Composite superconductor

Abstract: A LnBaCuO-series superconducting thin film is provided over a surface of a substrate of Y₂O₃ single crystal to form a composite superconductor. Ln stands for Y or a lanthanoid element. The composite superconductor has an improved interfacial diffusion.

Description: This invention relates to a composite superconductor composed of a substrate and a superconducting metal oxide film provided on the substrate. Superconductors of a YBaCuO type are known to show critical temperatures (Tc) as high as 90 K. According to a method suggested by Murakami et al, in which a superconductor phase is formed from a fused state, YBaCuO superconductors having a high critical current density (Jc) of 10,000 A/cm² at a magnetic flux density of 1 T (tesla) can be produced (Japanese Journal of Applied Physics, 29 , 1189 (1989)). When a YBaCuO superconducting material is used in the form of a thin film, it is necessary to form the film on a suitable substrate. Hitherto, a MgO substrate has been predominantly used. For the purpose of improving the quality of such a composite superconductor, SrTiO₃, NdGaO₃ and YAlO₃ have been proposed as a material for the substrate (see, for example, EP-A-0293836). These substrates are expected to provide a composite superconductor having good lattice consistency because the lattice constant thereof resembles that of the YBaCuO film and because the YBaCuO superconducting thin film may be formed thereon through epitaxial growth. It has been found, however, that the quality of the composite superconductor deteriorates when the thickness of the YBaCuO film is reduced. It is important that the YBaCuO film should have a very small thickness of less than 100 nm (1000 Å) in order to obtain optimum results. In accordance with the present invention there is provided a composite superconductor comprising a substrate of Y₂O₃ single crystal, and a superconducting layer provided on a surface of said substrate and formed of metal oxides containing Ln, Ba, Cu and O wherein Ln stands for an element selected from Y and elements belonging to the lanthanoid series. It has been found that interfacial diffusion of the elements constituting the substrate accounts for the deterioration of the superconducting characteristics of the known composite superconductors using a SrTiO₃, NdGaO₃ or YAlO₃ substrate for supporting a super-thin YBaCuO film. Namely, the substrate of the known composite superconductor contains one or more hetero-elements, such as Sr, Ti, Ga or Al, which are not present in the YBaCuO film. Such a hetero-element diffuses into the superconducting film during the formation thereof on the substrate. When the superconducting film is very thin, the influence of the interfacial diffusion of the hetero-element cannot be neglected. The use of a Y₂O₃ substrate has now been found to solve the problem of interfacial diffusion. Thus, the composite superconductor according to the present invention exhibits excellent superconducting characteristics even when the YBaCuO film has a thickness of 100 nm (1000 Å) or less. The present invention will now be described in detail below with reference to the accompanying drawings, in which: Fig. 1 is a schematical illustration of the crystal structure of Y₂O₃ single crystal; Fig. 2 is a graph showing temperature-resistivity characteristics of the YBa₂Cu₃O x film of the composite superconductor obtained in Example 1; Fig. 3 is an X-ray diffraction pattern of the composite superconductor obtained in Example 1; Fig. 4 shows rocking curves of the (018) and (108) planes of the YBa₂Cu₃O x film and the (311) plane of the Y₂O₃ substrate of the composite superconductor obtained in Example 1 with respect to the 〈011〉 plane of the Y₂O₃ substrate; Fig. 5 is an Auger depth profile of the composite superconductor obtained in Example 1; and Fig. 6 is an X-ray diffraction pattern of the composite superconductor obtained in Example 3. In the following, measurements are reported in ångstrom units. An ångstrom is 10⁻¹ nm. As shown in Fig. 1, the crystal structure of Y₂O₃ is a cubic system in which the lattice constant is 10.60 Å. Thus, the bond length r is 3.75 Å. On the other hand, the lattice constants of YBa₂Cu₃O x (x is a number of 6-7) are 3.82 Å in the a-axis, 3.89 Å in the b-axis and 11.68 Å (≃3.89x3 Å) in the c-axis. Therefore, the lattice mismatching in the a-, b- and c-axes are only 1.9 %, 3.7 % and 3.7 %, respectively. This indicates that the use of Y₂O₃ substrates permits epitaxial growth of YBa₂Cu₃O x thereon. In the case of YBa₂Cu₄O x (x is a number of 8-10), the lattice constants in the a-, b- and c-axes are 3.84 Å, 3.87 Å and 27.24 Å (≃3.89x7 Å), respectively. Therefore, this superconducting film too has good matching with the single crystal Y₂O₃ substrate. The single crystal Y₂O₃ substrate may be prepared in any known manner. Provided over a surface of the Y₂O₃ substrate is a superconducting layer of metal oxides containing Ln, Ba, Cu and O wherein Ln stands for an element selected from Y and elements belonging to the lanthanoid consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Preferably, the superconducting layer is formed of metal oxides having the following formula: LnBa₂Cu y O x wherein Ln stands for an element selected from Y, La, Ce, Nd, Pm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, y is a number between 3 and 4 and x is a number between 6 and 10. The superconducting layer may be formed on the Y₂O₃ substrate by the pulsed laser deposition method, the chemical vapor deposition method, the magnetron sputtering method, the molecular beam epitaxy method or any other known method. The superconducting layer preferably has a thickness in the range of from 60 to 5000 Å, more preferably 100-1000 Å. The following examples will further illustrate the present invention. Example 1 A Y₂O₃ single crystal substrate was prepared by the Verneuil method. On the (100) plane of the substrate a thin film of YBa₂Cu₃O x (x is a number of 6-7) was formed by the pulsed laser deposition method using an energy density of 1 J/cm², an oxygen partial pressure of 1x10² mTorr and a substrate temperature of 700 °C to obtain a composite superconductor. The thickness of the YBa₂Cu₃O x film was 700 Å. The film as deposited was tested for superconducting characteristics by the four-terminal resistivity measuring method. The results are shown in Fig. 2, from which the composite superconductor has a zero resistivity temperature T R=0 of 86.2 K and a superconducting transition temperature width ΔT of 1.2 K. The temperature-resistivity curve shown in Fig. 2 also indicates that the extraporated temperature in the normal conducting side is about 0 K. The composite superconductor was also subjected to an X-ray diffraction analysis to give the results shown in Fig. 3. From Fig. 3, it is seen that the superconducting film is a c-axis oriented film and contains no inclusions. The rocking curves of the (018) and (108) planes of the YBa₂Cu₃O x film and the (311) plane of the Y₂O₃ substrate of the composite superconductor obtained above with respect to the 〈011〉 plane of the Y₂O₃ substrate are shown in Fig. 4. The diffraction peak attributed to the (311) plane of the Y₂O₃ single crystal substrate is separated into several peaks, indicating that the Y₂O₃ is not a perfect crystal in the strict sense but contains deviation of the crystal orientation. The diffraction peaks attributed to the (018) and (108) planes of the YBa₂Cu₃O x superconducting film are present in the 〈011〉 direction of the substrate and are separated and distributed with nearly the same breadth. This suggests that the 〈110〉 direction of the YBa₂Cu₃O x film is parallel with the 〈010〉 direction of the Y₂O₃ substrate. Further, the results shown in Fig. 3 indicates that the 〈001〉 direction of the YBa₂Cu₃O x film is parallel with the 〈100〉 direction of the Y₂O₃ substrate. It follows that there is good lattice matching between the substrate and the superconducting film. Fig. 5 shows an Auger depth profile obtained by the Auger electron spectroscopy of the composite superconductor obtained above. It is also revealed that the depth (Δt) of the film/substrate interface is below 120 Å. This indicates that almost no interfacial diffusion or interfacial reaction occurs and the interface is very sharp. Example 2 Example 1 was repeated in the same manner as described except that the formation of a thin film of YBa₂Cu₃O x (x is a number of 6-7) was effected by the chemical vapor deposition method in lieu of the pulsed laser deposition method. A composite superconductor having properties similar to those of Example 1 was obtained. Example 3 Example 2 was repeated in the same manner as described except that the Y₂O₃ substrate was overlaid with a superconducting film of NdBa₂Cu₃O x , YbBa₂Cu₃O x , EuBa₂Cu₃O x or ErBa₂Cu₃O x . The thickness, Tc R=0 and ΔT of the superconducting film as deposited were as summarized in Table 1. Metal Oxides Film Thickness (Å) Tc R=0 (K) ΔT (K) NdBa₂Cu₃O x 920 85.7 1.0 YbBa₂Cu₃O x 220 79.4 1.3 EuBa₂Cu₃O x 130 86.0 1.3 ErBa₂Cu₃O x 340 80.1 1.2 Example 4 Example 1 was repeated in the same manner as described except that the vacuum deposition was performed at a substrate temperature of 650 °C. The composite superconductor showed good superconducting characteristic similar to that obtained in Example 1. The composite superconductor was subjected to an X-ray diffraction analysis to give the results shown in Fig. 6. From Fig. 6, it is seen that the superconducting film is an a-axis oriented film and contains no inclusions.

Claims: A composite superconductor comprising a substrate of Y₂O₃ single crystal, and a superconducting layer provided on a surface of said substrate and formed of metal oxides containing Ln, Ba, Cu and O, wherein Ln stands for an element selected from Y and elements belonging to the lanthanoid series. A composite superconductor as claimed in claim 1, wherein said superconducting layer is formed of metal oxides having the following formula: LnBa₂Cu y O x wherein Ln stands for an element selected from Y, La, Ce, Nd, Pm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, y is a number between 3 and 4 and x is a number between 6 and 10. A composite superconductor as claimed in claim 1 or claim 2, wherein said superconducting layer has a thickness in the range of from 6 to 500 nm (60 to 5000 Å).

Inventors: MORISHITA TADATAKA, OISHI AKIRA, MORISHITA, TADATAKA, OISHI, AKIRA
Applicants: INT SUPERCONDUCTIVITY TECH, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER
Application year: 1991
Main classification: C30B23
IPC codes: C01G1, C01G3, H01L39, C30B25, C30B11, C30B23, C30B29