Magnetoresistive heads and the reciprocity principle

The calculation of the reproduce signal from a magnetoresistive (MR) sensor is complicated by spatial non-uniformities in the sensor's magnetization
\bar{M}
and large variations of signal fields over the height of the sensor. In general, accurate performance calculations require nonlinear self-consistent computations, which must be repeated at numerous head-medium positions. However, if one is interested in the first-harmonic (i.e., linear) response of the MR head as a function of recording density, this information can be very quickly obtained to a high degree of accuracy using the reciprocity principle.