Time-Resolved Diamond Magnetic Microscopy of Superparamagnetic Iron-Oxide Nanoparticles

Superparamagnetic iron-oxide nanoparticles (SPIONs) are promising probes for biomedical imaging, but the heterogeneity of their magnetic properties is difficult to characterize with existing methods. Here, we perform wide-field imaging of the stray magnetic fields produced by hundreds of isolated ∼30 nm SPIONs using a magnetic microscope based on nitrogen-vacancy centers in diamond. By analyzing the SPION magnetic field patterns as a function of the applied magnetic field, we observe substantial field-dependent transverse magnetization components that are typically obscured with ensemble characterization methods. We found negligible hysteresis in each of the three magnetization components for nearly all SPIONs in our sample. Most SPIONs exhibit a sharp Langevin saturation curve, enumerated by a characteristic polarizing applied field, B_c. The B_c distribution is highly asymmetric, with a standard deviation (σ_c = 1.4 mT) that is larger than the median (0.6 mT). Using time-resolved magnetic microscopy, we directly record SPION Néel relaxation, after switching off a 31 mT applied field, with a temporal resolution of ∼60 ms, which is limited by the ring-down time of the electromagnet coils. For small bias fields |B_hold| = 1.5-3.5 mT, we observe a broad range of SPION Néel relaxation times - from milliseconds to seconds - that are consistent with an exponential dependence on B_hold. Our time-resolved diamond magnetic microscopy study reveals rich SPION sample heterogeneity and may be extended to other fundamental studies of nanomagnetism.