Physica C 193 (1992) 437-440 North-Holland PI ICA

Critical-current degradation by surface damage in high-temperature superconductors

AIvaro Sanchez and Du-Xing Chen Electromagnetism Group, Physics Department, Universitat Autbnoma Barcelona, 08193 Bellaterra (Barcelona), Spain

¢

Received 16 December 1991 Revised manuscript received 31 January 1992

h is proved that the critical-current density Jc ofsintered YBa2Cu~O~_6 samples is decreased by 70% in the cutting surface layer and that this degradation is extended to a depth of around 0.1 ram. This is deduced form a critical-state model fit to field- dependent AC susceptibility data of two samples with different thicknesses cut from a pellet by an ordinary, diamond saw. A spatial dependence of Jc in the critical-state model calculations is used to simulate the degradation. The local field-dependent Jc is also determined.

In order to determine the transport critical-current density Jc of sintered high-To superconductors, the sample is usually cut from a pellet to have a thin and long orthorhombic shape. Such a sample shape has a small cross-section so that the current correspond- ing to Jc is small and the contacts can be made easier. Besides, for most applications, cutting is also nec- essary to have an appropriate device shape and to avoid an over-large self-field effect that decreases the average Jc in the device. However, cutting involves a severe plastic deformation, which will destroy the Josephson junctions among the grains in a surface layer and degrade J~. There have been no quantita- tive studies on this surface damage. In this work, we analyze the AC susceptibility data of two samples with different thicknesses cut from the same YBa2Cu3Ov_a pellet by an ordinary diamond saw and study the degradation of Jc in a surface layer.

The samples and experimental details were pre- sented in [ 1 ], where the field-dependent complex AC susceptibility, X=Z ' - jX ' , of four samples with the same width (the pellet thickness), 2b= 2.6 ram, and different thicknesses, 2a ranging from 0.40 to 2.5 ram, was measured at 77 K and analyzed on the grounds of Kim's critical-state model. X was mea- sured using a lock-in analyzer with a mutual imped- ance bridge [2] in an AC field applied along the sample length. Demagnetization corrections were performed for both Z and the field amplitude Hm. All

the samples had a smooth surface, and the thick- nesses of the thinner samples were determined from the sample length, width and mass, and the pellet density. We found that, although Kim's model could explain some main features of the data, the local in- ternal field-dependent Jc, Jc(!-Ii), could not be ex- pressed by it. Thickness dependence from AC sus- ceptibility measurements has recently been studied by other groups with similar results [ 3 ].

We further tested other Jc (Hi) functions and found that the best simple one for many not-very-thin sam- ples is exponential [ 4-6 ]. This exponential model is good because it simulates well in a modest field-range a high-field power-law field dependence, that is often found from transport measurements, and has a roughly linear section at low fields. The power-law dependence has been phenomenologically derived from a network of Josephson junctions, the direc- tions and coupling strenghts of which are randomly distributed. Considering the diffraction pattern of

• I [ , ~ I - - [ . 5 elliptic Josephson jur~ctions, this power Is i, , ,i [ 7 ]. For low fields, this power-law behavior is not adequate, since the critical currents of the junctions are finite; a linear function is used as a first approx- imation in the low-field region.

However, it turns out that the exponential fitting parameters for different samples are rather different, especially for the thinnest and thickest ones. This

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

438 A. Sanchez, D.-X. Chen I Critical-current degradation by surface damage

makes us wonder whether a common Jc(H~) func- tion exists for the whole pellet or not.

To answer this question, we ftrst check ff there is a J¢(Hi) function by which the data for all the sam- ples can be fitted. This function should not be as simple as those for which analytical solutions of the critical-state hysteresis loops have been obtained [6,8 ]. It could be expressed by a combination of them. To treat such functions, a numerical tech- nique, that allows calculations for an arbitrary J¢ (Hi) function has been developed with high accuracy. We present the fitting results using this technique for the thinnest and thickest samples. For simplicity, we re- fer to refs. [ 1 ] and [ 5 ] for the fitting process and only give the X" plots.

The linear and Hi-LS-power-law Jc(Hi) function is our basis for making such a fit. Its validity has been argued above and tested by comparison with other, more complex functions. Using one such J¢ (Hi), we have fitted very well the data of the thick sample, but when the same J¢(H~) is applied to the thin sample, the calculaled Z" peak is remarl~ably higher than the experimental data and shifted to the high-field side (fig. 1 (a)). If, instead, we fit the Z" maximum of the thin sample, then the theoretical curve for the thick sample will be lower than the data (fig. l (b ) ) . Moreover, it was not possible to fit data of the thin sample completely: the calculated curve is always shifted to the high-field side if the maximum point is well-fitted (fig. 1 (b) ) . As a conclusion, there is no single J:(Hi) function that can simultaneously fit both samples with a reasonable good agreement.

We noticed that, after hydrostatic compression, the intergranular J~ of sintered high-T¢ superconductors is not enhanced, as originally expected, by increasing pinning defects, but degraded owing to the damage of Josephson junctions [ 9 ]. Cutting is accompanied by a severe plastic deformation; a similar J~ degra- dation should occt:r in a surface layer in this case.

To prove this idea, we assume Jc to be both Hi- and position-dependent, expressed by

~(H,, x)=J~(H,, oc) [1 - t r exp(

-x/5¢)b/(a+b)], (1)

where x is the distance from the sample surface, 6< is a distance constant characterizing the damage depth, J¢(H, oc) is the J~(Hi) without a surface

0 l~) r - ~ i . . . . . ~ - - ' r - + ' r ~ ' ~ ' - I ~ ~ " ' : ~ ' ; "

f . )

0i. [

i 0.00 ~-

0 . 0 6

0 0 3

0 . 0 0 i I I I I I I l l l ~ t t % 1 ; 1 " T " I " t ~

1 0 0 1 0 0 0 1 0 0 0 0

H~ ( A / m ) 0 15 [ . . . . ~ - v - . . . . ~ - - - ~ ~-~--~- , , ,~ . . . . r - - i

,I ( b ) !

OZ:2~ J !

o

i " 0 . 0 6 ~ /

f / \ \ o • o

0 . 0 3 ~" r ~" ~ ~ o ~ "° ~ /" \ " o i -- ¢/~ o\ { ~ / % " \ •

o ~ o " . o I

0 0 0 L_ ~ ~ ,t , ~ _ ~ _ , ~ < ~ L _ L " q " ' - ~ , ~ . - - , - ~ ~ J

I00 I000 I0000

Hm (A ."m)

0 1 2 I

oo0 ,, \ ,;: .. t t -YI

i ~ % , _ l \ " o i

0 . 0 3 o . " *o • a o

1 0 0 1 0 0 0 1 0 0 0 0

Hm ( A / ' m )

Fig. i. 7£" as a func t ion o~" the AC f ie ld a m p l i t u d e Hm fo r t he t h i n

( O ) arid th ick ( . ) sam,)les . ( a ) Fi t w i th the th ick s a m p l e p a - ~ m e t c ~ . t t . ' , , , , , , Ell in- t v ) F i t with the " ~ : . . . . I . s ^ ' , - • ;~¢tilipllo paramete r s , t , - I e l u d in g su r f ace d amag e effect.

damage, o" is the relative degradation of surface arc, and the factor b/(a+b) is the ratio of the cut sur- faces to the complete side surfaces.

Jc(H,, ~ ) is again chosen to be the above linear and power-law type. An excellent data fit is found for both samples by using eq. ( 1 ), as shown in fig.

A. Sanchez, D.-X. Chen/ Critical-,u,,ent degradation by surface damage 439

l (c); the above idea is therefore justified. We find tr=0.7 and $c=0.1 mm for our samples. We can see that the fitting curves pass through almost all the data points, except in the low-field part where they are higher.

The phenomenon of the low-field Z" being lower than our model fit has important significance. There has been a long-unanswered question: is there an in- tergranular lower critical field H~lfi To obtain ex- perimental evidence, it has been thought that it is best to use a thin sample. In this instance, the answer is negative, as can be seen from the good fit in fig. 1 (b) for the thin sample by the critical-state model, which assumes Hew=0. However, when the X" data of both samples are fitted by our model, an obvious departure is always seen in the low-field region. This suggests that there is an effective Hew which makes the low-field Z" lower than that predicted by the crit- ical-state model. We will discuss this point else- where, accompanied by a presentation of the mech- anism of the intergranular critical state.

Figure 2 shows the J¢(Hi) functions used for the fit in figs. 1 (a) and 1 (b), and the J~(Hi, oo) for fig. 1 (c). A comparison can be made from this figure: the J¢(Hi) curve corresponding to fig. 1 (a) for the thick-sample fit agrees with J~(H~, oo) for H,>600 A/m while the J¢(0) is only 12% lower than J~(0,

); whereas the J~ (0) obtained by the thin-sample fit is only 66% of J¢(0, oo).

This fact reminds us that, for determining .l~(Hi) of the material, i.e., J¢(H~, oo), the proper method

is to cut two samples with different thicknesses and to analyze data as performed in this work. If only one sample is measured, then the sample should be as thick as possible. This statement is the opposite of the common knowledge, that in order to determine Jc(Hi) at low fields the sample should be as thin as possible.

In conclusion, the critical-state model is a good ap- proximation for describing the intergranular super- currents of sintered high-T~ superconductors. How- ever, if the sample is cut during preparation, a severe surface damage has to be considered. The expression of J¢ should be a function of the local internal field Hi and the distance from the cut surface. For our samples, Jc is degraded by 70% at the cut surface and the degradation depth is of the order of 0.1 mm. Our finding is important for Jc measurements and ap- plications of high-T¢ superconductors, and the de- rived Jc(Hi, oo) function can be a good basis for the further study of the mechanism of intergranular J~. The same technique can be used for research into other surface effects, such as oxygen diffusion during annealing and aging.

Acknowledgements

W ~ ' ih~nk | q M U d . n 7 f a r he lp qOmP c!Innor1 {'rorn

the Spanish Superconductivity Program MIDAS and CIRIT is acknowledged.

300 [\.,, - - - ~ -

240[- ",. \

i

180 "-. E i ,

I I

L °o o'oo-

H, (A.. m)

t 1500 2000

Fig. 2. J¢(H, ) curves corresponding to the parameters of fig, I ( l ) (dotted line) and fig. 1 (b) (dashed lille), and J¢(Hi, oo) corre- sponding to fig. I (c) (solid line).

References

[ 1 ] D.-X. Chen. A. Sanchez, T. Puig, L.M. Martinez and J.S. Mufloz, Physica C 168 (1990) 652.

[2] D.-X. Chen in: Ballistic and bridge methods of magnetic measurements of materials, (China Metrology, Beijing, 1990 ) p.526.

[3] M. Forsthubert, F. Ludwig and G. Hilscher, Physica C i77 (1991) 401; V. Skumryev, M.R. Koblischka and H. KronmiJller, Physica C 184 (1991) 332.

[4] A. Sanchez, D.-X. Chen, J.S. Mufioz and Y.-Z. Li, Ph~rsica C 175 (1991) 33.

I5]A. Sanchez and D.-X. Chert, in: Susceptibility of Superconductors and Other Spin Systems, eds. T. Francavilla, R.A. Hein and D. Lieberger (Plenum, New York, 1992). D.-X. Chen and A. Sanchez, J. Appl. Phys. 70 ( 1991 ) 5463.

t ~ J

440 A. Sanchez, D.-X. Chert / Critical-current degradation by surface damage

[ 7 ] R.L. Peterson and J.W. Ekin, Phys. Rev. B37 (1988) 9848; Idem, Ph~ica C 157 (1989) 325~ Idem, Phys. Rev. B42 (1990) 8014.

[8] D.-X. Chen and R.B. Goldfarb, J. Appl. Phys. 66 (1989) 2510;

D.-X. Chen, A. Sanchez and J.S. Mufioz, J. Appl. Phys. 67 (1990) 3430.

[9] R. Puzniak, D.-X. Chert, E.M. Georgy and K.V. Rao, J. Appl. Phys. 65 (1989) 4344.