Alignment and small oscillation of superparamagnetic iron oxide nanoparticle in liquid under alternating magnetic field

It is important to understand the rotational behavior of magnetic nanoparticles (MNPs) in liquids under an alternating magnetic field for their successful application in biomedical fields. These applications include magnetic hyperthermia therapy and magnetic particle imaging. In particular, recent theoretical studies suggest that the physical rotation of MNP itself causes a significant change in the magnetization response of MNPs. However, because it is hard to distinguish the contribution of the physical rotation to the magnetization curve, its behavior under an alternating field has not been established to date. In this study, we measured the absorbance change of maghemite MNP suspension. We observed magnetically induced linear dichroism under a damped oscillatory magnetic field (DOMF) with Faraday configuration, where the propagation of light was parallel to the magnetic field. Accompanied by the application of DOMF, an absorbance change was observed. Through numerical simulations, it was revealed that the waveform of the absorbance change reflects the behavior of the physical rotation. From the analysis of the waveform, it was experimentally found that the easy axis of the MNP aligns partially and oscillates with the small amplitude under an alternating magnetic field. The rotation mechanism of the MNP in liquids under an alternating magnetic field is proposed based on the observed waveform of the absorbance change. We also investigated the influences of the viscosity and the field frequency on the rotation behavior and confirmed that these results were consistent with the mechanism. Published under license by AIP Publishing.