Magnetic nanoparticles of γ-Fe 2 O 3 coated by organic molecules and suspended in liquid and solid matrices, as well as a non-diluted magnetic fluid have been studied by electron magnetic resonance (EMR) at 77-380 K. Slightly asymmetric spectra observed at room temperature become much broader, symmetric, and shift to lower fields upon cooling. An The shift and broadening of the spectrum upon cooling are ascribed to the role of the surface layer, which is considered with taking into account the strong surface-related anisotropy. To describe the overall spectrum shape, a "quantization" model is used which includes summation of the resonances corresponding to various orientations of the particle's magnetic moment at a given temperature. This approach, supplemented with some phenomenological assumptions, provides satisfactory agreement with the experimental data.

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Absorption at multiple resonance frequencies is observed in magnetic nanoparticles in strong similarity with forbidden multiple quantum transitions known for paramagnetic ions. Temperature and concentration dependences have been studied to reveal the origin of the signals. The results are discussed using a "quantization" approach, considering resonance transitions between energy levels of a giant spin corresponding to the total magnetic moment of a nanoparticle. The multiple quantum transitions are ascribed to both the effects of anisotropy and dipole-dipole interactions between nanoparticles.

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 in 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.

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