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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We report on the fabrication of condensed and mesoporous silica coated CoFe2O4 and FeCo alloy magnetic nanocomposites. The CoFe2O4 magnetic nanoparticles were encapsulated by well defined silica layer with a uniform thickness of 5 nm. The mesoporous silica shells lead to a larger magnetic coercivity than that of the pure CoFe2O4 magnetic nanoparticles due to decrease of interparticle interactions and magneto-elastic anisotropy. In addition, the FeCo nanoparticles were coated with condensed and mesoporous silica. As a consequence, the condensed silica protects the reactive FeCo alloy from oxidation up to 300 °C, maintaining the high magnetization of the nanoparticles. However, saturation magnetization of silica coated FeCo nanoparticles is dramatically decreased after annealing at 400 °C due to the oxidation of the FeCo core.

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