A magnetoresistor is monolithically integrated with an active circuit by growing a thin film magnetoresistor on a semiconductor substrate after the substrate has been doped and annealed for the active devices. The magnetoresistor is grown through a window in a mask, with the mask and magnetoresistor materials selected such that the magnetoresistor is substantially non-adherent to the mask. InSb is preferred for the magnetoresistor, Si.sub.3 N.sub.4 for the mask and GaAs for the substrate. The non-adherence allows the mask to be substantially thinner than the magnetoresistor without impairing the removal of the mask after the magnetoresistor has been established.