Recognition of rotational modes of magnetic nanoparticles by frequency dependence of magnetic linear dichroism under AC field

This study shows that magnetic linear dichroism (MLD) measurement has great potential for characterizing magnetic nanoparticles (MNPs) used in recently developing biomedical applications or nanoscale mechanical measurement techniques. MLD of MNP suspension reflects the orientation of the MNP. We investigated the frequency dependence of the MLD of MNP suspension under a simple AC field and the effect of the MNP size and material. Under an AC field with the frequency f, the MLD oscillated with 2f for every MNP. The amplitude and phase of this 2f-oscillation were precisely measured with a lock-in amplifier, and we showed the MLD2f frequency spectrum, a plot of real and imaginary parts of the 2f-component of MLD as a function of f. We found that the shape of the MLD2f frequency spectrum can distinguish the rotational modes of MNPs in an AC field. Therefore, this spectrum is helpful for the selection of MNPs for each technique mentioned above. We propose two model functions to fit the MLD2f frequency spectra, consistent with each rotational mode. It is shown that one of the fitting parameters tau(0) is associated with the rotational Brownian relaxation time of MNP, and the other parameter beta would represent the distribution of the anisotropy energy. The frequency dependence of MLD will provide precise insights into the magnetization and orientational dynamics of the MNP in liquid.