Studying static and dynamic magnetic properties of individual 22.6-nm superparamagnetic nanoparticles using diamond magnetic microscopy*

Nitrogen-vacancy Centers (NVC) in diamond are presently being investigated as sensitive probes of the magnetic properties of materials at the nanometer scale. This is facilitated by their unique properties, including the sub-ms spin coherence time and superb photostability at room temperature. We developed a new strategy based on NVC for high throughput, high spatio-temporal resolution characterization of individual magnetic nanoparticles (MNPs) for biomedical imaging. We doped a diamond chip with a near-surface layer of NVC and used it to perform wide-field microscopy by optically detecting the NV magnetic resonance frequencies. We measured the static and dynamic magnetic properties of individual 22.6-nm superparamagnetic nanoparticles and correlated them with their morphology determined from atomic force microscopy. The static magnetic images reveal magnetic dipole field patterns from small clusters as well as weaker signatures (~4 uT) from individual MNPs. The dynamic properties, dominated by Néel relaxation, show a variation of relaxation times, explained by size variation (~5%). This study provides first step to understand the effect of size and surface structure on MNP magnetic properties.