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PrBa2Cu3Oy layered structures
Physica
C
1 8 5 - 1 8 9
(1991)
BU
2 0 4 t - 2 0 4 2
North-HoUand
MAGNETIZATION M E A S U R E M E N T S OF YB&2Cu3Oy [PtBR2Cu3Oy LAYERED STRUCTURES ttaruhiko OBARA, Shin KOSAKA, and Yoichi KIMURA, Electrotechnical Laboratory, 1-1-4, Umezono, Tsukuba-shi,
Ibaraki 305, JAPAN
We have successfully prepared YBa2Cu3Oy (YBCO)/PrBa2CusOy (PBCO) layered structures (multilayers) using molecular beam epitaxy (MBE). The structure of these multilayers was examined by x-ray diffraction spectra, and the coverage of YBCO by PBCO, i.e., the morphology of the layered structures, was estimated using in - s i t u x-ray photoelectron spectroscopy (XPS). The superconductivity of these multilayers was measured by a SQUID magnetometer, which is quite sensitive to homogeneity of films and enables us to estimate critical current properties of superconductors. From these experimental results, we have discussed the homogeneity and the superconductive properties of the YBCO/PBCO layered structures. Recently YBCO/PBCO layered structures have been extensively studied. One of the purposes of this research is to apply the high-To superconductors to electronic devices and another is to observe the intrinsic properties of very thin films of oxide superconductor. Several groups have reported the preparation of PBCO]YBCO layered structures using sputtering L2 and laser deposition 3. MBE is suitable for fabrication of good-quality layered structures, which have very thin layers and sharp interfaces. At the present stage, it is very important to evaluate quality of thin films. However, using a conventional transport measurement which depends on the current path, it is difficult to evaluate quality of films on the whole. Magnetization measurements are powerful method for evaluation of thin films because it is sensitive to inhomogeneity of samples. Moreover, critical current properties of films can be observed using the magnetization measurements. In the present work, we have examined the P B C O / Y B C O layered structures using x-ray diffraction spectra and in-situ XPS spectroscopy which enabled us to estimate the morphology of the layered structures. The superconductivity of the multilayers was measured using a SQUID magnetometer and we have discussed the homogeneity and the critical current properties of the YBCO/PBCO layered structures.
Films were prepared by coevaporation in an MBE system. The evaporation sources were four effusion cells (the maximum temperature is 1800°C ) and Y, Ba, Cu and Pr metal elements were individually evaporated. Ba and Cu were continuously ewporated and the layered structure was controlled by shutters of Y and Pr sources. The deposition rate was typically ~ 0.5 ~,/s and the substrate temperature wa~ about 650°C . For oxidation of the samples, we used an electron cyclotron resonance (ECR) ion source and oxygen
gas was introduced to the ECR ion source during deposition. The details of sample preparation have been described elsewhere 4 Figure 1 shows the x-ray diffraction spectra ;~ the region of (001) peaks from the layered structures of YBCO/PBCO on MgO (100) plane. The number of YBCO and PBCO layers in these multilayers was ten, and the thickness of each YBCO layer was constant. Only the thickness of each PBCO layer was changed. 1 0
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8 9 20 (degree)
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FIGURE 1 X-ray diffraction spectra of Y B C O / P B C O ~yered structures which have different modulation wavelength. The arrow~ itidicate "- . . . . 11:, ~.o t, lll[:~
~o,
Leill,
lt.~
p ~ ' ~ a o .
The films showed strong c-axis orientation. The satellite peaks on the sides of tile main (001) peaks indicates that the periodicity was achieved in this syslem and tile modulation wavelength D was calculated from tile separation of satellite peaks ~', and we got D ,-- 112.~ (sample A) and D --, 84~ (sample B). The modulation wavelength is sum of the individual layer thickness and almost agreed to the flux rate which was measured using a quartz crystal monitor.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.
H. Obara et at I Magnaimaon mea,mrements of YBa2Cupy / PrBazCu~Oylayeredstructures
2042
To evaluate the morphology of these layered structures, we have measured i n - s i t u x-ray photoelectron spectroscopy (XPS). In Fig. 2, the XPS spectra of the multilayered film, the top layer of which was PBCO, are shown. The small intensity of the Y 3d peak of sample B indicates that the films were not smooth in the scale of the PBCO thickness of sample B (,,-30~Q. On tae other hand, almost complete coverage was observed in sample A, the PBCO thickness of which is ~60/~.
films, which have very thin YBCO layers. The systematic measurements of the magnetization in layered structures will give the great information about the pintfing mechanism in oxide superconductors. For example, the layered structure of YBCO system can be considered as a model system of other materials which have longer lattice constant, such as Bi system. More extensive studies are needed to discuss the intrinsic Jc properties of layered structures. •
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180 160 140 120 Binding Energy (eV)
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FIGURE 2 XPS spectra of YBCO/PBCO layered structures. The top layer was PBCO. The superconductive properties of the YBCO/PBCO layered structures, which was measured using a SQUID magnetometer, are shown in Fig. 3. We have measured the temperature dependence of the remanent magnetization and critical current density J~ was calculated using a simple Bean critical state model. In this calculation, we considered that the supercurrent flows only in YBCO layers. T¢ and Jc were lower in sample A, which had the longer modulation wavelength than sample B. We have also prepared very thin films of single YBCO layer, which were sandwiched by PBCO layers, and thickness dependence of the YBCO layer on superconductive properties was also plotted in the inset of Fig. 3. In the inset of Fig. 3, T¢ was determined as the onset temperature of the magnetization. The rapid decrease of T¢ was observed in very thin films of YBCO (< 40A). T¢ of multilayered films, however, remains relatively high iv such very thin YBCO layers. One possible explanation of e_n.hanced T~ in the multilayered system is that there was some coupling effect between each YBCO layer. Another explanation is that the YBCO layer had contacts with other YBCO layers because the very thin PBCO layers is not perfect. At the present stage, the origin of the enhanced Tc in the multilayered system is still unknown because there is some defects in Y B C O / P B C O layered structures. As shown in Fig. 3, we have observed J¢ in multilayered
0
10
Temperature
40
50
(K)
FIGURE 3 Temperature dependences of J~ in layered structures, which were calculated from the remanent magnetization of films. Inset of the figure shows YBCO thickness dependence of Tc in P B C O / Y B C O / P B C O sandwiched systems. In summary, we have successfully prepared YBCO/PBCO layered structures using an MBE technique. The structure of the layered film was examined by x-ray diffraction measurements and XPS spectroscopy. The superconductive properties were measured using a SQUID magnetometer and compared with the data of single YBCO film. The J¢ properties of the layered structure were also calculated using a Bean critical state model. These experimental results will make it possible to discuss the intrinsic properties of very thin YBCO layers. REFERENCES 1. J.-M. Triscone, ¢. Fischer, O. Brunner, L. Antognazza, and A.D. Kent, Phys. Rev. Lett. 64(1990) 804. 2. C.L. Jia, B. Kabius, H. Soltner, U. Poppe, K. Urban, J. Schubert, and Ch. Buchal, Physica C 167 (1990) 463. T V,,,t-~ . . . . I l . . . . n n , , , ~ , D Ramesh, M.S. llegde, X.D. Wu, L. Nazar, C.C. Chang, J.B. Barner, D.M. |twang. and C.T. Rogers, Apph.>. Phys. Letl. 56 (199o) 391. H. Obara, S. Kosaka, M. Umeda and Y. Kiinura, Advances in Sapcrcon4uctivity II (Springer, Tokyo, 1990), p.801. X.W. Wu, X.X. XI. Q. Li, A. lnam, B. Dutta, L. DiDomernico, C. Weiss, .J,A. Martinez, B.J. Wilkens, S.A. Schwarz, J.B. Burner, C.C. ('hang, L. Nazar, and T . Venkal¢,san, A p t ) I . l~hy.~, l : , t l .
56 (1990).t00.
C
1 8 5 - 1 8 9
(1991)
BU
2 0 4 t - 2 0 4 2
North-HoUand
MAGNETIZATION M E A S U R E M E N T S OF YB&2Cu3Oy [PtBR2Cu3Oy LAYERED STRUCTURES ttaruhiko OBARA, Shin KOSAKA, and Yoichi KIMURA, Electrotechnical Laboratory, 1-1-4, Umezono, Tsukuba-shi,
Ibaraki 305, JAPAN
We have successfully prepared YBa2Cu3Oy (YBCO)/PrBa2CusOy (PBCO) layered structures (multilayers) using molecular beam epitaxy (MBE). The structure of these multilayers was examined by x-ray diffraction spectra, and the coverage of YBCO by PBCO, i.e., the morphology of the layered structures, was estimated using in - s i t u x-ray photoelectron spectroscopy (XPS). The superconductivity of these multilayers was measured by a SQUID magnetometer, which is quite sensitive to homogeneity of films and enables us to estimate critical current properties of superconductors. From these experimental results, we have discussed the homogeneity and the superconductive properties of the YBCO/PBCO layered structures. Recently YBCO/PBCO layered structures have been extensively studied. One of the purposes of this research is to apply the high-To superconductors to electronic devices and another is to observe the intrinsic properties of very thin films of oxide superconductor. Several groups have reported the preparation of PBCO]YBCO layered structures using sputtering L2 and laser deposition 3. MBE is suitable for fabrication of good-quality layered structures, which have very thin layers and sharp interfaces. At the present stage, it is very important to evaluate quality of thin films. However, using a conventional transport measurement which depends on the current path, it is difficult to evaluate quality of films on the whole. Magnetization measurements are powerful method for evaluation of thin films because it is sensitive to inhomogeneity of samples. Moreover, critical current properties of films can be observed using the magnetization measurements. In the present work, we have examined the P B C O / Y B C O layered structures using x-ray diffraction spectra and in-situ XPS spectroscopy which enabled us to estimate the morphology of the layered structures. The superconductivity of the multilayers was measured using a SQUID magnetometer and we have discussed the homogeneity and the critical current properties of the YBCO/PBCO layered structures.
Films were prepared by coevaporation in an MBE system. The evaporation sources were four effusion cells (the maximum temperature is 1800°C ) and Y, Ba, Cu and Pr metal elements were individually evaporated. Ba and Cu were continuously ewporated and the layered structure was controlled by shutters of Y and Pr sources. The deposition rate was typically ~ 0.5 ~,/s and the substrate temperature wa~ about 650°C . For oxidation of the samples, we used an electron cyclotron resonance (ECR) ion source and oxygen
gas was introduced to the ECR ion source during deposition. The details of sample preparation have been described elsewhere 4 Figure 1 shows the x-ray diffraction spectra ;~ the region of (001) peaks from the layered structures of YBCO/PBCO on MgO (100) plane. The number of YBCO and PBCO layers in these multilayers was ten, and the thickness of each YBCO layer was constant. Only the thickness of each PBCO layer was changed. 1 0
•
•
.
•
•
•
|
•
'
-.
.
.
i
.
'
-
.
.
.
.
.
.
i
.
.
•
'
•
•
"
x~
1
,
5
-
6
7
•
•
'
•
'
'
8 9 20 (degree)
10
FIGURE 1 X-ray diffraction spectra of Y B C O / P B C O ~yered structures which have different modulation wavelength. The arrow~ itidicate "- . . . . 11:, ~.o t, lll[:~
~o,
Leill,
lt.~
p ~ ' ~ a o .
The films showed strong c-axis orientation. The satellite peaks on the sides of tile main (001) peaks indicates that the periodicity was achieved in this syslem and tile modulation wavelength D was calculated from tile separation of satellite peaks ~', and we got D ,-- 112.~ (sample A) and D --, 84~ (sample B). The modulation wavelength is sum of the individual layer thickness and almost agreed to the flux rate which was measured using a quartz crystal monitor.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.
H. Obara et at I Magnaimaon mea,mrements of YBa2Cupy / PrBazCu~Oylayeredstructures
2042
To evaluate the morphology of these layered structures, we have measured i n - s i t u x-ray photoelectron spectroscopy (XPS). In Fig. 2, the XPS spectra of the multilayered film, the top layer of which was PBCO, are shown. The small intensity of the Y 3d peak of sample B indicates that the films were not smooth in the scale of the PBCO thickness of sample B (,,-30~Q. On tae other hand, almost complete coverage was observed in sample A, the PBCO thickness of which is ~60/~.
films, which have very thin YBCO layers. The systematic measurements of the magnetization in layered structures will give the great information about the pintfing mechanism in oxide superconductors. For example, the layered structure of YBCO system can be considered as a model system of other materials which have longer lattice constant, such as Bi system. More extensive studies are needed to discuss the intrinsic Jc properties of layered structures. •
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.
,
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,
,
i
•
•
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.
L
~.~
0
-
A
•
lo[
I
3d
,.,
"~
B
lo
loo
.
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_,_
lo 3
-
Thickness (A)
" .
i
"'"" "',¢.
20
30
. . . . . .
t
. . . . .
: : : . :
0
200
180 160 140 120 Binding Energy (eV)
100
FIGURE 2 XPS spectra of YBCO/PBCO layered structures. The top layer was PBCO. The superconductive properties of the YBCO/PBCO layered structures, which was measured using a SQUID magnetometer, are shown in Fig. 3. We have measured the temperature dependence of the remanent magnetization and critical current density J~ was calculated using a simple Bean critical state model. In this calculation, we considered that the supercurrent flows only in YBCO layers. T¢ and Jc were lower in sample A, which had the longer modulation wavelength than sample B. We have also prepared very thin films of single YBCO layer, which were sandwiched by PBCO layers, and thickness dependence of the YBCO layer on superconductive properties was also plotted in the inset of Fig. 3. In the inset of Fig. 3, T¢ was determined as the onset temperature of the magnetization. The rapid decrease of T¢ was observed in very thin films of YBCO (< 40A). T¢ of multilayered films, however, remains relatively high iv such very thin YBCO layers. One possible explanation of e_n.hanced T~ in the multilayered system is that there was some coupling effect between each YBCO layer. Another explanation is that the YBCO layer had contacts with other YBCO layers because the very thin PBCO layers is not perfect. At the present stage, the origin of the enhanced Tc in the multilayered system is still unknown because there is some defects in Y B C O / P B C O layered structures. As shown in Fig. 3, we have observed J¢ in multilayered
0
10
Temperature
40
50
(K)
FIGURE 3 Temperature dependences of J~ in layered structures, which were calculated from the remanent magnetization of films. Inset of the figure shows YBCO thickness dependence of Tc in P B C O / Y B C O / P B C O sandwiched systems. In summary, we have successfully prepared YBCO/PBCO layered structures using an MBE technique. The structure of the layered film was examined by x-ray diffraction measurements and XPS spectroscopy. The superconductive properties were measured using a SQUID magnetometer and compared with the data of single YBCO film. The J¢ properties of the layered structure were also calculated using a Bean critical state model. These experimental results will make it possible to discuss the intrinsic properties of very thin YBCO layers. REFERENCES 1. J.-M. Triscone, ¢. Fischer, O. Brunner, L. Antognazza, and A.D. Kent, Phys. Rev. Lett. 64(1990) 804. 2. C.L. Jia, B. Kabius, H. Soltner, U. Poppe, K. Urban, J. Schubert, and Ch. Buchal, Physica C 167 (1990) 463. T V,,,t-~ . . . . I l . . . . n n , , , ~ , D Ramesh, M.S. llegde, X.D. Wu, L. Nazar, C.C. Chang, J.B. Barner, D.M. |twang. and C.T. Rogers, Apph.>. Phys. Letl. 56 (199o) 391. H. Obara, S. Kosaka, M. Umeda and Y. Kiinura, Advances in Sapcrcon4uctivity II (Springer, Tokyo, 1990), p.801. X.W. Wu, X.X. XI. Q. Li, A. lnam, B. Dutta, L. DiDomernico, C. Weiss, .J,A. Martinez, B.J. Wilkens, S.A. Schwarz, J.B. Burner, C.C. ('hang, L. Nazar, and T . Venkal¢,san, A p t ) I . l~hy.~, l : , t l .
56 (1990).t00.