This paper reviews magnetoresistive sensors for detecting magnetic fields. These sensors are based on an effect which has been known for more than 100 years, but it has only come to prominence since 1971 when a thin-film magnetoresistive sensor was shown to have good signal-to-noise performance in a field-sensing application in magnetic recording. A large amount of research has been carried out to explore the limits of magnetoresistive sensors in their thin-him form and many novel structures have been published. Despite this there are application limits relating to non-linearity of response characteristics, presence of demagnetising fields, hysteresis and Barkhausen noise. The paper will present the basic theory of these sensors and deal with some technical limitations. A range of magnetic biasing techniques are discussed. The shape and structure of these sensors affects sensitivity through the form and magnitude of the demagnetising field. Improvements in performance can be gained by making thinner sensor films and films with transverse shape-anisotropy. Research has led to a reduction in coercivity of sensor film materials and a corresponding improvement in hysteresis. Barkhausen noise reduces in thinner films due to the transition from predominantly Bloch-type to predominantly Neel-type magnetic domain walls. This paper considers two types of sensor application. The first is for general magnetic field detection and the second, detecting magnetic fields on information storage media. (C) 1997 Published by Elsevier Science S.A.
