Optimisation of material and structure for a switched-bias magnetoresistive sensor

Abstract This paper describes research on a magnetoresistive sensor consisting of two differentially connected Permalloy stripes with an overlay bias conductor [D.J. Mapps et al., A double bifilar magnetoresistor for earth's field detection, IEEE Trans. Magn. 23 (5) (1987) 2413–2415.]. A magnetic field derived from current flowing in the bias conductor switches the magnetisation in the sensor stripes alternately in opposite directions for each half-cycle of the bias field current. The material of the sensor is optimised for lowest Barkhausen noise [M.A. Akhter, D.J. Mapps, Y.Q. Ma, A. Petford-Long, R. Doole, Thickness and grain size dependence of the coercivity of permalloy thin film, J. Appl. Phys. 81 (8) (1997) 4122–4124.] and lowest demagnetising field. A number of sensors have been prepared with film thicknesses of 5–40 nm, stripe widths varying between 10 and 80 μm and a standard length of 6 mm. Barkhausen noise and corresponding hysteresis is studied by analysis of MR response of the group of sensors, together with their magnetoresistive hysteresis, which may be reduced or improved by using a suitable external high-frequency field (5–20 kHz). Variation in the sensitivity of the sensor to the magnitude of a switched-biasing field was measured by applying a very small alternating field (variable from a few tens to hundreds of nano-Tesla) and a varying transverse DC field component ( δH x ). Advantages and limitations of the sensors will be discussed in the paper.