Principle of operation of magnetoresistive sensors, app note from Honeywell.
Magnetoresistivity is the ability of a material to change resistance under the influence of magnetic fields. There are several different magnetoresistive effects, but Honeywell sensors use the Anisotropic Magneto Resistive (AMR) effect which occurs in ferrous materials, including Permalloy. Permalloy is an alloy of nickel and iron and has been used as a sensing material since the early part of the 20th century in transformer designs. Low sensitivity to mechanical stress coupled with high sensitivity to magnetic fields provide sensors that vart from magneto meters to automotive engine applications.