Magnetic and viscous modes for physical rotation of magnetic nanoparticles in liquid under oscillating magnetic field

The response of magnetic nanoparticles (MNPs) in a liquid to an alternating magnetic field is complicated because the rotational motions of both the magnetic moment in the MNP (internal rotation) and the MNP itself (physical rotation) affect each other. It is necessary to elucidate the mechanisms of these rotation behaviors for the success of the recent bio-applications of MNPs. However, the experimental technique for the observation of the physical rotation has been lacking. In our previous work, we demonstrated that the physical rotation of MNPs could be measured quantitatively from the absorbance change with a Faraday configuration. Then, it was revealed that the ten nm-sized maghemite MNP kept aligning to some extent with small oscillation. This behavior is consistent with the magnetic mode that Usov and Liubimov proposed from their numerical simulation. In the present study, we improved the apparatus to measure the induced linear dichroism with a Voigt configuration. This improvement increased the signal-to-noise ratio by about 100 times and allowed us to investigate the dependence of the physical rotation on the size of magnetite MNPs. It was shown that smaller MNPs (11nm and 13nm) rotated in the magnetic mode. By contrast, the rotational motion of the biggest MNP (25nm) had different features compared to the smaller ones. These differences can be explained with viscous mode that Usov and Liubimov also predicted. Furthermore, we suggest a dimensionless parameter as a useful criterion for the transition of these modes.