The time of the NP decaying magnetic field for bound particles in SPMR measurements is on the order of seconds. Unbound particles of similar size decay on the order of milliseconds, contributing very little to the results.
The decay curve for bound NP is fit by an equation of the form
or
The constants are fit to the experimental data and a particular time point is used to extract the value of the magnetic field. The fields from all the sensor positions are then used to construct a field contour map.