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Critical current density improved by laser-irradiation on growing surface in YBa2Cu3Ox superconducting films
EI~SEVIER
PhysicaC231 (1994) 118-122
Critical current density improved by laser-irradiation on growing surface in YBa2Cu3Ox superconducting films T. Minamikawa i, ,, T. Suzuki ", A. Morimoto a.., T. Shimizu a, K. Segawa b • Department of Electrical and Computer Engineering, Faculty of Technology, ganazawa University, Kodatsuno, Kanazawa 920, Japan b Industrial Research Institute oflshikawa, Tomizu, Kanazawa 920-02, Japan Received 7 June 1994
Abstract S u p e r c o n d u c t i n g Y B a 2 C u 3 0 x films with c-axis orientation on (100) MgO substrates were prepared by pulsed laser ablation with pulsed laser irradiation onto the growing film surface. The irradiation gives rise to a-axis-oriented crystallites in c-axisoriented matrix, short c-axis length and smooth surface morphology without degradation in crystaUinity of c-axis-oriented crystal. Critical current density Jc of the film is improved by the laser irradiation, resulting in 2.6 × 106 A/cm 2 at 77 K and zero field, which is twice that of the non-irradiated one. In addition, an enhanced pinning effect was also observed at a high field.
1. Introduction Pulsed laser ablation (PLA) is one of the most popular preparation techniques for YBa2Cu3Ox (YBCO) superconducting films. High-To and large critical-current density (J~) superconducting films with c-axis orientation are expected to be used for power application [ 1 ] and microwave devices [2]. So, it is attractive for the microwave devices to prepare the high quality superconducting films on an inexpensive and low dielectric-constant substrate, such as single crystal MgO. In order to obtain a high quality film, PLA with pulsed laser irradiation on the growing film surface has shown promise [ 3-5 ]. The irradiation improves the film surface morphology, enhances the crystallization with a small fluence, and enhances a-axis-oriented crystallization with a large fluence as a result of the thermal and/or electronic excitation of the surface [6]. It * Corresponding author. i On leave from: the Industrial Research Institute of Ishikawa, Tomizu, Kanazawa 920-02, Japan. 0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved
SSD!0921-4534(94)00471-4
should be noted that the irradiation introduces a-axisoriented crystallites even into c-axis-oriented films and then a small amount of a-axis-oriented crystallites is expected to bring about pinning centers for flux [7]. Furthermore the surface roughness reduced by the laser irradiation is expected to lead to an improvement of the weak links between grain boundaries. In this letter we will report an improvement of Jc of c-axis-oriented films by pulsed laser irradiation on the growing surface of superconducting films.
2. Experimental Our apparatus was assembled with an ArF excimer laser (Shibuya SQL2240, 193 nm wavelength, 10 ns pulse duration) as the beam source. The beam was shot at 1 Hz and split into two beams for the ablation of the YBCO target and for irradiation on the growing film surface. The ablation beam was focused on the target with a fluence of 1.5 J / ( c m 2 shot). The irradiation
T. Minamikawa et al. / Physica C 231 (1994) 118--122
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0 10 20 Irradiation Fluence (mJ/(cm ~.shot)) Fig. 1. Irradiation-fluence dependence of the ratio of the diffraction intensity from the a-plane to the sum of intensities from the a- and c-planes.
fluence on film surface was varied over 0--25 mJ / ( c m 2 shot) by moving the position of the focusing lens. Distance between the ablated target and the substrate center was 36 mm. A detailed arrangement of our system was shown in our previous paper [3]. The film deposition was carried out on ( 100)MgO substrates held at temperature of 720°(= in an 02 ambient of 40 Pa for 1 h. The present preparation condition was found to bring
119
about a near "cube-on-cube" film growth on a ( 100)MgO substrate by the X-ray diffraction (XRD) d~ scan technique. In other words, the c-axis and the aaxis (b-axis) of the film were aligned with the c-axis and the a-axis (b-axis) of MgO, respectively, in spite of a large lattice mismatch of around 7% between YBCO and MgO, resulting in almost no large-angle grain boundaries in the film plane. Above this substrate temperature, large-angle (45 ° ) grain boundaries are created. After deposition, the films were cooled down to room temperature at 5°C/rain in O2 ambient of 10 kPa. The MgO substrates were annealed for 5 h at 1200°C in 02 ambient of 1 atm before deposition. This treatment gives us reliable data with a good reproducibility [5]. The thickness of the films was about 200 nm, measured by scanning electron microscopy (SEM). Jc was measured using the standard four-probe method in liquid nitrogen ( T = 7 7 K), after the film was patterned into a 15 ~,m wide and 160 I~m long microbridge by laser etching and gold contacts were evaporated on it. A voltage criterion of 1 ~V/cm was used for determining Jc. The characterization of the film structure was carded out by XRD with the 0-20 scan and the to-rocking scan techniques performed with a Rigaku RAD-rA X-ray diffractometer.
3. Results
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Irradiation Fluence (mJ/(cm2.shot)) Fig. 2. (a) Irradiation-flucnce dependence of the full width at half maximum (FWHM) of the w-rocking scan of the (005) peak and (b) irradiation-fluence dependence of the c-axis length of the film derived from the XRD spectra~
The irradiation-fluence dependence of the ratio of the diffraction intensity from the a-plane to the sum of intensities from the a- and c-planes is shown in Fig. 1. The intensity ratio is defined as 1(200)/ (1(200) +1(006) ), where 1(200) and 1(006) are the intensities of the XRD spectra for (200) and (006), respectively. The other phases were not observed on XRD spectra even for a larger fluence than the present one [6]. The intensity ratio increases slightly as the irradiation fluence increases up to a fluence of 22.5 mJ / (cm 2 shot) and then rapidly increases. This result reveals that the laser irradiation with a small fluence introduces a small fraction of a-axis-oriented crystallites in a c-axis oriented matrix. We estimated that the volume fraction of a-axis-oriented crystallites for nonirradiation and irradiation fluences of 22.5 m J / ( c m 2 shot) are around 0.4% and 1%, respectively, based on the result of the diffraction pattern of YBCO powder.
120
T. Minamikawa et aL / Physica C 231 (1994) 118-122
I
.
.
.
.
i
•
|
10'
"3
at 1/~77K V/cm criterion I 1o
. . . .
lb
. . . .
2'0
Irradiation Fluence (mJ/(cm2.shot)) Fig. 3. Dependence of Jc of the films on the irradiation fluence at 77 K and zero field.
Fig. 2(a) shows the irradiation-fluence dependence of the full width at half maximum (FWHM) of the corocking scan of the (005) peak. The FWHM is kept at a rather small value (around 0.3°), independent of the irradiation fluence. In addition, the FWHM of the (005) diffraction in the normal 0-20 scan was found to be almost constant around 0.1 ° independent of the laser fluence. This suggests that laser irradiation does not degrade the c-axis orientation normal to the film plane despite inducing the a-axis orientation in the caxis-oriented matrix. The peak intensity of the (005) diffraction is found to be decreased slightly by the laser irradiation, suggesting that the volume of the c-axisoriented matrix is slightly decreased by the irradiation. Fig. 2(b) shows the irradiation-fluence dependence of the c-axis length derived from the XRD spectra. It is found that the c-axis length of the film decreases toward a fully-oxidized one with an increase in the irradiation fluence. This suggests that the laser irradiation increases the oxygen content in the film. No obvious change was, however, observed in T¢. This result can be explained by the saturation behavior in Tc vs. the caxis length around 1.169 nm [8]. Fig. 3 shows the irradiation-fluence dependence of Jc of the films at zero field. Jc increases up to 2.6 X 106 A/cm 2 with an increase in the laser fluence. Above the threshold value, J¢ dramatically decreases down to 1.3 × 10~ A/cm 2. Thus J¢ has a maximum around a fluence of 20 m J / ( c m 2 shot), the maximum value
being twice as much as the Jc value of the non-irradiated film. Jc is largely decreased by the irradiation with the fluence of 25 m J / ( c m 2 shot) in accordance with the dramatic enhancement in the a-axis orientation. This suggests that the transport Jc value is degraded by an extra incorporation of a-axis-oriented crystallites or the formation of micro-grain boundaries by laser irradiation, though no sign of the degradation of the c-axis oriented matrix is seen in the XRD data. The magnetic field dependences of arc for film nonirradiated and film irradiated at a fluence of 20 mJ/ (cm 2 shot) are shown in Fig. 4. The magnetic field is applied perpendicularly to a film plane: parallel to the c-axis. The Jc value of the irradiated film is larger than Jc of the non-irradiated one at zero field and high magnetic field. The latter result suggests that the irradiation enhances the formation of pinning centers for the flux in the film. Fig. 5 shows SEM photographs for the film nonirradiated (a) and irradiated with a fluence of 18 mJ/ (cm 2 shot) (b). The lengths of the white bars on the photographs correspond to 500 nm. The white particles are the droplets from the target which are often observed in films prepared by PLA. The crystal grains of about 300 nm sizes are recognized in the photograph of the non-irradiated film, but no obvious grains can be recognized in the photograph of the irradiated film. This suggests that the irradiation improves the grain boundary structure. The formation of c-axis-oriented crystallites deduced from the XRD measurement is not .i
.
.
.
.
.
.
.
.
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.
.
.
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.
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.
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o o 1
. . . . . . .
Magnetic Field (kOe) Fig. 4. Magnetic field dependonce of Jc for non-irradiated film (open circle) and irradiated film (closed circle) at 77 K.
T. Minamikawa et al. / Physica C 231 (1994) 118-122
10K
U
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OK:=:
1 s • ,.
.~/~
'-~i .
50$9n
~-, E, ~S~-
-,(. : .~,:
Fig. 5. SEM photographs for (a) the non-irradiated film and (b) the film irradiated with a fluence of 18 mJ/(cm 2 shot).
confirmed by SEM. This result can be ascribed to a small crystallite-size and/or the above-mentioned morphology improvement by the laser irradiation.
4. Discussions We offer two explanations for the improvement in J¢: an improvement of the weak links and an introduction of pinning centers. First, at the initial stage of the film growth, YBCO film grows with an island structure on the MgO substrate. The islands coalesce to form a continuous film. Fig. 5(a) shows the island structure of the film. Irradiation with a small fluence enhances the migration, promoting the two-dimensional growth as shown in Fig. 5(b). As a result, the grain boundary is filled with superconducting grains, and then both J¢ and the surface morphology are improved. The improvement of the weak links is consistent with the
121
increased oxygen content because degraded weak links often have oxygen deficiencies. Therefore the improvement Of Jc at zero field is attributed to the improvement of weak links. Second, let us consider the possibility that the irradiation introduces a-axis crystallites in a caxis-oriented film, leading to effective pinning centers for the flux. The improvement of J¢ of the irradiated film at the high magnetic field in Fig. 4 is attributed to the introduction of the pinning centers for the flux. The magnetic characterization by the SQUID magnetometer reveals that the magnetization hysteresis for the irradiated film is larger than the non-irradiated one at a high field. Employing Bean's critical state model [9], we estimated intragrain Jc'S of 4 x 108 and 8 × l0 s A/ cm 2 for films without irradiation and with irradiation, respectively, at a field of 4 kOe parallel to the film plane and at 77 K. The configuration of applied field parallel to the film plane was adopted for estimating the intragrain Jc. In this configuration, we can use film thickness as a size of current loop. Detailed explanation was described in the previous paper [ 10]. This result also supports the above result of transport measurement. In any case, the experiment revealed that with the extra irradiation J¢ decreases before the crystal degradation takes place. We suppose that a large amount of a-axis crystallites can be barriers to the superconductive current. This suggests that there exists an optimal amount of a-axis-oriented crystallites.
5. Conclusion The superconducting properties of the c-axis-oriented YBCO films on (100)MgO, which were prepared by PLA, were improved by pulsed laser irradiation on the growing film surface. The largest Jc of the irradiated films was 2.6 × 106 A/cm 2, i.e., twice as much as that of the non-irradiated film. In addition, an enhanced pinning effect was also observed at a high field.
Acknowledgements We would like to thank M. Kumeda of Kanazawa University for his helpful discussion. We would also like to thank H. Yoshida and Y. Yonezawa of the Industrial Research Institute of Ishikawa for their help
122
T. Minamikawa et al. /Physica C 231 (1994) 118-122
in o u r e x p e r i m e n t . W e are g r a t e f u l to the S h i b u y a K o g y o Co., Ltd. for s u p p l y i n g t h e e x c i m e r laser system.
References
[ 1 ] X.D. Wu, R.E. Muenchausen, S. Foltyn, R.C. Estler, R.C. Dye, A.R. Garcia, N.S. Nogar, P. England, R. Ramesh, D.M. Hwang, T.S. Ravi, C.C. Chang, T. Venkatesan, X.X. Xi, Q. Li and A. Inam, Appl. Phys. Lett. 57 (1990) 523. [2] K.H. Young, G.V. Negrete, R.B. Hammond, A. Inam, R. Ramesh, D.L. Hart and Y. Yonezawa, Appl. Phys. Lett. 58 (1991) 1789.
[3] T. Minamikawa, Y. Yonezawa, S. Otsubo, T. Maeda, A. Morimoto and T. Shimizu, Proc. 2nd Int. Symp. on Superconductivity, Tsukuba, 1989, in: Advances in Superconductivity lI (Springer. Tokyo, 1990) p. 857. [4] T. Minamikawa, Y. Yonezawa, K. Segawa, S. Otsubo, S. Mizukami, A. Morimoto and T. Shimizu, Physiea C 185-189 (1991) 1969. [5] A. Morimoto, S. Mizakami, T. Shimiza, T. Minamikawa, Y. Yonezawa, K. Segawa and S. Otsubo, Mater. Res. Soc. Symp. Vol. 275 (Mater. Res. Soc., Pittsburgh, 1992) p. 371. [6] T. Minamikawa, T. Suzuki, K. Segawa, A. Morimoto and T. Shimizu, Jpn. J. Appl. Phys. 33 (1994) L98. [ 7 ] H. Fuke, H. Yoshino, M. Yamazaki, T.D. Thanh, S. Nakamura, K. Ando and Y. Kobayashi, Appl. Phys. Lett. 60 (1992) 2686. [ 8 ] M. Ohkubo, T. Kachi, T. Hioid and J. Kawamoto, Appl. Phys. Lett. 55 (1989) 899. [9] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [ 10] A. Morimoto, T. Matsuki, T. Minamikawa and T. Shimizu, Physica C 220 (1994) 332.
PhysicaC231 (1994) 118-122
Critical current density improved by laser-irradiation on growing surface in YBa2Cu3Ox superconducting films T. Minamikawa i, ,, T. Suzuki ", A. Morimoto a.., T. Shimizu a, K. Segawa b • Department of Electrical and Computer Engineering, Faculty of Technology, ganazawa University, Kodatsuno, Kanazawa 920, Japan b Industrial Research Institute oflshikawa, Tomizu, Kanazawa 920-02, Japan Received 7 June 1994
Abstract S u p e r c o n d u c t i n g Y B a 2 C u 3 0 x films with c-axis orientation on (100) MgO substrates were prepared by pulsed laser ablation with pulsed laser irradiation onto the growing film surface. The irradiation gives rise to a-axis-oriented crystallites in c-axisoriented matrix, short c-axis length and smooth surface morphology without degradation in crystaUinity of c-axis-oriented crystal. Critical current density Jc of the film is improved by the laser irradiation, resulting in 2.6 × 106 A/cm 2 at 77 K and zero field, which is twice that of the non-irradiated one. In addition, an enhanced pinning effect was also observed at a high field.
1. Introduction Pulsed laser ablation (PLA) is one of the most popular preparation techniques for YBa2Cu3Ox (YBCO) superconducting films. High-To and large critical-current density (J~) superconducting films with c-axis orientation are expected to be used for power application [ 1 ] and microwave devices [2]. So, it is attractive for the microwave devices to prepare the high quality superconducting films on an inexpensive and low dielectric-constant substrate, such as single crystal MgO. In order to obtain a high quality film, PLA with pulsed laser irradiation on the growing film surface has shown promise [ 3-5 ]. The irradiation improves the film surface morphology, enhances the crystallization with a small fluence, and enhances a-axis-oriented crystallization with a large fluence as a result of the thermal and/or electronic excitation of the surface [6]. It * Corresponding author. i On leave from: the Industrial Research Institute of Ishikawa, Tomizu, Kanazawa 920-02, Japan. 0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved
SSD!0921-4534(94)00471-4
should be noted that the irradiation introduces a-axisoriented crystallites even into c-axis-oriented films and then a small amount of a-axis-oriented crystallites is expected to bring about pinning centers for flux [7]. Furthermore the surface roughness reduced by the laser irradiation is expected to lead to an improvement of the weak links between grain boundaries. In this letter we will report an improvement of Jc of c-axis-oriented films by pulsed laser irradiation on the growing surface of superconducting films.
2. Experimental Our apparatus was assembled with an ArF excimer laser (Shibuya SQL2240, 193 nm wavelength, 10 ns pulse duration) as the beam source. The beam was shot at 1 Hz and split into two beams for the ablation of the YBCO target and for irradiation on the growing film surface. The ablation beam was focused on the target with a fluence of 1.5 J / ( c m 2 shot). The irradiation
T. Minamikawa et al. / Physica C 231 (1994) 118--122
0.06
l
l
l
l
l
I
I
I
I
I
l
l
r
l
l
l
A
m 0.04 o tr
0.021 ¢..
"E
I
I
I
l
l
l
l
0 10 20 Irradiation Fluence (mJ/(cm ~.shot)) Fig. 1. Irradiation-fluence dependence of the ratio of the diffraction intensity from the a-plane to the sum of intensities from the a- and c-planes.
fluence on film surface was varied over 0--25 mJ / ( c m 2 shot) by moving the position of the focusing lens. Distance between the ablated target and the substrate center was 36 mm. A detailed arrangement of our system was shown in our previous paper [3]. The film deposition was carried out on ( 100)MgO substrates held at temperature of 720°(= in an 02 ambient of 40 Pa for 1 h. The present preparation condition was found to bring
119
about a near "cube-on-cube" film growth on a ( 100)MgO substrate by the X-ray diffraction (XRD) d~ scan technique. In other words, the c-axis and the aaxis (b-axis) of the film were aligned with the c-axis and the a-axis (b-axis) of MgO, respectively, in spite of a large lattice mismatch of around 7% between YBCO and MgO, resulting in almost no large-angle grain boundaries in the film plane. Above this substrate temperature, large-angle (45 ° ) grain boundaries are created. After deposition, the films were cooled down to room temperature at 5°C/rain in O2 ambient of 10 kPa. The MgO substrates were annealed for 5 h at 1200°C in 02 ambient of 1 atm before deposition. This treatment gives us reliable data with a good reproducibility [5]. The thickness of the films was about 200 nm, measured by scanning electron microscopy (SEM). Jc was measured using the standard four-probe method in liquid nitrogen ( T = 7 7 K), after the film was patterned into a 15 ~,m wide and 160 I~m long microbridge by laser etching and gold contacts were evaporated on it. A voltage criterion of 1 ~V/cm was used for determining Jc. The characterization of the film structure was carded out by XRD with the 0-20 scan and the to-rocking scan techniques performed with a Rigaku RAD-rA X-ray diffractometer.
3. Results
(a)
A v
o.8
',60m 0.6 •
O0
•
•
nO-° 0.2 t
i(b) 11.6~
•
£
_~11.680
10
20
Irradiation Fluence (mJ/(cm2.shot)) Fig. 2. (a) Irradiation-flucnce dependence of the full width at half maximum (FWHM) of the w-rocking scan of the (005) peak and (b) irradiation-fluence dependence of the c-axis length of the film derived from the XRD spectra~
The irradiation-fluence dependence of the ratio of the diffraction intensity from the a-plane to the sum of intensities from the a- and c-planes is shown in Fig. 1. The intensity ratio is defined as 1(200)/ (1(200) +1(006) ), where 1(200) and 1(006) are the intensities of the XRD spectra for (200) and (006), respectively. The other phases were not observed on XRD spectra even for a larger fluence than the present one [6]. The intensity ratio increases slightly as the irradiation fluence increases up to a fluence of 22.5 mJ / (cm 2 shot) and then rapidly increases. This result reveals that the laser irradiation with a small fluence introduces a small fraction of a-axis-oriented crystallites in a c-axis oriented matrix. We estimated that the volume fraction of a-axis-oriented crystallites for nonirradiation and irradiation fluences of 22.5 m J / ( c m 2 shot) are around 0.4% and 1%, respectively, based on the result of the diffraction pattern of YBCO powder.
120
T. Minamikawa et aL / Physica C 231 (1994) 118-122
I
.
.
.
.
i
•
|
10'
"3
at 1/~77K V/cm criterion I 1o
. . . .
lb
. . . .
2'0
Irradiation Fluence (mJ/(cm2.shot)) Fig. 3. Dependence of Jc of the films on the irradiation fluence at 77 K and zero field.
Fig. 2(a) shows the irradiation-fluence dependence of the full width at half maximum (FWHM) of the corocking scan of the (005) peak. The FWHM is kept at a rather small value (around 0.3°), independent of the irradiation fluence. In addition, the FWHM of the (005) diffraction in the normal 0-20 scan was found to be almost constant around 0.1 ° independent of the laser fluence. This suggests that laser irradiation does not degrade the c-axis orientation normal to the film plane despite inducing the a-axis orientation in the caxis-oriented matrix. The peak intensity of the (005) diffraction is found to be decreased slightly by the laser irradiation, suggesting that the volume of the c-axisoriented matrix is slightly decreased by the irradiation. Fig. 2(b) shows the irradiation-fluence dependence of the c-axis length derived from the XRD spectra. It is found that the c-axis length of the film decreases toward a fully-oxidized one with an increase in the irradiation fluence. This suggests that the laser irradiation increases the oxygen content in the film. No obvious change was, however, observed in T¢. This result can be explained by the saturation behavior in Tc vs. the caxis length around 1.169 nm [8]. Fig. 3 shows the irradiation-fluence dependence of Jc of the films at zero field. Jc increases up to 2.6 X 106 A/cm 2 with an increase in the laser fluence. Above the threshold value, J¢ dramatically decreases down to 1.3 × 10~ A/cm 2. Thus J¢ has a maximum around a fluence of 20 m J / ( c m 2 shot), the maximum value
being twice as much as the Jc value of the non-irradiated film. Jc is largely decreased by the irradiation with the fluence of 25 m J / ( c m 2 shot) in accordance with the dramatic enhancement in the a-axis orientation. This suggests that the transport Jc value is degraded by an extra incorporation of a-axis-oriented crystallites or the formation of micro-grain boundaries by laser irradiation, though no sign of the degradation of the c-axis oriented matrix is seen in the XRD data. The magnetic field dependences of arc for film nonirradiated and film irradiated at a fluence of 20 mJ/ (cm 2 shot) are shown in Fig. 4. The magnetic field is applied perpendicularly to a film plane: parallel to the c-axis. The Jc value of the irradiated film is larger than Jc of the non-irradiated one at zero field and high magnetic field. The latter result suggests that the irradiation enhances the formation of pinning centers for the flux in the film. Fig. 5 shows SEM photographs for the film nonirradiated (a) and irradiated with a fluence of 18 mJ/ (cm 2 shot) (b). The lengths of the white bars on the photographs correspond to 500 nm. The white particles are the droplets from the target which are often observed in films prepared by PLA. The crystal grains of about 300 nm sizes are recognized in the photograph of the non-irradiated film, but no obvious grains can be recognized in the photograph of the irradiated film. This suggests that the irradiation improves the grain boundary structure. The formation of c-axis-oriented crystallites deduced from the XRD measurement is not .i
.
.
.
.
.
.
.
.
I
• •
.
.
.
.
.
.
.
.
i
Olrr.(20mJ/(crn'.sho,)~ Non-I,.
106 O o~• o
•
o
• •o © • o
10~0 . 1
H//c-axis at 77K ........
o o 1
. . . . . . .
Magnetic Field (kOe) Fig. 4. Magnetic field dependonce of Jc for non-irradiated film (open circle) and irradiated film (closed circle) at 77 K.
T. Minamikawa et al. / Physica C 231 (1994) 118-122
10K
U
2(I
OK:=:
1 s • ,.
.~/~
'-~i .
50$9n
~-, E, ~S~-
-,(. : .~,:
Fig. 5. SEM photographs for (a) the non-irradiated film and (b) the film irradiated with a fluence of 18 mJ/(cm 2 shot).
confirmed by SEM. This result can be ascribed to a small crystallite-size and/or the above-mentioned morphology improvement by the laser irradiation.
4. Discussions We offer two explanations for the improvement in J¢: an improvement of the weak links and an introduction of pinning centers. First, at the initial stage of the film growth, YBCO film grows with an island structure on the MgO substrate. The islands coalesce to form a continuous film. Fig. 5(a) shows the island structure of the film. Irradiation with a small fluence enhances the migration, promoting the two-dimensional growth as shown in Fig. 5(b). As a result, the grain boundary is filled with superconducting grains, and then both J¢ and the surface morphology are improved. The improvement of the weak links is consistent with the
121
increased oxygen content because degraded weak links often have oxygen deficiencies. Therefore the improvement Of Jc at zero field is attributed to the improvement of weak links. Second, let us consider the possibility that the irradiation introduces a-axis crystallites in a caxis-oriented film, leading to effective pinning centers for the flux. The improvement of J¢ of the irradiated film at the high magnetic field in Fig. 4 is attributed to the introduction of the pinning centers for the flux. The magnetic characterization by the SQUID magnetometer reveals that the magnetization hysteresis for the irradiated film is larger than the non-irradiated one at a high field. Employing Bean's critical state model [9], we estimated intragrain Jc'S of 4 x 108 and 8 × l0 s A/ cm 2 for films without irradiation and with irradiation, respectively, at a field of 4 kOe parallel to the film plane and at 77 K. The configuration of applied field parallel to the film plane was adopted for estimating the intragrain Jc. In this configuration, we can use film thickness as a size of current loop. Detailed explanation was described in the previous paper [ 10]. This result also supports the above result of transport measurement. In any case, the experiment revealed that with the extra irradiation J¢ decreases before the crystal degradation takes place. We suppose that a large amount of a-axis crystallites can be barriers to the superconductive current. This suggests that there exists an optimal amount of a-axis-oriented crystallites.
5. Conclusion The superconducting properties of the c-axis-oriented YBCO films on (100)MgO, which were prepared by PLA, were improved by pulsed laser irradiation on the growing film surface. The largest Jc of the irradiated films was 2.6 × 106 A/cm 2, i.e., twice as much as that of the non-irradiated film. In addition, an enhanced pinning effect was also observed at a high field.
Acknowledgements We would like to thank M. Kumeda of Kanazawa University for his helpful discussion. We would also like to thank H. Yoshida and Y. Yonezawa of the Industrial Research Institute of Ishikawa for their help
122
T. Minamikawa et al. /Physica C 231 (1994) 118-122
in o u r e x p e r i m e n t . W e are g r a t e f u l to the S h i b u y a K o g y o Co., Ltd. for s u p p l y i n g t h e e x c i m e r laser system.
References
[ 1 ] X.D. Wu, R.E. Muenchausen, S. Foltyn, R.C. Estler, R.C. Dye, A.R. Garcia, N.S. Nogar, P. England, R. Ramesh, D.M. Hwang, T.S. Ravi, C.C. Chang, T. Venkatesan, X.X. Xi, Q. Li and A. Inam, Appl. Phys. Lett. 57 (1990) 523. [2] K.H. Young, G.V. Negrete, R.B. Hammond, A. Inam, R. Ramesh, D.L. Hart and Y. Yonezawa, Appl. Phys. Lett. 58 (1991) 1789.
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