Spin Amplification for Magnetic Sensors Employing Crystal Defects
  Recently there have been several theoretical and experimental studies of the prospects for magnetic field sensors based on crystal defects, especially nitrogen vacancy (NV) centres in diamond. Such systems could potentially be incorporated into an AFM-like apparatus in order to map the magnetic properties of a surface at the single spin level. In this Letter we propose an augmented sensor consisting of an NV centre for readout and an `amplifier' spin system that directly senses the local magnetic field. Our calculations show that this hybrid structure has the potential to detect magnetic moments with a sensitivity and spatial resolution far beyond that of a simple NV centre, and indeed this may be the physical limit for sensors of this class. 
Marcus Schaffry and Erik M. Gauger and John J. L. Morton and Simon C.   Benjamin
cond-mat.mtrl-sci quant-ph