Static and Dynamic Magnetization Responses of Self-Assembled Magnetic Nanoparticle Chains

The dynamic magnetization responses of magnetic nanoparticles (MNPs) subjected to alternating magnetic fields have been exploited for many biomedical applications, such as hyperthermia therapy, magnetic biosensing, and imaging. This dynamic process is governed by the combined Brownian and Néel relaxations via various energy terms. Both extrinsic factors, such as external alternating fields, dipolar fields, and the properties of the MNP medium, and intrinsic factors, such as the shape, size, and the magnetic properties of the MNPs, can affect their dynamic magnetization responses. However, due to the complex energy terms and interparticle interactions involved, it can be challenging to characterize how each factor influences the dynamic magnetization responses. In this study, we systematically examined the static and dynamic magnetization responses of an ensemble of MNPs. By solidifying the MNP suspension under a fixation field, the immobilized MNPs form long chains, and their easy axes are artificially tuned. In this simplified model, factors such as relative orientations of MNPs' easy axes to the external field and the dipolar interactions of MNPs are studied. Using a magnetic particle spectroscopy (MPS) platform, the time domain dynamic magnetization responses, dynamic hysteresis loops, high harmonics (which are of interest for MPS and magnetic particle imaging applications), and phase lag of MNPs' magnetizations to external fields were recorded. A strong correlation between the phase lag of MNPs and the nonlinearity in AC magnetization loops was established.