Dynamic Magnetic Properties of Fe₃O₄ Nanoparticle Assembly in High-frequency Range

Abstract

The magnetic nanoparticle assembly is the candidate as a high-frequency magnetic material with isotropic magnetic properties. For practical applications, the dynamic properties of the superparamagnetic nanoparticle assembly, especially the upper limit of frequency for superparamagnetic properties, should be clarified. According to the phenomenological thermal relaxation theory, the upper limit of frequency has been assumed to be the blocking resonance frequency, f_b. However, considering that the superparamagnetic nanoparticle is one of the ferromagnetic materials, the influence of the ferromagnetic resonance of magnetic nanoparticles should be considered in high frequency range near the ferromagnetic resonance frequency, f_r. In this study, in order to clarify the upper limit of frequency for the superparamagnetic properties, we investigated its dynamic magnetic properties at high frequencies close to f_r using Fe₃O₄ nanoparticles. As a result, we found that magnetic nanoparticle assembly can be made to maintain its superparamagnetic properties almost up to approximately f_r by reducing the nanoparticle volume.