Diamond Magnetic Microscopy of Individual Superparamagnetic Nanoparticles*

Nitrogen-Vacancy (NV) centers in diamonds are currently being used as a quantum sensor for imaging magnetic fields at the nanometer scale due to their magnetic field dependent fluorescence. We have applied this method to characterize Super Paramagnetic Iron Oxide Nanoparticles (SPIONs) for biomedical applications. Isolated 22 nm SPIONs were dispersed on a diamond substrate with a 200 nm thick NV layer, and images of SPION stray magnetic fields were obtained by optically detected magnetic resonance imaging. When subjected to a bias magnetic field the SPIONs form their own magnetic dipole, which was imaged and analyzed with a Mathematica code that fit the magnetic dipole equation to the image. Magnetic images for dozens of individual SPIONs were obtained as a function of applied field and as a function of time after abruptly turning off an applied field. From this analysis we seek a more fundamental understanding of how minor differences in size and shape of SPIONs can have drastic changes to their superparamagnetic behaviors.