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

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.