In order to better understand the origin of multiple quantum transitions observed in superparamagnetic nanoparticles, electron magnetic resonance (EMR) studies have been performed on iron oxide nanoparticles assembled inside the anodic alumina membra
ne. The positions of both the main resonance and forbidden (double-quantum, 2Q) transitions observed at the half-field demonstrate the characteristic angular dependence with the line shifts proportional to 3cos2q-1, where q is the angle between the channel axis and external magnetic field B. This result can be attributed to the interparticle dipole-dipole interactions within elongated aggregates inside the channels. The angular dependence of the 2Q intensity is found to be proportional to sin2qcos2q, that is consistent with the predictions of quantum-mechanical calculations with the account for the mixing of states by non-secular inter-particle dipole-dipole interactions. Good agreement is demonstrated between different kinds of measurements (magnetization curves, line shifts and 2Q intensity), evidencing applicability of the quantum approach to the magnetization dynamics of superparamagnetic objects.
In order to better understand the transition from quantum to classical behavior in spin system, electron magnetic resonance (EMR) is studied in suspensions of superparamagnetic magnetite nanoparticles with an average diameter of ~ 9 nm and analyzed i
n comparison with the results obtained in the maghemite particles of smaller size (~ 5 nm). It is shown that both types of particles demonstrate common EMR behavior, including special features such as the temperature-dependent narrow spectral component and multiple-quantum transitions. These features are common for small quantum systems and not expected in classical case. The relative intensity of these signals rapidly decreases with cooling or increase of particle size, marking gradual transition to the classical FMR behavior.
