Spin-Valve Film for Sensitive Giant Magnetoresistive Sensor Using AC Bias Magnetic Field

A sensor element material for a new sensitive magnetic sensor was developed by a NiO spin-valve film. The magnetic sensor is composed of a multilayered structure that includes the spin-valve element and a bias current line. High sensitivity is obtained by using only the (rising or falling) edge of the magnetoresistive (MR) loop. An AC bias magnetic field induced by a square-wave bias current (plus or minus) is applied to the spin-valve element. The bias magnetic field is set to be slightly smaller than the rising field. The resistance of the spin-valve element is constant, if an external magnetic field is not applied. The output signal synchronizes with the input bias current, if the bias field is made to cross the rising edge of the MR loop by applying an external magnetic field. The spin-valve film: [NiO/NiFeCo/Cu/NiFeCo] exhibits the appropriate properties for the sensor element, such as a MR ratio of 5.1%, a sensing field of 0.2 kA/m, a driving field of 1.4 kA/m, an element destroying field of 32 kA/m and a high corrosion resistance. Moreover, the spin-valve element, patterned to a microscopic size, exhibits a step-shaped MR loop in which a free-layer magnetization switches like a single domain. The magnetic sensor using this spin-valve film detects an external field change of 0.04 kA/m.