the present invention will be described in detail below by way of its examples with reference to the accompanying drawings . mgal 2 o 4 was epitaxially grown on a ( 100 ) si single - crystal substrate , and bi x ( sr , ca ) y cu z o . sub . δ was formed thereon by sputtering . fig1 a and 1b show manufacturing steps of this example . as shown in fig1 a , an mgal 2 o 4 epitaxial film 2 was grown on a ( 100 ) si single - crystal substrate 1 by epitaxial growth . as shown in fig1 b , a bi x ( sr , ca ) y cu z o . sub . δ film 3 was formed on the mgal 2 o 4 film 2 by sputtering . epitaxial growth of the mgal 2 o 4 film was performed by a method disclosed in japanese patent laid - open no . 59 - 26999 . that is , an mgcl 2 gas , an alcl 3 gas obtained by reacting al with hcl , a co 2 gas and an h 2 gas were used as reaction gases , and an n 2 gas was used as a carrier . a production reaction of mgal 2 o 4 is represented as follows : mgal 2 o 4 was grown to have a thickness of 0 . 1 μm at a growth temperature of 950 ° c ., and it was confirmed by x - ray diffraction and electron beam diffraction that ( 100 ) mgal 2 o 4 was epitaxially grown . the bi x ( sr , ca ) y cu z o . sub . δ film was fabricated to have a thickness of 1 μm by rf magnetron sputtering . powders were prepared by mixing oxides to obtain compositions listed in table 1 . the resultant mixtures were calcined at 800 ° c . and used as targets to perform film formation in an ar - o 2 gas mixture at a substrate temperature of 700 ° c . after film formation , changes in electrical resistance of the bi x ( sr , ca ) y cu z o . sub . δ upon temperature changes were measured by a four - probe method , and critical temperatures listed in table 1 were obtained . although the bi x ( sr , ca ) y cu z o . sub . δ film whose composition falls within the above range consists of both a superconducting substance and a nonsuperconducting substance , the film has superconductivity with good reproducibility as a whole because a volume ratio of the superconducting substance is larger . if the composition of the bi x ( sr , ca ) y cu z o . sub . δ film does not fall within the above range , however , a volume ratio of the nonsuperconducting substance becomes larger , and the film exhibits semiconducting or insulating electrical characteristics . therefore , the composition of the bi x ( sr , ca ) y cu z o . sub . δ film preferably falls within the ranges of 0 . 08 ≦ x /( x + y + z )≦ 0 . 41 , 0 . 29 ≦ y /( x + y + z )≦ 0 . 47 and 1 ≦ sr / ca ≦ 3 . table 1______________________________________bi (%) sr + ca (%) cu (%) sr / ca t . sub . c ( k ) ______________________________________1 8 47 45 1 . 5 54k2 8 29 63 1 . 5 47k3 41 29 30 1 . 5 68k4 41 47 12 1 . 5 59k5 41 40 30 1 . 0 85k6 30 40 30 1 . 0 80k7 30 40 30 3 . 0 67k8 20 40 40 1 . 0 85k9 20 40 40 1 . 5 72k______________________________________ an si substrate was thermally oxidized via a 0 . 1 - μthick mgal 2 o 4 film epitaxially grown on a ( 100 ) si single - crystal substrate to form sio 2 having a thickness of 0 . 5 μm between the mgal 2 o 4 film and the si substrate , and bi x ( sr , ca ) y cu z o . sub . δ was grown on the mgal 2 o 4 epitaxial film to have a thickness of 1 μm . fig2 a to 2c are manufacturing steps of this example . as shown in fig2 a , an mgal 2 o 4 epitaxial film 5 was formed on a ( 100 ) si substrate 4 similar to that used in example 1 . as shown in fig2 b , an sio 2 film 6 was formed between the si substrate 4 and the film 5 by thermal oxidation . in this case , thermal oxidation was steam oxidation performed at 1 , 100 ° c . a bi x ( sr , ca ) y cu z o . sub . δ film was then formed on the mgal 2 o 4 film 5 . by thermal oxidation , a full - width at half maximum of an x - ray rocking curve of the mgal 2 o 4 film was reduced by 30 %, and its crystallinity was improved . mgal 2 o 4 and bi x ( sr , ca ) y o . sub . δ were grown following the same procedures as in example 1 . in any composition , a critical temperature characteristic as in example 1 was obtained . a 800 - å thick mgal 2 o 4 film was epitaxially grown on a ( 100 ) si single - crystal substrate , and a 0 . 5 - μm thick mgo film was epitaxially grown thereon . thereafter , a 2 - μm thick bi x ( sr , ca ) y cu z o . sub . δ film was grown following the same procedures as in example 1 . in any composition , a critical temperature as in example 1 was obtained . fig3 shows a structure according to this example . referring to fig3 reference numeral 8 denotes an si single - crystal substrate ; 9 , an mgal 2 o 4 epitaxial film ; 10 , an mgo epitaxial film ; and 11 , a bi x ( sr , ca ) y cu z o . sub . δ film . it was confirmed that the same effect could be obtained by forming bi x ( sr , ca ) y cu z o . sub . δ on a substrate consisting of mgo / mgal 2 o 4 / sio 2 / si single - crystal . a tl x ( ba , ca ) y cu z o . sub . δ film was epitaxially grown in place of a bi x ( sr , ca ) y cu z o . sub . δ film following the same procedures as in example 1 . growth was performed following the same procedures as in example 1 , and a 1 - μm thick film was prepared by an rf magnetron sputtering method . powders were prepared by mixing oxides to obtain compositions listed in table 2 , and the resultant mixtures were calcined at 850 ° c . and used as targets to perform film formation in an ar - o 2 gas mixture at a substrate temperature of 700 ° c . after film formation , changes in electrical resistance of the tl x ( ba , ca ) y cu z o . sub . δ film upon temperature changes were measured by a four - probe method , and superconducting characteristics of critical temperatures listed in table 2 were obtained . table 2______________________________________tl (%) ba + ca (%) cu (%) ba / ca t . sub . c ( k ) ______________________________________1 30 40 30 1 . 5 87k2 20 40 40 1 . 0 88k3 8 47 45 1 . 5 52k______________________________________ as shown in fig4 a , an mgal 2 o 4 film 22 was epitaxially grown on a ( 100 ) si single - crystal substrate 21 . as shown in fig4 b , batio 3 23 was epitaxially grown on the resultant structure . as shown in fig4 c , bi x ( sr , ca ) y cu z o . sub . δ 24 was formed on the batio 3 film 23 by sputtering . epitaxial growth of the mgal 2 o 4 film 22 was performed by a method already proposed ( japanese patent laid - open no . 59 - 26999 ). that is , an mgcl 2 gas , an alcl 3 gas obtained by reacting al and hcl , a co 2 gas and an h 2 gas were used as reaction gases , and an n 2 gas was used as a carrier gas , thereby forming mgal 2 o 4 by a reaction of mgcl 2 + 2alcl 3 + 4co 2 4h 2 → mgal 2 o 4 + 4co + 8hcl . the film was grown at a temperature of 950 ° c . to have a thickness of 0 . 1 μm . it was confirmed by x - ray diffraction and electron beam diffraction that ( 100 ) mgal 2 o 4 was epitaxially grown . the batio 3 epitaxial film was prepared by a method already proposed ( japanese patent laid - open no . 59 - 17358 ). that is , a batio 3 powder was used as a target to perform film formation in a 10 % o 2 - ar gas mixture at a substrate temperature of 600 ° c . by an rf magnetron sputtering method . the film thickness was 0 . 4 μm . similar to mgal 2 o 4 , it was confirmed by x - ray diffraction and electron beam diffraction that a ( 100 ) film was epitaxially grown . the bi x ( sr , ca ) y cu z o . sub . δ film was prepared by the rf magnetron sputtering method to have a thickness of 1 μm . powders were prepared by mixing oxides to obtain compositions listed table 3 , and the resultant mixtures were calcined at 850 ° c . and used as targets to perform film formation in an ar - o 2 gas mixture at a substrate temperature of 700 ° c . after film formation , changes in electrical resistance of the bi x ( sr , ca ) y cu z o . sub . δ film upon temperature changes were measured by a four - probe method , and superconducting characteristics of critical temperatures t c listed in table 3 were obtained . table 3______________________________________bi (%) sr + ca (%) cu (%) sr / ca t . sub . c ( k ) ______________________________________1 8 47 45 1 . 5 56k2 8 29 63 1 . 5 48k3 41 29 30 1 . 5 70k4 41 47 12 1 . 5 60k5 30 40 30 1 . 5 86k6 30 40 30 1 . 0 82k7 30 40 30 3 . 0 69k8 20 40 40 1 . 0 88k9 20 40 40 1 . 5 71k______________________________________ although the bi x ( sr , ca ) y cu z o . sub . δ film whose composition falls within the above range consists of both a superconducting substance and a nonsuperconducting substance , the film has superconductivity with good reproducibility as a whole because a volume ratio of the superconducting substance is larger . if the composition of the bi x ( sr , ca ) y cu z o . sub . δ film does not fall within the above range , however , a volume ratio of the nonsuperconducting substance becomes larger , and the film exhibits semiconducting or insulating electrical characteristics . therefore , the composition of the bi x ( sr , ca ) y cy z o . sub . δ film preferably falls within the ranges of 0 . 08 ≦ x ( x + y + z )≦ 0 . 41 , 0 . 29 ≦ y /( x + y + z )≦ 0 . 47 and 1 ≦ sr / ca ≦ 3 . a ( ba , sr ) tio 3 film was epitaxially grown in place of batio 3 following the same procedures as in example 5 . growth was performed following the same procedures as in example 5 by a magnetron sputtering method to form a film having a film thickness of 0 . 4 μm . a ( ba x , sr 1 - x ) tio 3 powder was used as a target , and three types of compositions x = 0 . 25 , 0 . 50 and 0 . 75 were used . a bi x ( sr , ca ) y cu z o . sub . δ film was grown on the ( ba , sr ) tio 3 epitaxial film following the same procedures as in example 5 by an rf magnetron sputtering method . in any composition , the same superconductivity as in example 5 was obtained . an si substrate was thermally oxidized via an mgal 2 o 4 film epitaxially grown on a ( 100 ) si single - crystal substrate , and sio 2 was formed between the mgal 2 o 4 film and the si substrate . thereafter , a batio 3 film was epitaxially grown on the mgal 2 o 4 epitaxial film , and bi x ( sr , ca ) y cu z o . sub . δ was grown thereon . fig5 a to 5d show manufacturing steps of this example . as shown in fig5 a , mgal 2 o 4 26 was grown on an si substrate 25 . as shown in fig5 b , an sio 2 film 27 was formed between the si substrate 25 and the mgal 2 o 4 26 by steam oxidation at 1 , 100 ° c . as shown in fig5 c , a batio 3 film 28 was epitaxially grown on the mgal 2 o 4 26 . as shown in fig5 d , a bi x ( sr , ca ) y cu z o . sub . δ film 29 was formed on the film 28 . the single - crystalline property of mgal 2 o 4 was not degraded upon thermal oxidation , but a full - width at half maximum of an x - ray rocking curve was reduced by about 30 % to improve the crystallinity . mgal 2 o 4 , batio 3 and bi x ( sr , ca ) y cu z o . sub . δ were grown following the same procedures as in example 1 . in any composition , the same superconductivity as in example 5 was obtained . a 0 . 1 - μm thick mgal 2 o 4 film was epitaxially grown on a ( 100 ) si single - crystal substrate , and 0 . 1 - μm thick mgo was epitaxially grown thereon . following the same procedures as in example 5 , a 0 . 4 - μm thick batio 3 film was epitaxially grown , and a 1 - μm thick bi x ( sr , ca ) y cu z o . sub . δ film was grown thereon . fig6 shows a structure of the film according to this example . referring to fig6 reference numeral 30 denotes an si single - crystal substrate ; 31 , an mgal 2 o 4 epitaxial film ; 32 , an mgo epitaxial film ; 33 , a batio 3 epitaxial film ; and 34 , a bi x ( sr , ca ) y cu z o . sub . δ film . in any composition , the same superconductivity as in example 5 was obtained . in this case , an sio 2 layer may be formed between the si single crystal and mgal 2 o 4 . a tl x ( ba , ca ) y cu z o . sub . δ film was epitaxially grown in place of a bi x ( sr , ca ) y cu z o . sub . δ film following the same procedures as in example 5 . growth was performed following the same procedures as in example 5 , and the film having a thickness of 1 μm was prepared by an rf magnetron sputtering method . powders were prepared by mixing oxides to obtain compositions listed in table 4 , and the resultant mixtures were calcined at 850 ° c . and used as targets to perform film formation in an ar - o 2 gas mixture at a substrate temperature of 700 ° c . after film formation , changes in electrical resistance of the tl x ( ba , ca ) y cu z o . sub . δ film upon temperature changes were measured by a four - probe method , and superconducting characteristics of critical temperatures t c listed in table 4 were obtained . table 4______________________________________tl (%) ba + ca (%) cu (%) ba / ca t . sub . c ( k ) ______________________________________1 30 40 30 1 . 5 87k2 20 40 40 1 . 0 86k3 8 47 45 1 . 5 50k______________________________________