Harmonic signals of magnetic nanoparticle in suspension for biosensing application: Comparison of properties between Brownian- and Néel-dominant regions

Abstract

We have studied harmonic signals caused by the nonlinear AC magnetization (M-H) curve of magnetic nanoparticles (MNPs) suspended in solution. First, numerical simulations were performed for wide ranges of the MNP parameters and the excitation fields that are commonly used in biosensing applications. As an example, the third harmonic signal was mainly studied. Because the AC M-H curve for the suspended MNPs is affected by both Brownian and Néel relaxations, we demonstrated the condition that determines the dominant relaxation. We emphasize that the MNP’s behavior changes from Brownian-dominant to Néel-dominant relaxation when the amplitude of the excitation field is increased, even when the MNP parameters are fixed. The properties of the harmonic signals in the Brownian- and Néel-dominant cases were compared, and the differences between the two cases were clarified. The relationships among the higher harmonic signals were also presented. Next, an analytical expression for the third harmonic signal was obtained for both the Brownian- and Néel-dominant cases. In the Néel-dominant case, the alignment of the easy axes caused by the AC field was taken into account. We demonstrated good agreement between the simulation and the analysis results. Finally, harmonic signals were measured using a commercial MNPs sample. Reasonable agreement was obtained between experiment and analysis. The results obtained will be useful for biosensing applications.