Overview of progress in giant magnetoresistive sensors based on NiFe/Ag multilayers

The evolution of devices based on NiFe and Ag in thin film giant magnetoresistance (GMR) structures is reviewed and traced from the early continuous multilayer (CML) structures, through granular alloys (GAs) of NiFe and Ag, spin valves (SVs), discontinuous multilayers (DMLs), and the most recent patterned multilayer (PML) structures. The technological motivation far the development of these various structures, based in particular on field sensitivity as an important figure of merit, and their limitations is discussed. Although DMLs are shown to possess the highest sensitivity, 1.2%/Oe, it is shown that they are limited by noise. Various issues affecting the implementation of NiFe/Ag DML sensors are addressed with particular emphasis on noise, since presently noise gates the development of a successful sensor. The MR characteristics of DML sensors are examined as a function of different biasing schemes to reduce noise and improve linearity. It is shown that the transverse response of ten-layer DML sensors when transversely biased may give acceptable noise performance. This represents one of the two fundamental approaches to the reduction of noise in NiFe/Ag MLs, i.e. one based on the concept of increasing the number of easily switchable ferromagnetic "grains" in the structure. The other approach is based on the concept of a sensor consisting of single large domains in each NiFe layer which provided the impetus for the development of patterned multilayer (PML) devices. Finally, the GMR response of these PML structures is briefly discussed.