Designing a giant magnetoresistance field sensor via inferences from a giant magnetoresistance hysteresis model

Abstract

By using a model for giant magnetoresistance (GMR) hysteresis and a model for Barkhausen noise, a magnetic hysteresis model is used to map the effects of magnetic hysteresis on GMR and on accompanying magnetic noise in a GMR magnetic field sensor. Two magnetic noise sources potentially exist in the field sensor: (1) the GMR multilayer material and (2) permalloy field concentrators. The GMR material typically shows a coercivity at Hc≅1 kA/m, much larger than Hc=0.012 kA/m of permalloy. Because permalloy has a very large maximum permeability, it is a much larger magnetic noise source than the GMR material, and would greatly affect sensor operation if operated near H=0. By operating with a bias field well above 1 kA/m, one can operate away from both noise sources. Because the increasing and decreasing arms of the magnetic hysteresis tail of the GMR material are each approximately proportional to |H|1/2 over a large field range, the model shows that increasing and decreasing portions of the GMR vs H hysteresis curve turn out to be linear, parallel to each other, for the operating field region. If the GMR curve is linear in a region away from Hc, a bias field operation in this range would mean that the sensor would not only be nearly noiseless (save for Johnson noise) but also would exhibit linear change in field under an applied field H and could easily be calibrated to measure H. Thus, the modeling points the way to low noise GMR field sensor design. In practice, it is found that biasing at fields halfway down the GMR curve does indeed produce a much quieter measurement than near zero bias field.