Ferromagnetic resonance of superparamagnetic nanoparticles: The effect of dipole-dipole interactions

摘要

Superparamagnetic iron-oxide nanoparticles play an important role in a wide range of applications and determining their magnetic state is crucial. Typically, it is assumed that dipole-dipole interactions are not relevant in the superparamagnetic state due to thermal fluctuations. Here, we show evidence of how dipole-dipole interactions modify the collective magnetic state. Ferromagnetic resonance spectroscopy of iron-oxide nanoparticles with a diameter of 10 nm reveals that the configuration of the particles has a strong effect on their effective magnetic anisotropy in short time scales. Fits of the experimental spectra with a theoretical model enable the quantification of these anisotropy fields. Particles in suspension exhibit an easy-axis anisotropy due to the field-induced alignment, whereas condensed dry particles exhibit easy-plane anisotropy due to clustering, and the difference between uniaxial anisotropy in suspension and in the condensed state is on the order of 0.1 T. These findings highlight that dipole-dipole interactions have a strong effect on the collective magnetic state despite thermal fluctuations and should be taken into account in any high-frequency application because in short time scales, the configuration of the particles exhibits effective anisotropy that is an order of magnitude larger than the intrinsic magnetocrystalline anisotropy.