Effect of easy axis alignment on dynamic magnetization of immobilized and suspended magnetic nanoparticles

Magnetic nanoparticles (MNPs) have been widely studied for use in biomedical applications suchas magnetic particle imaging and magnetic hyperthermia. We studied the effect of easy axis alignment on dynamic magnetization of both immobilized and suspended MNPs. For immobilized samples, easy axes of magnetization were aligned by applying a dc field during immobilization. For suspended samples, the applied ac field was set so that the alignment of the easy axes was caused by the ac field. Magnetization (M-H) curves of MNPs with different degrees of easy axis alignment were measured withan applied ac field. We found that the easy axis alignment of MNPs considerably affects their M-H curve and harmonic spectra, which indicates the importance of this alignment for their biomedical applications. The experimental results were quantitatively explained with numerical simulation that took account of easy axis alignment and core size distribution in the sample. We can express the degree of easy axis alignment using a distribution function of the easy axis angle for both immobilized and suspended cases. The distribution function will be useful for quantitatively evaluating MNP performance in biomedical applications.