The high sensitivity detection of magnetic fields was demonstrated using the oscillatory domain-wall displacing method for a GMR sensor. A micro-wire with a width of 30 µm and a length of 200 µm of a spin valve film, Al(20 nm)/MnIr(20 nm)/CoFe(3 nm)/Cu(2.2 nm)/CoFeB(10 nm), was fabricated as a magnetic sensor device. The domain walls in the free layer, whose easy axis was perpendicular to the wire, were oscillatorily displaced by an alternating magnetic field produced by a current with 140 kHz

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... hrough the micro-wire. The oscillatory wall displacement reduced the influence of wall coercivity and the linear relationship between the external field with 1 kHz and an output signal for the sensor circuit was obtained down to 10 -3 Oe.