eds. T. Francavilla and R. Hein (Plenum, New York, 1992), to be published. [ 11 ] A. Fiory, A. Hebard, P. Mankiewich and R. Howard, Appl. Phys. Lett. 52 (1988) 2165. [ 12 ] G. Koren, E. Polturak, B. Fisher, D. Cohen and G. Kimel, Appl. Phys. Lett. 53 (1988) 2330. [ 13 ] G. Koren, A. Gupta, R. Baseman, M. Lutwyche and R. Laibowitz, Appl. Phys. Lett. 55 (1989) 2450. [ 14] J. Sun, M. Scharen, L. Bourne and J. Schrieffer, Phys. Rev. B44 (1991) 5275.

[ 15 ] Daniel Cohen, MSc Thesis (Technion, 1991 ). [ 16 ] G. Deutscher and P. de Gennes, in: Superconductivity, vol. 2, ed. R. Parks (Marcel Dekker, New York, 1969). [ 17] M. Ashida, J. Hara and K. Nagai, Phys. Rev. B 45 (1992) 828. [ 18 ] O. Pla and F. Nori, Phys. Rev. Lett. 67 ( 1991 ) 919. [ 19 ] C. Bean and J. Livingston, Phys. Rev. Lett. 12 ( 1964 ) 14. [20] M. Konczykowski, L. Burlachkov, Y. Yeshurun and F. Holtzberg, Phys. Rev. B 43 (1991) 13707.