Sputtered permanent magnet arrays for MR sensor bias

Magnetic field sensors consisting of four magnetoresistive elements in a bridge configuration are capable of detecting very low magnetic fields. These sensors require the application of a bias field to maximise the sensitivity and prevent domain wall formation which gives rise to Barkhausen noise. The design of thin film permanent magnet arrays for the production of sensor bias fields is considered. A simple charge sheet model is used to calculate the field from thin-film PtCo magnet arrays and the criteria for the production of sufficiently large and uniform bias fields are discussed. The use of these magnet arrays is shown to enable the simple construction of sensors with excellent vector response and allow the possibility of producing sensors capable of measuring fields up to 100 Oe with a linearity better than 1%.