the invention will be described in more detail with reference to the following examples . a typical giant magnetoresistive sensor of the present invention is shown in section in fig1 . there is shown a substrate 5 . on the substrate 5 are formed sequentially a lower shield film ( nife film ) 10 , an insulating film ( al 2 o 3 film ) 20 for magnetic gap , and a magnetoresistive film 30 . the magnetoresistive film 30 is composed of a first free ferromagnetic film ( nifeco ) 35 , a copper layer 40 , a composite ferromagnetic film ( pinned layer of self - pinned type ) 50 , a copper layer 40 , and a second free ferromagnetic film ( nifeco ) 45 , which are sequentially formed one over another . the composite ferromagnetic film ( pinned layer of self - pinned type ) 50 is composed of co 51 ( 25 å ), ru 52 ( 6 å ), co 53 ( 30 å ), ru 52 ( 6 å ), and co 54 ( 25 å ), which are laminated sequentially . during lamination a magnetic field is applied in the direction of the arrow 200 . owing to the applied magnetic field , the composite ferromagnetic film has its axis of easy magnetization oriented in the direction perpendicular to the paper surface . an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) 80 is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode 90 . an insulating film ( al 2 o 3 film ) 100 for magnetic gap is formed . an upper shield film ( nife film ) 110 is laminated and then it is fabricated in a desired shape . thus there is obtained a magnetic head . the electrode spacing tw is 0 . 5 μm and the sensor height hmr is 0 . 4 μm . the arrows 55 in the figure denote the direction of magnetization of each magnetic film . in the composite ferromagnetic film 50 , the first , second , and third ferromagnetic films are strongly coupled with one another in an antiferromagnetic manner , and hence magnetization takes place in the direction shown . the composite ferromagnetic film 50 has a large effective coercive force and forms the pinned layer of self - pinned type . the head of the present invention was compared with a giant magnetoresistive sensor of conventional type which has the same electrode spacing and sensor height ( shown in fig7 ). it was found that the head of the present invention produces 1 . 5 times as large read - back output as the conventional one . a large number of heads varying in hmr were prepared , and they were tested for peak - asymmetry of read - back waveform ( asym .) and read - back output . the results are shown in fig1 . asym . is defined as follows : ( where v + denotes the peak value of the positive output and v − denotes the peak value of the negative output .) the read - back output is the value normalized such that the value of the conventional head ( with hmr being 0 . 4 μm ) is 1 . in the case of the conventional head , it is difficult to make asym . null . in addition , asym . greatly changes as hmr changes . by contrast , the head of the present invention keeps asym . almost null . even when hmr changes from 0 . 2 μm to 0 . 7 μm , asym . changes very little . this makes it possible to reduce the fluctuation of asym . due to tolerance of hmr . the read - back output is about 1 . 5 times larger than that in the conventional case . as mentioned above , this example gives a giant magnetoresistive sensor with small peak asymmetry and large read - back output . in addition , this example gives a giant magnetoresistive sensor superior in electro - static durability . this example demonstrates another giant magnetoresistive sensor which differs in the thickness of the pinned layer from the one in example 1 . as in example 1 shown in fig1 it consists of a substrate 5 , a lower shield film ( nife film ) 10 , an insulating film ( al 2 o 3 ) 20 for magnetic gap , and a magnetoresistive film 30 . the magnetoresistive film 30 consists of a first free ferromagnetic layer ( nifeco ) 35 , a copper layer 40 , a composite ferromagnetic film ( pinned layer of self - pinned type ) 50 , and a copper layer 40 , and a second free ferromagnetic film nifeco ) 55 , which are laminated sequentially . the composite ferromagnetic film ( pinned layer of self - pinned type ) 50 consists of co 51 ( 20 å ), ru 52 ( 6 å ), co 53 ( 35 å ), ru 52 ( 6 å ), and co 54 ( 20 å ) , which are laminated sequentially . during lamination a magnetic field is applied in the direction of the arrow 200 . owing to the applied magnetic field , the composite ferromagnetic film has its axis of easy magnetization oriented in the direction perpendicular to the paper surface . an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) 80 is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode 90 . an insulating film ( al 2 o 3 film ) 100 for magnetic gap is formed . an upper shield film ( nife film ) 110 is laminated and then it is fabricated in a desired shape . thus there is obtained a magnetic head . the electrode spacing tw is 0 . 5 μm and the sensor height hmr is 0 . 4 μm . it was found that in this example , too , the composite ferromagnetic film 50 has a large effective coercive force and forms the pinned layer of self - pinned type for stable operation . it was also found that the giant magnetoresistive sensor produces 1 . 5 times as large read - back output as the conventional one and has good electro - static durability and good asym . this example demonstrates another giant magnetoresistive sensor whose sectional view is shown in fig2 . it consists of a substrate 5 , a lower shield film ( nife film ) 10 , an insulating film ( al 2 o 3 ) 20 for magnetic gap , and a magnetoresistive film 30 . the magnetoresistive film 30 consists of a first composite ferromagnetic film ( pinned layer ) 50 , a copper layer 40 , a free ferromagnetic film ( nifeco ) 35 , a copper layer 40 , and a second composite ferromagnetic film ( pinned layer ) 60 , which are laminated sequentially . the first composite ferromagnetic film ( pinned layer ) 50 consists of co 51 ( 20 å ), ru 52 ( 6 å ), and co 53 ( 35 å ), which are laminated sequentially . during lamination a magnetic field is applied in the direction of the arrow 200 . owing to the applied magnetic field , the composite ferromagnetic film 50 has its axis of easy magnetization oriented in the direction perpendicular to the paper surface . the co film 51 and the co film 53 are strongly coupled with each other in an antiferromagnetic manner , and hence magnetization takes place in the direction shown . the composite ferromagnetic film 50 has a large effective coercive force and forms the pinned layer of self - pinned type . likewise , the second composite ferromagnetic film ( pinned layer ) 60 consists of co 61 ( 20 å ), ru 62 ( 6 å ), and co 63 ( 35 å ), which are laminated sequentially . during lamination a magnetic field is applied in the direction of the arrow 201 . owing to the applied magnetic field , the composite ferromagnetic film 60 has its axis of easy magnetization oriented in the direction perpendicular to the paper surface , and magnetization takes place in the direction shown . as in the case of the composite ferromagnetic film 50 , the composite ferromagnetic film 60 forms the pinned layer of self - pinned type . the first and second ferromagnetic composite films produce effective magnetic moments in their antiparallel direction . this cancels out the magnetic field applied to the free layer from the composite ferromagnetic film . subsequently , an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) 80 is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode 90 . an insulating film ( al 2 o 3 film ) 100 for magnetic gap is formed . an upper shield film ( nife film ) 110 is laminated and then it is fabricated in a desired shape . thus there is obtained a magnetic head . the electrode spacing tw is 0 . 5 μm and the sensor height hmr is 0 . 4 μm . it was found that in this example , too , the giant magnetoresistive sensor produces 1 . 5 times as large read - back output as the conventional one and has good electro - static durability and good asym . further , this example also demonstrates another giant magnetoresistive sensor which is identical in structure with the above - mentioned one , except that the thickness of the co film in the first and second composite ferromagnetic film varies so that the effective magnetic moment of the composite ferromagnetic film varies . in the first case , the first and second composite ferromagnetic films produce the effective magnetic moment in the same direction . in the second case , the first and second magnetic composite layers have the two co films of the same thickness so that their effective magnetic moment is nearly null . in the third case , either of the composite ferromagnetic films has an effective magnetic moment which is almost null . in all the cases , the resulting giant magnetoresistive sensor produces a large read - back output and has good electro - static durability and good asym . this example demonstrates another giant magnetoresistive sensor whose sectional view is shown in fig3 . it consists of a substrate 5 , a lower shield film ( nife film ) 10 , an insulating film ( al 2 o 3 ) 20 for magnetic gap , and a magnetoresistive film 30 . the magnetoresistive film 30 consists of a composite ferromagnetic film ( pinned layer ) 50 , a copper layer 40 , a free ferromagnetic film nifeco ) 35 , a copper layer 40 , a pinned ferromagnetic film ( cofe ) 65 , and an antiferromagnetic film ( crmnpt ) 70 , which are laminated sequentially . the composite ferromagnetic film ( pinned layer ) 50 consists of co 51 ( 20 å ), ru 52 ( 6 å ), and co 53 ( 35 å ), which are laminated sequentially . during lamination a magnetic field is applied in the direction of the arrow 200 . owing to the applied magnetic field , the composite ferromagnetic film 50 has its axis of easy magnetization oriented in the direction perpendicular to the paper surface . the co film 51 and the co film 53 are strongly coupled with each other in an antiferromagnetic manner , so that they form a pinned layer of self - pinned type . subsequently , an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) 80 is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode 90 . an insulating film ( al 2 o 3 film ) 100 for magnetic gap is formed . an upper shield film ( nife film ) 110 is laminated and then it is fabricated in a desired shape . thus there is obtained a magnetic head . the electrode spacing tw is 0 . 5 μm and the sensor height hmr is 0 . 4 μm . in this example , the pinned ferromagnetic layer 65 is magnetized in the direction of the arrow 55 . however , this direction may be reversed ( 180 °). it was found that the giant magnetoresistive sensor in this example produces 1 . 5 times as large read - back output as the conventional one and has good electro - static durability and good asym . further , this example also demonstrates another giant magnetoresistive sensor which is identical in structure with the above - mentioned one , except that the thickness of the ferromagnetic film is changed ( co 20 å , ru 6 å , co 25 å ) so that the effective magnetic moment of the composite ferromagnetic film 50 is almost null . the giant magnetoresistive sensor produces a large read - back output and has good electro - static durability and good asym . this example demonstrates another giant magnetoresistive sensor whose sectional view is shown in fig4 . it consists of a substrate 5 , a lower shield film ( nife film ) 10 , an insulating film ( al 2 o 3 ) 20 for magnetic gap , and a magnetoresistive film 30 . the magnetoresistive film 30 consists of an antiferromagnetic layer ( crmnpt ) 70 , a pinned ferromagnetic layer ( cofe ) 65 , a copper layer 40 , a free ferromagnetic layer ( nifeco ) 35 , a copper layer 40 , and a composite ferromagnetic film ( pinned layer ) 50 , which are laminated sequentially . the composite ferromagnetic film ( pinned layer ) 50 consists of co 51 ( 35 å ), ru 52 ( 6 å ), and co 53 ( 20 å ), which are laminated sequentially . during lamination a magnetic field is applied in the direction of the arrow 200 . owing to the applied magnetic field , the composite ferromagnetic film 50 has its axis of easy magnetization oriented in the direction perpendicular to the paper surface . the co film 51 and the co film 53 are strongly coupled with each other in an antiferromagnetic manner , so that they form a pinned layer of self - pinned type . subsequently , an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) 80 is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode 90 . an insulating film ( al 2 o 3 film ) 100 for magnetic gap is formed . an upper shield film ( nife film ) 110 is laminated and then it is fabricated in a desired shape . thus there is obtained a magnetic head . the electrode spacing tw is 0 . 5 μm and the sensor height hmr is 0 . 4 μm . in this example , the pinned ferromagnetic layer 65 is magnetized in the direction of the arrow 55 . however , this direction may be reversed ( 180 °). the composite ferromagnetic film 50 produces an effective coercive force and hence forms the pinned layer of self - pinned type . it was found that the giant magnetoresistive sensor in this example produces 1 . 5 times as large read - back output as the conventional one and has good electro - static durability and good asym . further , this example also demonstrates another giant magnetoresistive sensor which is identical in structure with the above - mentioned one , except that the thickness of the ferromagnetic layer is changed ( co 25 å , ru 6 å , co 20 å ) so that the effective magnetic moment of the composite ferromagnetic film 50 is almost null . the giant magnetoresistive sensor produces a large read - back output and has good electro - static durability and good asym . this example demonstrates another giant magnetoresistive sensor having an antiferromagnetic layer as in example 4 . it consists of a substrate , a lower shield film ( nife film ) 5 , an insulating film ( al 2 o 3 ) 10 for magnetic gap , and a magnetoresistive film 30 . the magnetoresistive film 30 consists of a composite ferromagnetic film ( pinned layer ) 50 , a copper layer 40 , a pinned ferromagnetic layer ( cofe ) 65 , and an antiferromagnetic layer ( ptmn ) 70 , which are laminated sequentially . the composite ferromagnetic film ( pinned layer ) 50 consists of co 51 ( 20 å ), ru 52 ( 6 å ), and co 53 ( 35 å ), which are laminated sequentially . the composite ferromagnetic film 50 has its axis of easy magnetization oriented in the direction perpendicular to the paper surface . the co film 51 and the co film 53 are strongly coupled with each other in an antiferromagnetic manner , so that they form a pinned layer of self - pinned type . subsequently , an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) 80 is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode . an insulating film ( al 2 o 3 film ) for magnetic gap is formed . an upper shield film ( nife film ) is laminated and then it is fabricated in a desired shape . thus there is obtained a magnetic head . the giant magnetoresistive sensor in this example produces 1 . 5 times as large read - back output as the conventional one and has good electro - static durability and good asym . this example demonstrates a thin - film magnetic head ( of read / write separate type ) in which the magnetoresistance element of the present invention is used as the reading head and the conventional inductive thin - film head is used as the writing head . fig5 is a partly cut away perspective view showing this thin - film magnetic head . there is shown a substrate 5 ( as a slider ) which is a sintered body composed mainly of al 2 o 3 . tic . on the substrate are formed a lower shield film 10 and an insulating film ( al 2 o 3 film ) for magnetic gap . on them is further formed a magnetoresistive film 30 , which consists of a first free ferromagnetic film ( nifeco ), a copper layer , a composite ferromagnetic film ( pinned layer of self - pinned type ), a copper layer , and a second free ferromagnetic film ( nifeco ), which are laminated sequentially . the ferromagnetic composite film ( pinned layer of self - pinned type ) is composed of co ( 20 å ), ru ( 6 å ), co ( 35 å ), ru ( 6 å ), and co ( 20 å ), which are laminated sequentially . subsequently , an organic resist film is laminated , and then it is patterned as desired . a film of permanent magnet ( cocrpt film ) is laminated . it is fabricated in a desired shape . a layer of nb / au / nb is laminated , and then it is fabricated to form an electrode . an insulating film ( al 2 o 3 film ) for magnetic gap is formed . a magnetic shield film ( nife film ) 110 is formed . the part produced as mentioned above functions as the reading head . an inductive thin - film head is formed as the magnetic writing head , which consists of an upper magnetic pole 120 and a coil 130 . the upper magnetic pole 120 is a 3 . 0 - μm thick film of ni - 20 at % fe alloy which is formed by sputtering . the gap between the upper shield film 110 and the upper magnetic pole 120 is filled with a 0 . 2 - μm thick al 2 o 3 film formed by sputtering . the coil 130 is a 3 . 0 - μm thick copper film . the lower magnetic pole 110 and the upper magnetic pole 120 are magnetically connected with each other to form a magnetic circuit . the coil 130 intersects the magnetic circuit . it was found that the thin - film magnetic head of this example produces 1 . 5 times as large read - back output as the conventional one and has good peak symmetry of read - back waveform , with asym . depending on hmr very little . this example demonstrates a magnetic disk apparatus equipped with the magnetic head produced in the aforesaid examples of the present invention . it is schematically shown in fig6 . there is shown a magnetic recording medium 140 , which is made of a co — ni — pt — ta alloy having a residual magnetic flux density of 0 . 45 t . the magnetic recording medium 140 is driven by a drive unit 150 . the magnetic head 160 is driven by the drive unit 170 so that it selects any track on the magnetic recording medium 140 . signals for the magnetic head 160 are processed by the read / write signal processing system 180 . the magnetoresistive sensor built into the magnetic head 160 produces a larger reproduction output and better peak symmetry of read - back waveform than the magnetoresistive sensor of conventional structure . therefore , the magnetic disk apparatus equipped with it has a narrow track width and a high recording density . as mentioned above , the present invention provides a giant magnetoresistive sensor which produces a large reproduction output , good peak symmetry of read - back waveform , and good electro - static durability .