Characterization of magnetic nanoparticle ensembles via Thermal Noise Magnetometry*

Accurate characterization of magnetic nanoparticles (MNPs) is essential because of their biomedical applications, such as magnetic particle hyperthermia, magnetic particle imaging, and drug targeting. Most magnetic methods characterize MNPs by measuring their response to an externally applied excitation field, which can affect the state of the particles in the ensemble, thereby complicating the characterization of the individual particle properties.

We recently proposed a passive method called Thermal Noise Magnetometry (TNM) which captures the dynamics of the particles in thermal equilibrium without external excitation. This purely observative measurement reduces the impact of external influences on the measurement results to a minimum, thereby giving further insights into the system's fundamental magnetization dynamics. 

In this contribution, we discuss our recent advances that turned this emerging technique into a more practical and accessible magnetic characterization method. The scaling behavior of the stochastic TNM signal with sample properties [1] and the impact of temperature on the TNM signal [2] are discussed and verified with experiments and simulations. Until now, all TNM experiments have been performed with SQUID sensors, requiring cryogenic cooling and limiting the geometrical freedom of the experiment. We propose an alternative TNM setup based on Optically Pumped Magnetometers that is ideally suited to monitor clustering and aggregation processes in biological media. The performance of the tabletop setup is compared with an established SQUID-based TNM setup [3].