Griessen, R.

Ren, Z.F.

Rossel, C.

Surdeanua, R.

Wang, J.H.

Wijngaarden, R.J.

DSpace at VU

ELSE’v’IER Physica C 282-287 (1997) 2219-2220 
PHYSICA 8 
Crossover from Ractal to Non-Fractal Flux Penetration In Vicinal 
Tl:!Ba#uO6+6 Thin Films 
R. Surdeanua, R.J. Wijngaardena; R. Griessena, C. Rosselb, Z.F. RenC and J.H. Wang” 
“Institute COMPAS and Faculty of Physics and Astronomy, 
Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands 
bIBM Research Division, Zurich Research Laboratory, 8803 Riischlikon, Switzerland 
“Superconductive Materials Laboratory, Department of Chemistry and New York State Institute 
on Superconductivity, State University of NY at Buffalo, Buffalo, NY 14260-3000, U.S.A. 
The role of anisotropy on magnetic flux penetration and critical current density j, in c-axis epitaxial 
TlzBa2CuOs+s (T12201) films is investigated by magneto-optics. Thin films of T12201 are deposited by a sput- 
tering technique on vicinal (001) S r 1 T‘O s substrates with miscut angles of 0.5 and 2.5 degrees and patterned into 
various shapes. Circular samples are used to determine the current anisotropy axis, whereas square samples allow 
to evaluate the amplitude of this anisotropy. A crossover from fractal to uniform anisotropic flux penetration is 
observed with increasing miscut angle. 
1. INTRODUCTION 
Magneto-optical (MO) investigations of mag- 
netic flux penetration in high-T, superconducting 
thin films show a flux front with a very irregular 
fractal shape even in high quality films [l] This 
behavior is in contrast with the smooth and well- 
defined flux penetration observed in most single 
crystals [l-3]. In this paper we study the cross- 
over from fractal to non-fractal flux penetration 
in thin films as a function of anisotropy in the 
critical current density. Previous work [3] on 
anisotropic thin films was done on YBazCusO,_6. 
In this paper we study epitaxial T12BazCuOs+6 
(T12201) thin films. An advantage of such films is 
the simplicity of their structure: tetragonal sym- 
metry with only one CuO2 plane per unit cell and 
no CuO chain [4]. 
2. EXPERIMENTAL 
Epitaxial T12201 thin films are deposited on 
vicinal (001) SrTiOa substrates with different 
miscut angles by RF magnetron sputtering and 
subjected to a two-step post-deposition anneal- 
ing as described elsewhere [4]. The miscut angle 
of the substrates was determined with X-ray Laue 
diffraction. The films are patterned in shapes of a 
disk, 2 squares with the sides oriented parallel (P) 
0921-4534/97/$17.00 0 Elsr~~i~: Xence B.V. All rights reserved. 
PII SO921-4534(97)01177-5 
with the edges of the substrate, and one square 
tilted (T) with respect to the edges with 45’. The 
T, of the samples is about 83K as determined by 
resistivity. 
To determine magneto-optically the magnetic 
flux in the sample we used Bi-doped YIG films 
with in-plane anisotropy. A complete description 
of the MO experimental setup and method can 
be found in refs. [l] and 121. 
3. RESULTS AND DISCUSSION 
We present first the results on a 0.5’ miscut 
T12201 thin film, with a thickness of 500 nm. Fig- 
ure 1 shows an image at 57 mT, after a zero field 
cooling down to 4.2 K for one of the (P) square 
samples. Three features can be observed: (1) the 
flux penetration is very irregular and fractal like. 
Possibly small defects at the edges lead to prefer- 
ential flux penetration, but further penetration in 
the film is observed to take place in ‘tree’-like pat- 
terns for which the reason at present is unclear. 
The shape of these ‘trees’ remains the same when 
ramping down the field, the flux leaving the sam- 
ple faster in those regions; (2) similar behavior 
is observed in all the samples deposited on the 
substrate, regardless of the shape of the samples 
and their position on the substrate; (3) because 
2220 R. Surdeanu et al./Physica C 282-287 (1997) 2219-2220 
of the very high irregularity of the flux penetra- 
tion, no so-called current discontinuity lines are 
observed. Furthermore no preferential direction 
for the superconducting current is observed. For 
this reason an estimate of the current’s anisotropy 
cannot be made. 
As an example of the behavior for larger miscut 
angles we now consider the 2.5’ miscut angle film 
with a thickness of 200 nm. Figure 2 shows the 
MO image at 28 mT, after a zero-field cooling 
down to 4.2K. The edge of the square is parallel 
with the edge of the substrate. On the basis of the 
flux pattern shown schematically in fig. 3, we can 
determine the ratio between the current densities 
flowing in the directions parallel to the edges, as 
being jr/j2 = tga. This is, however, not the ‘real 
anisotropy’ Areal since the directions of minimal 
and maximal critical current are at an angle /3 
with respect to the sides of the square as is clear 
from the flux penetration in the disk in Fig. 2. 
From the ‘apparent anisotropy’ Aapp=ji/j2, the 
‘real anisotropy’ Area/ can be calculated by the 
formula: 
A tan P - A,, 
real = Aapp tan p - 1 (1) 
which gives, for the case of figure 2, the value 
A&l= 11. (Note that we are not in the case 
described in ref. [5], fig. 2h.) 
Another striking feature which appears in the 
2.5’ miscut sample is that the flux front is now 
completely regular (not fractal) as observed for 
single-crystals. This very important observation 
is the main result of this paper. 
The fact that the direction of the ‘real 
anisotropy’ is systematic for one substrate was 
demonstrated by a sample (with 4’ miscut) 
on which we patterned 4 disks. The axis of 
anisotropy was found to be the same for all disks. 
Fig.1. (a) MO image of 0.5’ miscut T12201 thin 
film, /.&Z,,t=56mT, T=4.2 K. 
The origin of the critical current anisotropy is 
probably related to the steps at the growth inter- 
face with the vicinal substrate since the direction 
of lower j, corresponds to the direction of the 
steepest descent of the substrate. 
4. CONCLUSIONS 
From an analysis of the behavior of the flux 
front in several T12201 thin films grown on vici- 
nal substrates with miscut angles of 0.5’and 2.5’ 
we find that the flux penetration changes dramat- 
ically from a fractal! very irregular one, to a regu- 
lar, uniform penetration for increasing miscut an- 
gle. The cross-over is correlated with an increase 
in the critical current anisotropy and takes place 
in the interval 0.5’-2.5’ for the miscut angle. 
REFERENCES 
1. 
2. 
3. 
4. 
5. 
M.R. Koblischka and R.J. Wijngaarden, Su- 
percond. Sci. and Technol. 8, 199 (1995) and 
references therein 
R.J. Wijngaarden et al., Phys. Rev. B 55, 
3268 (1997) 
T. Haage et al., Solid State Comm. 99, 8, 553 
(1996) 
C.A. Wang et al., Physica C 262,98(1996) 
Th. Schuster et al., Phys. Rev. B 50, 9499 
(1994) 
Fig. 2. MO image of 2.5’ miscut T12201 thin 
film, &Y,,t=28 mT, T=4.2 K. 
Fig. 3. Schematic picture for the current flow 
in a sample with anisotropy in the critical cur- 
rent. 


Crossover from Ractal to Non-Fractal Flux Penetration In Vicinal Tl:! Ba# uO6+ 6 Thin Films

Surdeanu, R.C.

Griessen, R.P.

VU Research Portal

Crossover from fractal to non-fractal flux penetration in vicinal Tl2Ba2CuO6+d thin films.

NARCIS 

VU Research PortalCrossover from Ractal to Non-Fractal Flux Penetration In Vicinal Tl:! Ba# uO6+ 6 ThinFilmsSurdeanua, R.; Wijngaarden, R.J.; Griessen, R.; Rossel, C.; Ren, Z.F.; Wang, J.H.published inPhysica C. Superconductivity and its Applications1997document versionPublisher's PDF, also known as Version of recordLink to publication in VU Research Portalcitation for published version (APA)Surdeanua, R., Wijngaarden, R. J., Griessen, R., Rossel, C., Ren, Z. F., & Wang, J. H. (1997). Crossover fromRactal to Non-Fractal Flux Penetration In Vicinal Tl:! Ba# uO6+ 6 Thin Films. Physica C. Superconductivity andits Applications, 282(287), 2219-2220.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.E-mail address:vuresearchportal.ub@vu.nlDownload date: 22. May. 2021ELSE’v’IER Physica C 282-287 (1997) 2219-2220 PHYSICA 8 Crossover from Ractal to Non-Fractal Flux Penetration In Vicinal Tl:!Ba#uO6+6 Thin Films R. Surdeanua, R.J. Wijngaardena; R. Griessena, C. Rosselb, Z.F. RenC and J.H. Wang” “Institute COMPAS and Faculty of Physics and Astronomy, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands bIBM Research Division, Zurich Research Laboratory, 8803 Riischlikon, Switzerland “Superconductive Materials Laboratory, Department of Chemistry and New York State Institute on Superconductivity, State University of NY at Buffalo, Buffalo, NY 14260-3000, U.S.A. The role of anisotropy on magnetic flux penetration and critical current density j, in c-axis epitaxial TlzBa2CuOs+s (T12201) films is investigated by magneto-optics. Thin films of T12201 are deposited by a sput- tering technique on vicinal (001) S r 1 T‘O s substrates with miscut angles of 0.5 and 2.5 degrees and patterned into various shapes. Circular samples are used to determine the current anisotropy axis, whereas square samples allow to evaluate the amplitude of this anisotropy. A crossover from fractal to uniform anisotropic flux penetration is observed with increasing miscut angle. 1. INTRODUCTION Magneto-optical (MO) investigations of mag- netic flux penetration in high-T, superconducting thin films show a flux front with a very irregular fractal shape even in high quality films [l] This behavior is in contrast with the smooth and well- defined flux penetration observed in most single crystals [l-3]. In this paper we study the cross- over from fractal to non-fractal flux penetration in thin films as a function of anisotropy in the critical current density. Previous work [3] on anisotropic thin films was done on YBazCusO,_6. In this paper we study epitaxial T12BazCuOs+6 (T12201) thin films. An advantage of such films is the simplicity of their structure: tetragonal sym- metry with only one CuO2 plane per unit cell and no CuO chain [4]. 2. EXPERIMENTAL Epitaxial T12201 thin films are deposited on vicinal (001) SrTiOa substrates with different miscut angles by RF magnetron sputtering and subjected to a two-step post-deposition anneal- ing as described elsewhere [4]. The miscut angle of the substrates was determined with X-ray Laue diffraction. The films are patterned in shapes of a disk, 2 squares with the sides oriented parallel (P) 0921-4534/97/$17.00 0 Elsr~~i~: Xence B.V. All rights reserved. PII SO921-4534(97)01177-5 with the edges of the substrate, and one square tilted (T) with respect to the edges with 45’. The T, of the samples is about 83K as determined by resistivity. To determine magneto-optically the magnetic flux in the sample we used Bi-doped YIG films with in-plane anisotropy. A complete description of the MO experimental setup and method can be found in refs. [l] and 121. 3. RESULTS AND DISCUSSION We present first the results on a 0.5’ miscut T12201 thin film, with a thickness of 500 nm. Fig- ure 1 shows an image at 57 mT, after a zero field cooling down to 4.2 K for one of the (P) square samples. Three features can be observed: (1) the flux penetration is very irregular and fractal like. Possibly small defects at the edges lead to prefer- ential flux penetration, but further penetration in the film is observed to take place in ‘tree’-like pat- terns for which the reason at present is unclear. The shape of these ‘trees’ remains the same when ramping down the field, the flux leaving the sam- ple faster in those regions; (2) similar behavior is observed in all the samples deposited on the substrate, regardless of the shape of the samples and their position on the substrate; (3) because 2220 R. Surdeanu et al./Physica C 282-287 (1997) 2219-2220 of the very high irregularity of the flux penetra- tion, no so-called current discontinuity lines are observed. Furthermore no preferential direction for the superconducting current is observed. For this reason an estimate of the current’s anisotropy cannot be made. As an example of the behavior for larger miscut angles we now consider the 2.5’ miscut angle film with a thickness of 200 nm. Figure 2 shows the MO image at 28 mT, after a zero-field cooling down to 4.2K. The edge of the square is parallel with the edge of the substrate. On the basis of the flux pattern shown schematically in fig. 3, we can determine the ratio between the current densities flowing in the directions parallel to the edges, as being jr/j2 = tga. This is, however, not the ‘real anisotropy’ Areal since the directions of minimal and maximal critical current are at an angle /3 with respect to the sides of the square as is clear from the flux penetration in the disk in Fig. 2. From the ‘apparent anisotropy’ Aapp=ji/j2, the ‘real anisotropy’ Area/ can be calculated by the formula: A tan P - A,, real = Aapp tan p - 1 (1) which gives, for the case of figure 2, the value A&l= 11. (Note that we are not in the case described in ref. [5], fig. 2h.) Another striking feature which appears in the 2.5’ miscut sample is that the flux front is now completely regular (not fractal) as observed for single-crystals. This very important observation is the main result of this paper. The fact that the direction of the ‘real anisotropy’ is systematic for one substrate was demonstrated by a sample (with 4’ miscut) on which we patterned 4 disks. The axis of anisotropy was found to be the same for all disks. Fig.1. (a) MO image of 0.5’ miscut T12201 thin film, /.&Z,,t=56mT, T=4.2 K. The origin of the critical current anisotropy is probably related to the steps at the growth inter- face with the vicinal substrate since the direction of lower j, corresponds to the direction of the steepest descent of the substrate. 4. CONCLUSIONS From an analysis of the behavior of the flux front in several T12201 thin films grown on vici- nal substrates with miscut angles of 0.5’and 2.5’ we find that the flux penetration changes dramat- ically from a fractal! very irregular one, to a regu- lar, uniform penetration for increasing miscut an- gle. The cross-over is correlated with an increase in the critical current anisotropy and takes place in the interval 0.5’-2.5’ for the miscut angle. REFERENCES 1. 2. 3. 4. 5. M.R. Koblischka and R.J. Wijngaarden, Su- percond. Sci. and Technol. 8, 199 (1995) and references therein R.J. Wijngaarden et al., Phys. Rev. B 55, 3268 (1997) T. Haage et al., Solid State Comm. 99, 8, 553 (1996) C.A. Wang et al., Physica C 262,98(1996) Th. Schuster et al., Phys. Rev. B 50, 9499 (1994) Fig. 2. MO image of 2.5’ miscut T12201 thin film, &Y,,t=28 mT, T=4.2 K. Fig. 3. Schematic picture for the current flow in a sample with anisotropy in the critical cur- rent. 

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Crossover from Ractal to Non-Fractal Flux Penetration In Vicinal Tl:! Ba# uO6+ 6 Thin Films

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