A circuit for controlling an electromagnetic head module to detect antigens on a biosample track comprising a processor for receiving position-error-servo signal from the PES read sensor, a write head for magnetizing nanoparticles attached to antigens, and a read sensor for detecting the nanoparticle-labeled antigens. The circuit may further comprise an X-axis actuator for controlling the head module in the direction perpendicular to the track and an Y-axis actuator coupled to the head module and the X-axis actuator for controlling the head module in the direction of the track. Target antigens are attached to the biosample track and nanoparticles via antibodies.