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Critical current density across 45° misoriented grain boundaries in YBCO thin films
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Physica C 185-189 (1991) 2291-2292 North-Holland
CRITICAL CURRENT DENSITY ACROSS 45 ° MISORIENrI'ED GRAIN BOUNDARIES IN YBCO THIN FILMS
H.S. KWOK and D.H. KIM Institute on Superconductivity, State University of New York at Buffalo, Amherst, NY 14260, USA The question of critical current reduction across 45 ° misoriented grain boundaries was addressed in this study. The reduction in Jc for a polycrystalline c-axis perpendicular film with complete and partial in plane a-axis texturing was calculated using a limiting-pathe model. The numerical results were then compared to experimental results obtained with YBCO on YSZ substrates, which had a particular texturing that is most suited for this study. The results indicated that the Jc reduction was not so large as indicated in the bicrystal studies of Dimos et al.
Most electronic applications of high Tc materials
the lattice matching conditions, there are two sets of
are based on the availability of good quality thin films.
grains for YBCO films deposited on YSZ(0OD. The a-
It is well-known that even the best YBCO films are
axis of the YBCO grains can be aligned either along
polycrystalline with grain sizes of 0.05-0.5/zm. For a c-
YSZ[100] or the YSZ[116] directions. Therefore the
axis perpendicular film, all the grains have c-perp align-
current along such YBCO film,~ has to flow either
ment. The in plane a-axis texture depends on the crys-
across 0 ° or 45
tallinity of the substrates and in most cases are highly
reduction across such fi}ms therefore shouid provide in-
aligned 1. There is a good correlation between the de-
formation on the reduction of Jc across naturally occur-
gree of texturing and the maximum Jc. The study of
ring 45 ° GB's.
o
QB. A study of the critical current
critical currents across such polycrystalline films in-
"['he fiim~ v,,c:~: l~!~e:- depot; ted using the ArF laser
evitably involves the question of critical current across
at a substrate temperature of 680eC. At this tempera-
misoriented grain boundaries (GB). It is the purpose of
ture, all the grains are c-axis perpendicular to the sub-
the present investigation to examine one of the most
strate 4. The texture of the YBCO film depends on the
important case of GB, the 45 ° misorientation.
deposition rate.
Dimos et al provided the first set of quantiative data
In one film, the texture was 33%
YBCO[100]//YSZ[1001
and 67% Y B C O [ 1 0 0 ] / /
on the decrease in Jc across misoriented GB artificially
YSZ[ll0]. in another sample, the texture was totally
produced on bicrystal substrates 2. They noted a drop of
YBCO[100]//YSZ[ll0]. The measured Jc were 0.65x106
almost 1/50 in Jc across 45 ° mlsoriented GB. Other
A/cm 2 and 2x106 A/cm 2 respectively at 77 K. Hence the
studies have shown however, that high angle GB's do
reduction of Jc due to the 45 ° GB is about 0.33 for
not necessarily destroy Jc 3. At issue is the question whether artifically produced GB is representative of the
these samples. A iimit[ng path model was used to ca~cu}atc ',:h;:: i~
natural GB found in high quality epitaxially YBCO
across the YSZ film5. The major modification we have
films. It will be shown that our resuffs are consistent
made was that the film was assumed to be made up of a
with a smaller reduction in Jc than the bicrystal data.
hexagonal lattice. Each grain was assigned randomly an
YSZ is uniquely suited for resolving this issue of reduction in Jc across 45 ° misoriented GB. Bacause of
a-axis alignment, with a probability of 0.33 for 0° ;rod 0.67 for 4_5" orieniation. ]
0921-4534/91/$03.50 © ]991 - Elsevier Science Publishers B.V. ?d] righls reserved
aen
the .Ic was calcut;ued
2292
H.S. Kwok, D.H. Kim ] Critical current density across 45 ° misotiented grain boundaries
using a linear optimization method as discussed by Rhyner et al 5. When the current flow across adjacent
1.0
grains with the same alignment, the critical current was Jco, which was the normalization factor in all eaiculations. When the current flows across grains with 45 °
0.8
misalignment, the critical current was assumed to be r x
El
Jco where r is the reduction factor and is the only adjustable parameter in this calculation.
0.6
Fig. 1 summarizes the results of the simplex calculation. The reduction in Je for the entire film JdJco, is
Irl [] [] []
0.4 []
plotted against the factor r. When r is 1, the should be
in
no reduction at all which is indeed the case. When r is 0, then effectively 33% of the grains can be considered
0.2
nonsuperconducting. The fact that there is still a finite Jc from the calculation implies that there always exists a pathway for the critical current to meander around these bad grains. The spread among the data points for the same value of r in Fig. 1 is due to the random assigranent of orientation ('.Ath the same probability distributions) for each simulation ran.
0.0 0.0
0'.2
0'.4
" 0'.6
o'.8
1.o
i"
FIGURE 1 Je reduction across the film as a function of Je reduction across 45 ° GB for YBCO/YSZ.
We can compare the computer simulation to the real experimental results to extract the value of r. From the experiment, the reduction of Jc in YSZ samples is 0.33. From Fig. 1 this corresponds to a value of 0.15 for r. As a comparison, the bicrystal data of Dimos et al
REFERENCES 1.J.P. Zheng, S.Y. Dong and H.S. Kwok, Appl. Phys. Lett. 58, 540 (1991).
gave a value of 0.017 for 45° GB. Therefore, it is confirmed that across naturally
2.D. Dimos, P. Chaudhari and J. Mannhart, Phys. Rev. B41, 4038 (1990).
produced 45 ° rnisoriented GB, the reduction in Jc is significantly smaller than those obtained in artificial GB. The bicrystal results overestimated this Jc reduction probably because of imperfections in the GB of the bicrystal substrate itself. Since 45 ° misorientation is
3.S.W. Chan, D.M. Hwang, R. Ramesh, S. M. Sampere, L Nazar, R Gerhardt and P. Pruna, p. 172 in High Tc Superconducting Thin Films: Processing, Characterization and Applications, ed. by R. Stockbaur, AIP Conference Proc. 200, New York 1989.
the largest possible misorientation in c-axis perpendicular YBCO films, the present result explains why such films have large Jc's.
4.Q. Li, O. Meyer, X.X. Xi, J. Geerk and G. Linker, Appl. Phys. [~tt. 55, 1792 (t989).
This research was supported by the National
5.J. Rhyner and G. Blotter, Phys. Rev. B40, 829 (1989).
Science Foundation and New York State Institute on Superconductivity.
Physica C 185-189 (1991) 2291-2292 North-Holland
CRITICAL CURRENT DENSITY ACROSS 45 ° MISORIENrI'ED GRAIN BOUNDARIES IN YBCO THIN FILMS
H.S. KWOK and D.H. KIM Institute on Superconductivity, State University of New York at Buffalo, Amherst, NY 14260, USA The question of critical current reduction across 45 ° misoriented grain boundaries was addressed in this study. The reduction in Jc for a polycrystalline c-axis perpendicular film with complete and partial in plane a-axis texturing was calculated using a limiting-pathe model. The numerical results were then compared to experimental results obtained with YBCO on YSZ substrates, which had a particular texturing that is most suited for this study. The results indicated that the Jc reduction was not so large as indicated in the bicrystal studies of Dimos et al.
Most electronic applications of high Tc materials
the lattice matching conditions, there are two sets of
are based on the availability of good quality thin films.
grains for YBCO films deposited on YSZ(0OD. The a-
It is well-known that even the best YBCO films are
axis of the YBCO grains can be aligned either along
polycrystalline with grain sizes of 0.05-0.5/zm. For a c-
YSZ[100] or the YSZ[116] directions. Therefore the
axis perpendicular film, all the grains have c-perp align-
current along such YBCO film,~ has to flow either
ment. The in plane a-axis texture depends on the crys-
across 0 ° or 45
tallinity of the substrates and in most cases are highly
reduction across such fi}ms therefore shouid provide in-
aligned 1. There is a good correlation between the de-
formation on the reduction of Jc across naturally occur-
gree of texturing and the maximum Jc. The study of
ring 45 ° GB's.
o
QB. A study of the critical current
critical currents across such polycrystalline films in-
"['he fiim~ v,,c:~: l~!~e:- depot; ted using the ArF laser
evitably involves the question of critical current across
at a substrate temperature of 680eC. At this tempera-
misoriented grain boundaries (GB). It is the purpose of
ture, all the grains are c-axis perpendicular to the sub-
the present investigation to examine one of the most
strate 4. The texture of the YBCO film depends on the
important case of GB, the 45 ° misorientation.
deposition rate.
Dimos et al provided the first set of quantiative data
In one film, the texture was 33%
YBCO[100]//YSZ[1001
and 67% Y B C O [ 1 0 0 ] / /
on the decrease in Jc across misoriented GB artificially
YSZ[ll0]. in another sample, the texture was totally
produced on bicrystal substrates 2. They noted a drop of
YBCO[100]//YSZ[ll0]. The measured Jc were 0.65x106
almost 1/50 in Jc across 45 ° mlsoriented GB. Other
A/cm 2 and 2x106 A/cm 2 respectively at 77 K. Hence the
studies have shown however, that high angle GB's do
reduction of Jc due to the 45 ° GB is about 0.33 for
not necessarily destroy Jc 3. At issue is the question whether artifically produced GB is representative of the
these samples. A iimit[ng path model was used to ca~cu}atc ',:h;:: i~
natural GB found in high quality epitaxially YBCO
across the YSZ film5. The major modification we have
films. It will be shown that our resuffs are consistent
made was that the film was assumed to be made up of a
with a smaller reduction in Jc than the bicrystal data.
hexagonal lattice. Each grain was assigned randomly an
YSZ is uniquely suited for resolving this issue of reduction in Jc across 45 ° misoriented GB. Bacause of
a-axis alignment, with a probability of 0.33 for 0° ;rod 0.67 for 4_5" orieniation. ]
0921-4534/91/$03.50 © ]991 - Elsevier Science Publishers B.V. ?d] righls reserved
aen
the .Ic was calcut;ued
2292
H.S. Kwok, D.H. Kim ] Critical current density across 45 ° misotiented grain boundaries
using a linear optimization method as discussed by Rhyner et al 5. When the current flow across adjacent
1.0
grains with the same alignment, the critical current was Jco, which was the normalization factor in all eaiculations. When the current flows across grains with 45 °
0.8
misalignment, the critical current was assumed to be r x
El
Jco where r is the reduction factor and is the only adjustable parameter in this calculation.
0.6
Fig. 1 summarizes the results of the simplex calculation. The reduction in Je for the entire film JdJco, is
Irl [] [] []
0.4 []
plotted against the factor r. When r is 1, the should be
in
no reduction at all which is indeed the case. When r is 0, then effectively 33% of the grains can be considered
0.2
nonsuperconducting. The fact that there is still a finite Jc from the calculation implies that there always exists a pathway for the critical current to meander around these bad grains. The spread among the data points for the same value of r in Fig. 1 is due to the random assigranent of orientation ('.Ath the same probability distributions) for each simulation ran.
0.0 0.0
0'.2
0'.4
" 0'.6
o'.8
1.o
i"
FIGURE 1 Je reduction across the film as a function of Je reduction across 45 ° GB for YBCO/YSZ.
We can compare the computer simulation to the real experimental results to extract the value of r. From the experiment, the reduction of Jc in YSZ samples is 0.33. From Fig. 1 this corresponds to a value of 0.15 for r. As a comparison, the bicrystal data of Dimos et al
REFERENCES 1.J.P. Zheng, S.Y. Dong and H.S. Kwok, Appl. Phys. Lett. 58, 540 (1991).
gave a value of 0.017 for 45° GB. Therefore, it is confirmed that across naturally
2.D. Dimos, P. Chaudhari and J. Mannhart, Phys. Rev. B41, 4038 (1990).
produced 45 ° rnisoriented GB, the reduction in Jc is significantly smaller than those obtained in artificial GB. The bicrystal results overestimated this Jc reduction probably because of imperfections in the GB of the bicrystal substrate itself. Since 45 ° misorientation is
3.S.W. Chan, D.M. Hwang, R. Ramesh, S. M. Sampere, L Nazar, R Gerhardt and P. Pruna, p. 172 in High Tc Superconducting Thin Films: Processing, Characterization and Applications, ed. by R. Stockbaur, AIP Conference Proc. 200, New York 1989.
the largest possible misorientation in c-axis perpendicular YBCO films, the present result explains why such films have large Jc's.
4.Q. Li, O. Meyer, X.X. Xi, J. Geerk and G. Linker, Appl. Phys. [~tt. 55, 1792 (t989).
This research was supported by the National
5.J. Rhyner and G. Blotter, Phys. Rev. B40, 829 (1989).
Science Foundation and New York State Institute on Superconductivity.