High-sensitivity magnetoresistive transducers

Magnetoresistive transducers, combined with appropriate signal conditioning, can be used to detect magnetic-field changes from small currents, geomagnetic anomalies, and perturbations in the earth’s field from ferromagnetic objects. Two different designs of transducers are reported on−herringbone type, and barberpole biased type. Sensitivities as high as 3.5 mV/V/Oe have been obtained. The highest reported value seen in published literature is 1.75 mV/V/Oe (Philips Technical Publication 268), whereas values around 1.0 mV/V/Oe are typical. These high-sensitivity transducers are designed and fabricated based on previously published theoretical analysis [B. Pant, J. Appl. Phys. 67, 414 (1990)] of the response of thin magnetoresistive strips to external fields. This analysis indicates that to increase the sensitivity of these transducers one must reduce the ratio of strip thickness to its width and rotate the easy axis away from the long dimension of the strip wherever appropriate. The theoretical and experimental curves of bias field versus sensitivity for the herringbone design, and the transfer function for the barberpole design, are reported.