Comparing Magnetization Fluctuations and Dissipation in Suspended Magnetic Nanoparticle Ensembles

Recent research has demonstrated that thermal fluctuations on the net zero magnetization of a magnetic nanoparticle (MNP) ensemble can serve as a valuable tool for characterizing the sample's magnetic properties. These spontaneous fluctuations are intrinsically linked to the MNP system's response to small perturbations, as described by the fluctuation-dissipation theorem. We experimentally compare fluctuations and dissipation in both the linear and non-linear response regimes. Notably, a strong correspondence between the power spectral density (PSD) of the fluctuations and the out-of-phase dynamic susceptibility in the linear response regime was observed over a 500-kHz frequency range, facilitating interchangeability between these two characterization methods. This work contributes to the advanced characterization of MNPs for biomedical applications.