Analysis of magnetoresistors with high coercivity biassing films

It is necessary to bias permalloy thin-film magnetoresistive sensors in order to increase their sensitivity to applied fields and prevent domain wall formation giving rise to Barkhausen noise. This is possible by patterning the device from a three layer film consisting of the NiFe magnetoresistive layer and a high coercivity Co-25at.%Pt over layer separated by an insulating SiO2layer. Magnetic measurements have been made on this multilayer system and show that the spacing layer must be a minimum of 2000 Å thick in order to prevent increases in anisotropy magnitude and dispersion of the NiFe layer. A model has been developed for the behaviour of this multilayer system. The model predicts an increase in performance for overlayer biassed sensors compared with linear bias. The increase in zero bias sensitivity is found to be due to the non linear bias field from the permanent magnet layer partially compensating for the increase in demagnetising field near to the edge of the magnetoresistor strip. Both single strip sensors and strip arrays are considered and the sensitivity is found to depend strongly on spacer thickness for strip arrays but is much less dependent on spacer thickness for single strips.