Magnetic resonance in nanoparticles: between ferro- and paramagnetism

Magnetic nanoparticles of γ-Fe(2)O(3) coated with organic molecules and suspended in liquid and solid matrices, as well as 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 and symmetric, and shift to lower fields upon cooling. An additional narrow spectral component (with a line-width of 30 G) is found in diluted samples; its magnitude obeys the Arrhenius law with an activation temperature of about 850 K. The longitudinal spin-relaxation time, T(1)≈10 ns, is determined by a specially developed modulation method. The angular dependence of the EMR signal position in field-freezing samples points to substantial alignment, suggesting the formation of dipolar-coupled aggregates. The shift and broadening of the spectrum upon cooling are assigned to the effect of the surface-related anisotropy. To describe the overall spectral shape, the 'quantization' model is used which includes summation of resonance transitions over the whole energy spectrum of a nanoparticle considered as a giant exchange cluster. This approach, supplemented with some phenomenological assumptions, provides satisfactory agreement with the experimental data.