Magnetoresistive devices with increased response sensitivity to external magnetic fields and an increased magnetoresistive ratio (MR ratio) to cope with rapid advances made in a highdensity magnetic recording device. A patterned dielectric layer is laminated in a flat shape on a substrate capable of being doped with carriers (holes) in an electric field, and an FET structure with gate electrodes is then fabricated on that dielectric layer, and the substrate spatially modulated by applying a nonuniform electrical field to induce a first ferromagnetic domain, a nonmiagnetic domain and a second ferromagnetic domain.