the first magnetic layer 15 nm cu as the non magnetic layer 3 nm nife as the second magnetic layer 15 nm femn as the antiferromagnetic layer 15 nm and si 3 n 4 as the protective layer and by changing the film thickness of the si 3 n 4 protective layer as shown in table 26 the values of the reproducing outputs of the shield and yoke reproducing heads are also shown in table 26 respectively even when the film thickness of the protective layer is varied as described above there are almost no changes in the head reproducing outputs however if the film thickness is made less than 2 nm the femn antiferromagnetic layer is conspicuously oxidized thus causing a problem with respect to reliability 2 in the case where an al 2 o 3 alloy is used for the non metallic protective layer a spin valve film was produced using glass as the substrate al 2 o 3 as the buffer layer 80 nm fesial as the first magnetic layer 14 nm cu as the non magnetic layer 3 nm nife as the second magnetic layer 16 nm femn as the antiferromagnetic layer 15 nm and al 2 o 3 as the protective layer and by changing the film thickness of the al 2 o 3 protective layer as shown in table 27 the values of the reproducing outputs.